It’s already been a good year for comets — and it just got astronomically better. A newly discovered comet is poised to wow us in the coming week: Comet C/2023 P1 (Nishimura). It’s already readily visible and the best views are just days away.
Japanese amateur astronomer Hideo Nishimura discovered the new comet August 12 near the star Zeta (ζ) Geminorum in Gemini the Twins, according to Seiichi Yoshida’s page on the object. At that time, the comet was already magnitude 10.4 and closing in on the Sun at a distance of just 1 astronomical unit, or AU (1 AU is the average Earth-Sun distance). Nishimura is continuing into the inner solar system on its way toward perihelion, the closest point in its orbit to the Sun. It will reach that point in less than two weeks, on September 17, when it will swing around our star at a distance of just 0.23 AU — some 40 percent closer to the Sun than the planet Mercury.
Along the way, Nishimura is expected to brighten rapidly, potentially reaching magnitude 2. Although that should be an easy naked-eye magnitude, the comet’s low altitude in the Northern Hemisphere means it’s really best seen with binoculars or a telescope. But through those optics, it will dazzle. Keep reading to learn where it is now, when to view it, and what to look for.
How to see Comet Nishimura
Comet Nishimura is now visible in the predawn skies for Northern Hemisphere observers. It’s rising later each morning as it tracks quickly through the stars of Leo the Lion. On September 7, it will rise around 4:20 A.M. local daylight time. By Sunday morning (the 10th), it will rise around 5 A.M. local daylight time. This is because the comet is rapidly approaching the Sun, so it appears to be moving quickly through our sky. By perihelion, it will rise with the Sun and won’t be visible at all (or again) above the equator.
What this means is, now is the time to see it!
Look east one hour before sunrise on September 7 to catch Comet Nishimura low in the eastern sky near Epsilon Leonis. Credit: Alison Klesman (via TheSkyX)
Step outside an hour before sunrise tomorrow morning and you’ll see the front half of Leo rising above the eastern horizon. Look for the bright star Regulus (magnitude 1.4), which is just 2° high at that time. (Don’t confuse it for blazingly bright Venus, which is magnitude –4.8 and much higher in the east to Regulus’ far upper right, in Cancer.) Fortunately, Nishimura is a bit higher than Regulus: On the 7th, it is 12° high an hour before sunrise, located 1.7° due east of 3rd-magnitude Epsilon (ϵ) Leonis, the endpoint of the curve that forms the famous Sickle asterism.
But Nishimura is moving fast. It will continue tracking east, “falling” toward the Sun in the predawn sky. It passes less than 3° north of Gamma (γ) Leonis on the 9th and then slips 1.5° south of Delta (δ) Leonis on the 12th. By the 12th, the comet will rise around 5:40 A.M. local daylight time, almost exactly an hour before sunrise. You can try to spot it some 4.5° high 30 minutes before the Sun peeks above the horizon. The comet will be about 15° northeast of (to the left and slightly above) Mercury, which by that time will be magnitude 2.7 and 3° high.
From there, Nishimura will slide just under Denebola, the star marking Leo’s tail, and cross into Virgo on the 15th, which is likely the last day Northern Hemisphere observers will be able to catch it. After that, it will be too close to the Sun for observation.
Comet Nishimura moves quickly through Leo. It is visible for about another week in the predawn sky. Credit: Alison Klesman (via TheSkyX)
A bright comet
Comet Nishimura on August 19, while still in Gemini. To the left of the comet is the Eskimo Nebula (NGC 2392). Credit: Marion Haligowski
Although the comet will be dropping lower each morning as the sky grows brighter, there’s an important caveat: Nishimura will be growing brighter as well. Observers are currently reporting on the Comet Observation database (COBS) that Nishimura is 5th magnitude, and it’s predicted to reach magnitude 2 to 3. Stunning astrophotos are already circulating, featuring the comet’s glowing green head and long, thin tail.
Although its diminishing altitude in the brightening morning sky reduces the likelihood of spotting it with the naked eye even at its brightest, Nishimura should remain readily visible in binoculars or any small telescope. It’s also an excellent target for astrophotographers; if you’re interested in how to best photograph the comet, we’ve got advice for photographing comets from highly experienced astrophotographer Damian Peach. (You can send your photos to readergallery@astronomy.com; we’d love to feature them online or in print!)
While observing Nishimura, always take care to stop viewing through binoculars or a telescope several minutes before the Sun rises from your location — and note the time of exact sunrise will vary by location, so check this information specifically for your observing site.
Comets are unpredictable objects, and Nishimura has never been identified before. It may or may not survive its close trip around the Sun, and it may or may not brighten as expected during that time. There’s always the chance it could outburst and brighten suddenly or even more than expected, as 12P/Pons-Brooks did in July.
If all goes well, Nishimura will swing past the Sun and quickly head back for the outer reaches of the solar system, never to return. Although it came from our Oort Cloud, it’s now a hyperbolic comet, meaning it has enough energy to escape the Sun’s gravity and rocket off into interstellar space. So now is truly your best — and only — time to see it. Get out there!
Our universe is a secretive empire that burdens many astronomers with misconceptions. And nothing is as amazing, confusing, and elusive as the colors of the cosmos.
Take everyone’s favorite binary star, Albireo, whose components shine in a gorgeous contrasting yellow and blue. Science explains that compared with its golden counterpart, the blue star is hotter because its greater mass creates awesome gravitational pressure and a boosted burn rate in its core.
But few astronomers know that those colors don’t exist when no one’s looking. That’s because light is really just an energy morsel composed of alternating magnetic and electric fields. Neither field has brightness nor color. Instead, when that invisible electromagnetic energy strikes an animal’s cone-shaped retina cell, it inaugurates a biological process where millions of neurons cooperatively fashion the sensation of “blue.” Creating visual experiences consumes half the brain’s capacity. So, while Albireo is some 400 light-years away, its colorful image occurs solely within the skull.
What’s more, usually-gorgeous Albireo is colorless if it’s not optically intensified by a lens or mirror. Our retina has about 100 million specialized rod-shaped cells that solely function in low-energy situations and deliver their sensations in grayscale alone. It’s the less-sensitive cones, numbering only 6 million, that register color. That’s why the Pleiades look gray or white to the naked eye but pastel blue through binoculars.
When light is faint, the human mind won’t create any sort of color, which is why galaxies are always visually gray no matter the telescope size. And even the gray is sometimes jeopardized: At bright levels, the left eye’s blind spot — where the optic nerve sits — never coincides with the right eye’s, so the image remains intact. But at low light levels, a different situation occurs. Our rod-cell-based scotopic vision suffers a huge blind spot twice the Full Moon’s width lying straight ahead, with the areas of both eyes where no rods are present matching up. It’s an important reason to observe faint celestial objects by looking slightly to the side. A 15° offset is ideal.
And even that retinal rod-versus-cone business is a simplification. There are three different cone types — named L, M, and S — and only the S variety can show objects as blue. That’s why 8 percent of all males, missing one of the three types of cones since birth, perceive the cosmos differently from the rest of us. They see rainbows as simplified bands of blue and yellow. These people with deuteranopia (aka colorblindness), happily see Albireo as we do, though the contrasting hues of other binaries like Antares elude them.
If you chose a single intriguing cosmic color, it would be hard to beat green. It’s the wavelength of peak energy emission of the Sun. The topmost sensitivity of the human eye. The chief and often only visual color of most aurorae, thanks to oxygen’s emission at 557.7 nanometers. The main color of planetary nebulae. Yet amazingly, while you’ll find stars that are red, orange, yellow, blue, brown, black, or even purply, there are no green ones.
Why? Our Sun emits electromagnetic energy that creates in our minds the sensation of every spectral color, as rainbows vividly demonstrate. All the universe’s “living” stars with active fusion cores emit those same colors and no others. None fail to provoke human visual systems into perceiving red, green, and blue — light’s primary colors, which appear white when combined. That’s why the universe’s overall color is white or beige.
If a star is unusually hot, it emits energy we perceive as a blue excess. Cool stars like Betelgeuse create a red surplus. But stars never emit solely green. And since nearly all stars still emit a lot of green, red, and blue, white remains the main takeaway, with any extra blue or red constituting a pallid embellishment. This white flood explains why stars rarely appear richly hued but are only pastels.
The toughest celestial color is red, which can’t be seen at all when faint, not even as gray. That’s why the reds in the Orion Nebula (M42), so stunning in astroimages, are rarely visible to the eye, even with backyard equipment. Considering M42’s ruddy source — excited hydrogen, the most abundant element — it’s ironic and unfair that this hue is withheld from our eager eyes.
It’s yet another quirk in a cosmos crowded with them.
The Andromeda Galaxy (M31) makes for a great late summer target, as a naked-eye object or with binoculars or a telescope. It’s the closest major galaxy to us at 2.5 million light-years. It’s also the most distant thing that most people can see with their naked eyes alone! (In the right conditions, some people can see M33, the Triangulum Galaxy).
Viewed with a moderate-sized telescope, M31 will reveal a bright condensed hub, well-defined dark lanes, and numerous satellite galaxies. M32 and M110 are in the same field, but don’t forget about NGC 147 and NGC 185 in Cassiopeia, the next constellation over. The fact that they are 7° away — roughly 14 times the diameter of the Full Moon — and still gravitationally bound to M31 is a dramatic indication of just how close Andromeda is, and how large it looms in our sky.
In fact, the Milky Way and Andromeda are moving toward each other — and in about 4 billion years, the two galaxies will meet in a collision that will warp their spiral structures and light them up with bursts of star formation. But by that time, if there is any life around to see this spectacular smash-up, it won’t be on Earth. We have only about a billion years before the Sun’s evolution renders Earth uninhabitable. So get out there and see Andromeda while you can!
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UPDATE: XRISM will now launch WEDNESDAY, SEPTEMBER 6, at 7:42 P.M. EDT (23:42 UTC). The August 28 launch was scrubbed due to inclement weather. All dates and times in the article have been updated to reflect the new launch date and time.
A new X-ray observing mission will change how we see and understand the ultra-hot universe. The X-ray Imaging and Spectroscopy Mission (XRISM), led by the Japan Aerospace Exploration Agency (JAXA), will now launch this week on the morning of September 7. (Note the launch occurs in the evening of the 6th in the U.S.; for those interested in watching the launch, see the details at the end of the story. An earlier launch attempt on August 28 was scrubbed shortly before the scheduled time due to inclement weather at the launch site.) After liftoff, XRISM will move into a low Earth orbit at a height of 340 miles (550 kilometers) and an inclination of 31°.
The universe is filled with hot gas that emits energetic light — X-rays — invisible to the naked eye and Earth-bound telescopes. Because energetic and extreme processes produce such light, X-rays hold critical information about the formation and evolution of the universe. XRISM (pronounced “crism”) will help experts understand how clusters of galaxies formed and evolved, how the universe produced and distributed chemical elements, what the structure of space-time looks like under gravity’s intense pull, and how massive black holes affect star formation in their host galaxies.
Capturing energetic light
X-rays are released in the universe’s biggest explosions and hottest places, such as supermassive black holes pulling matter inward into an accretion disk and ejecting jets at high speeds. By observing X-rays from these sources, XRISM can determine the velocities and energies radiating from the gas both swirling in and shooting away from the black hole.
Other sources of cosmic X-rays include the hot gas that lies between galaxies in massive clusters, called the intracluster medium. This gas can reach temperatures of tens of millions of degrees. Studying its composition, which is built up from the continual explosion of massive stars as they die, spreading the elements they forged in their cores out into space, will teach researchers how the chemistry of the universe has changed over time.
Credit: ESA, CC BY-SA 3.0 IGO
XRISM transforms space into an observational laboratory with its two identical mirrors, called X-ray mirror Assemblies (XMA). Unlike classic telescope mirrors, which are polished glass or metal, X-rays utilize a cylindrical construction of thin aluminum foils nestled one inside the other. In total, 1,624 segments make up each XMA. The unique mirrors, assembled at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, reflect those X-rays into a sensor on the spacecraft 18 feet (5.6 meters) away.
One of XRISM’s two instruments is a spectrometer called Resolve. A collaboration between JAXA and NASA, the instrument is kept 50 times colder than deep space in order to measure tiny changes in temperature imparted to its small 6×6 pixel detector by incoming X-ray photons. Temperature information can be converted into light intensity over varying ranges of energies between 400 to 12,000 electron volts. Resolve measures light hundreds to thousands of times more energetic than visible light, which has energies of just a few electron volts.
The instrument is kept this cold via a mechanical cooling process that takes place inside a container the size of a fridge, which is filled with liquid helium. The helium is expected to last for three years.
Current instruments can only observe X-ray spectra “in a comparatively blurry way,” said Brian Williams, NASA’s XRISM project scientist at the Goddard Space Flight Center, in a statement. “Resolve will effectively give X-ray astrophysics a spectrometer with a magnifying glass.” The James Webb Telescope, in comparison, gathers similar data but in infrared light.
To aid Resolve, another instrument developed by JAXA, dubbed Xtend, will give XRISM the ability to image X-ray sources with a bigger field of view than any other X-ray imaging satellite to date. Xtend can observe over an area some 60 percent larger than the average size of the Full Moon. The instrument will both monitor nearby stars that give of variable X-rays and also map the properties of X-ray sources in the background while Resolve is working.
Paving the way
XRISM is a collaboration between the Japan Aerospace Exploration Agency (JAXA), NASA, and the European Space Agency (ESA). About 8 percent of observational time was allotted to scientists at European institutions during XRISM’s three-year mission. JAXA’s Smart Lander for Investigating Moon (SLIM) will also catch a ride with XRISM aboard the H-IIA Launch Vehicle No.47 from the Tanegashima Space Center in Japan.
XRISM is not only a major accomplishment on its own, but will also serve as a pathfinder and test drive for future missions as well. “XRISM will be a valuable bridge between ESA’s other X-ray missions: XMM-Newton, which is still going strong after 24 years in space, and Athena, which is due to launch in the late 2030s,” said Matteo Guainazzi, ESA’s project scientist for XRISM in a statement.
How to catch the launch
The launch will be livestreamed on JAXA’s YouTube channel and is currently scheduled to occur at 8:42:11 A.M. JST (23:42:11 UTC) on September 7, which is 7:42 P.M. EDT on September 6. The broadcast will begin about half an hour earlier, at 8:10 A.M. JST on the 7th, or 7:10 P.M. EDT Wednesday night in the US. You can check JAXA’s XRISM Special Site for launch timing and updates.
Editor’s note: This article was originally published August 25, 2023. All times and dates have been updated to reflect XRISM’s current launch schedule.
The more we discover about Mercury, the weirder it seems. For instance, despite the fact that daytime temperatures there soar to 800 degrees Fahrenheit (427 degrees Celsius), ice encases the shadowed crater floors on its poles. The tiny planet should be devoid of the ice and other volatiles — compounds that can easily vaporize — that stuck to the larger terrestrial planets. After all, it has spent most of its existence close to the Sun, where fierce solar winds strip away atmosphere and even solid rock over time. But in fact, Mercury is rich in volatiles, perhaps more so than early Earth or Venus.
Those puzzling compounds (like water, carbon monoxide, and sodium) have led to some of the strangest features in the solar system. As vapors break through the surface of the planet, they leave structures typical of volcanic eruptions. These include vents, fissures, flows of material, chains of collapsed pits, and raised mounds topped by craters.
But strangest of all are Mercury’s hollows, unique sunken regions encircled by bright halos. First noticed as bright splotches decades ago, their apparent volcanic nature was revealed by NASA’s MESSENGER mission, which orbited the planet from 2011 to 2015. The problem with these hollows is that they shouldn’t be there — Mercury’s interior gases should have disappeared long ago, and the hollows are geologically fresh.
Resolving this contradiction is forcing scientists to contemplate some fundamental mysteries about how Mercury — and the solar system itself — formed. As Mercury expert Mark Robinson of Arizona State University quips, “I think the MESSENGER mission proved that Mercury cannot exist!”
Alien landscape
The Central Peak of the 163-milewide (263 km) Raditladi impact basin is marked by hollows in this mosaic from the MESSENGER orbiter. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Beneath its rarified skies, Mercury displays alien landscapes, both elegant and desolate, that reflect several geological processes.
Dramatic cliffs, called rupes, span hundreds of miles and tower over a mile high in places. These record Mercury’s planetary contraction: Scientists estimate that the planet’s radius may have shrunk by as much as 4.4 miles (7 kilometers) since it formed. It is probably still contracting even today.
Mercury’s surface also displays a record of the assault it has taken under a drizzle of rock, metal, and ice. Meteorites, asteroids, and comets have left dramatic rayed craters and colossalimpact scars. The largest is the Caloris basin, a 950-mile-wide (1,525 km) wound surrounded by mile-high mountains. Some basins have multiple concentric mountain chains like ripples in a frozen pond.
And then there is Mercury’s history of volcanism. Major flows and eruptions of molten rock appear to have ceased around 3.5 billion years ago. After that time, Mercury’s global contraction pinched off many of the volcanic sites. But smaller-scale volcanism carried on at locations where the crust was weakened, most commonly by impacts or faulting of the crust. Among the plethora of volcanic features parading across the face of Mercury are the enigmatic hollows. Mercury’s hollows were first spotted in images taken by Mariner 10 during its three flybys of the planet in 1974 and 1975. But the resolution of its images was too low to understand what Mariner scientists then classified as “bright, ill-defined patches.”
A 2014 paper in the journal Icarus by a team led by Rebecca Thomas, then a graduate student at The Open University in Milton Keynes, U.K., defines the hollows as “sub-kilometer scale, shallow, flat-floored, steep-sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters.” The hollows often congregate within craters, sometimes eating away the summits of central peaks or crater rims. Chains of smaller depressions melt into each other, forming wide areas of shallow, irregular cavities. They range in size from tens of yards to over a mile (1.6 km).
The authors and others have noted that the hollows are quite different from volcanic collapse pits, which are plentiful across the little planet. Volcanic pits are deep with rounded edges and irregular, rough floors. But the exotic hollows are shallow with smooth floors, scalloped margins, and often a blue coloration. Coronas of bright material composed of unknown substances surround these mysterious pits.
The lowlands of northern Mercury are represented in this false-color view based on MESSENGER data. Terrain at low elevation appears as purple, with the highest elevations appearing white. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Fresh features
By looking at hollows in regions of varying age, scientists can piece together how these features evolve. The indentations appear to start as a localized darkening of the surface. A central region begins to collapse into several pits and as it does, a brightening builds around the assemblage of hollows. This brightening is often quite dramatic, developing into a halo surrounding the depressions. As the hollows age, the activity that led to the halos trails off and the ground darkens again. Over time, the surface relaxes. Micrometeorites and solar wind erode the walls until the hollows fade away.
Investigators have come to realize that hollows are fairly recent in Mercury’s geological record. The youngest craters on Mercury have crisp rims and bright rays. Prominent hollows reside within many craters, including Balanchine, Degas, and Dominici. The hollows that have been imaged in detail lack any craters overlaying them, so they occurred after Mercury’s major cratering eras.
The hollows appear almost exclusively in Mercury’s darkest areas, blanketed by what scientists call Low Reflectance Material (LRM). Something about the LRM regions must be favorable for hollow formation. LRM is high in magnesium, calcium, and sulfur, so it may be that one or more of these elements are involved in creating hollows. LRM also has a greater abundance of carbon, probably in the form of graphite — another substance that may be responsible for the creation of hollows. While it is evident that hollows form through some type of sublimation (when a solid turns directly into vapor) that causes the surface to sink, the specific mechanism is still a mystery.
“It’s clear that the planet’s building blocks included materials that formed at relatively low temperatures,” says cosmochemist Larry Nittler of Arizona State University in Tempe, Arizona. But were they there when the planet formed — or were they delivered later in the planet’s
life by impacting comets? “It could well be that many of the planetesimals that formed Mercury formed further out in the [protoplanetary] disk,” says Nittler. “The ice and organic volatiles in polar craters are certainly a sign of recent delivery, likely by comets. But Mercury is also relatively rich in moderately volatile rock-forming elements like sodium, potassium, and chlorine, which must reflect how the planet formed.” In short, he says, “We really don’t know … why it is so volatile-rich.”
Some possibilities can be found in planet formation models, which show that early in the solar system’s history, Jupiter, Saturn, Uranus, and Neptune migrated toward and away from the inner solar system. As they did so, their gravity acted as snowplows, moving material around the solar system.
One model, called the Grand Tack, indicates that Jupiter robbed Mars and its surroundings of icy and rocky planet building material, sending it toward the inner system. During its planetary migration, Jupiter also cast many of the most water-rich asteroids inward, delivering water to the terrestrial planets. This would explain the diminutive size of Mars, the structure of the asteroid belt, the birth of terrestrial seas — and Mercury’s abundance of volatile materials.
Analysts have also been discussing a model in which Mercury itself formed much farther out and was transported to its current orbit.
Something in the air
The Central Peak of the crater Eminescu is surrounded by hollows, imaged here
by MESSENGER. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
The history of Mercury’s volatile compounds is also inextricably tied to that of the planet’s atmosphere — what little remains of it.
Mercury’s first tenuous atmosphere came directly from the solar nebula, containing mostly hydrogen and helium. The Sun also contributed by blasting atoms off surface rocks. Volcanic eruptions further added to the diaphanous layer of thin gases as the planet matured. But Mercury’s magnetic field and weak gravity could not hold on to the thin, hot air, and soon only a trace remained — a loosely bound layer of gas that scientists refer to as an exosphere.
As with the other terrestrial planets, Mercury lost its initial atmosphere quite early in its development, leaving scientists to try to piece together its complicated history. “Knowing what it may or may not have been doing in the past is not an easy thing,” says Ron Vervack, a planetary atmospheres researcher at Johns Hopkins University’s Applied Physics Lab in Laurel, Maryland. “I think I can pretty safely say that it never had anything like a martian atmosphere in terms of either pressure levels or global presence.”
Vervack suggests that even in its prime, Mercury’s atmosphere may have been a somewhat transient feature, produced when the young planets were bombarded by asteroids and comets left over from the solar system’s formation. “You could imagine that a bunch of comets raining down on Mercury might have led to temporary mini-exospheres at places where a comet struck Mercury,” he says. At that position, water vapor mixed with other compounds like carbon monoxide and carbon dioxide would form a temporary local exosphere over the region and eventually disperse.
According to Vervack’s research, some of this gas would make its way to the poles, settling into the permanently shadowed regions in deep crater floors and valleys. There, it could have been preserved in the form of the ice deposits we see today. In that sense, the polar deposits are very likely the remnants of Mercury’s earliest exosphere.
Today, the planet stands as a battered world offering us a rich chronicle of the early solar system. Its exosphere contains 42 percent oxygen, 29 percent sodium, 22 percent hydrogen, 6 percent helium, and traces of potassium, argon, neon, carbon dioxide, water, nitrogen, xenon, and krypton. Many of these ancient elements may play a role in the formation of the hollows.
The polar areas of Mercury contain craters with regions that remain permanently shadowed, regardless of the time of day. These regions show up in this map of Mercury’s north polar area, which displays the maximum biannual surface temperature. Regions shown in red are hot, reaching temperatures over 260 degrees Fahrenheit (127 degrees Celsius); regions shown in purple are as cold as minus 370 F (minus 223 C). These shadowed areas may contain deposits of ice that are the remnants of Mercury’s original exosphere. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Misty glows
In Washington Irving’s short story “The Legend of Sleepy Hollow,” the author describes a Dutch-settled valley as a shadowy glen with a haunted atmosphere. Mercury’s hollows are also places of mystery haunted by unseen forces.
“Hollows are just strange,” says Vervack. He envisions a fascinating vista above the hollows near twilight, as volatiles waft into the sky and replenish the planet’s exosphere. “The exosphere glows in various colors and shapes owing to how it is generated and which atomic species comprise it,” Vervack explains. “The sodium glows with the same color that sodium streetlamps do on Earth — that sort of yellowish-amber color.”
The sodium that evaporates to form hollows is a contributor to one of Mercury’s most curious features — its cometlike sodium tail, which extends away from the Sun (anti-sunward) for 15 million miles (24 million km). “When the sodium tail is extended anti-sunward strongly, you would easily see [it] in the night sky,” says Vervack. “Imagine being on the equator at midnight. The sodium atoms stream around from the dayside all around the terminator in the anti-sunward direction, so you would see this yellowish glow near the horizon in all directions.”
Sodium is not the only actor on the Mercury sky stage. Calcium would season the sky with a violet glow. Its light is strongest at dawn because this is the direction in which Mercury moves through interplanetary dust. “The wispy vapor over the hollows wouldn’t glow at night, but would perhaps be visible in the predawn hours,” Vervack says.
These fluorescent glows would paint the sky above the planet’s depressions a ghostly green and purple — a display that must only add to the unusual enigma of Mercury’s sleepy hollows.
For many, the night sky is a personal wonder — a journey that starts by looking up, and ends by looking in. “You are a child of the universe,” the poem Desiderata tells us, “no less than the trees and the stars.” We don’t need a telescope to feel the pull of the stars on our souls. A simple glance at a truly dark sky brings the cold truth of science — the visible naked-eye universe — down to you at a personal level. You feel something when you see it, if only for an instant.
This melding of science and art was at the forefront of the Romantic thinkers. “What sort of science is that which enriches the understanding, but robs the imagination?” asked Henry David Thoreau in his Journal.
Recently I attended a Zoom talk by U.K. astronomer Andy Newsam, a professor of astronomy education and engagement at Liverpool John Moore University. Newsam shared how he takes forays into the world of art to explore challenging concepts with unsuspecting audiences. In media including music, dance, theater, and sculpture, he has tackled scientific topics ranging from gravity and the nature of dark matter to the physics of breakfast. “People think about art and science being totally different things,” Newsam said, “but I have found working with artists that we really have a lot in common, and there is much we can learn from each other.”
Canadian artist and amateur astronomer Julian Samuel expresses what he sees through telescopes or in astrophotos. These oil paintings are about 40 by 60 inches
(100 by 150 cm) or smaller. They depict (counterclockwise from upper left) open cluster M11 in Scutum, globular cluster M13 in Hercules, and solar prominences. Credit: Julian Samuel
Composite astrophotography is one of the most popular ways amateur astronomers experiment with science and art. For example, Guatemalan astrophotographer Sergio Montúfar photographs the Milky Way and combines it with landscapes of national monuments in Guatemala, such as Mayan temples, to instill a sense of ancient wonder. Guatemala’s Ministry of Foreign Affairs has recognized his work and organized some of his exhibitions, which have traveled to several countries. He has been declared part of the Cultural Ambassador of Guatemala program and promotes the concept of “how lucky we are to have evolved consciousness, to understand where we are and what surrounds us.”
In January 2009, Jennifer Wu of Nevada City, California, was selected to be a member of Canon U.S.A.’s Explorers of Light program — an elite group of internationally recognized visual artists across all genres of photography. Her images, which combine the stars with some of the world’s most unique locations, skirt the boundary between astronomy and imagination.
“I love expressing my creativity by reaching for the stars and capturing them in my photographic images,” Wu says. “One of my favorite things about being a creator is the freedom to explore nature. I love photographing the night sky in particular because no moment is the same. Whether it’s a Full Moon, a comet passing by, or just the Milky Way shining through the trees, there’s so much beauty to be captured, and figuring out the best way to do it justice is a creative challenge I’ve always enjoyed.”
Julian Samuel, a member of the Royal Astronomical Society of Canada, Toronto Centre, expresses himself with abstract, expressionist methods. Samuel depicts forms of astronomical reality with unstructured abandon, reminiscent of Jackson Pollock or Franz Kline. He emphasizes a personal or emotional feeling through the free, spontaneous expression of what he sees when looking at astroimages or peering through a telescope.
“Nineteenth-century photography played a key role in liberating painting from realism to abstraction,” Samuel says. “I am using astrophotography and science to make nearly abstract paintings — reversing the link between photography and painting.”
So before you put your eye to the telescope, take a moment to took up at the stars and see how they shine for you. As always, express yourself by writing to me at sjomeara31@gmail.com
The region surrounding Sagittarius A*, the Milky Way’s own supermassive black hole. Eventually, black holes will be the last remaining matter in the universe.
NASA/JPL-Caltech/Judy Schmidt
The cosmos may never end. But if you were immortal, you’d probably wish it would. Our cosmos’ final fate is a long and frigid affair that astronomers call the Big Freeze, or Big Chill.
It’s a fitting description for the day when all heat and energy is evenly spread over incomprehensibly vast distances. At this point, the universe’s final temperature will hover just above absolute zero.
Until a few decades ago, it looked like that expansion would eventually end. Astronomers’ measurements suggested there was enough matter in the universe to overcome expansion and reverse the process, triggering a so-called Big Crunch. In this scenario, the cosmos would collapse back into an infinitely dense singularity like the one it emerged from. Perhaps this process could even spark another Big Bang, the thinking went.
We’d be gone, but the Big Bang/Big Crunch cycle could infinitely repeat.
In the years since then, the discovery of dark energy has robbed us of a shot at this eternal rebirth. In 1998, two separate teams of astronomers announced that they’d measured special exploding stars in the distant universe, called a type Ia supernova, which serves as “standard candles” for calculating distances. They found that the distant explostions — which should all have the same intrinsic brightness — were dimmer, and therefore farther away, than expected. Some mysterious force was pushing the cosmos apart from within.
This dark energy is now thought to make up some 69 percent of the universe’s mass, while dark matter accounts for another roughly 26 percent. Normal matter — people, planets, stars, and anything else you can see — comprises just about 5 percent of the cosmos.
The most important impact of dark energy is that the universe’s expansion will never slow down. It will only accelerate.
Heat death of the universe
Decades of observations have only confirmed researchers’ findings. All signs now point to a long and lonely death that peters out toward infinity. The scientific term for this fate is “heat death.”
But things will be rather desolate long before that happens.
“Just” a couple trillion years from now, the universe will have expanded so much that no distant galaxies will be visible from our own Milky Way, which will have long since merged with its neighbors. Eventually, 100 trillion years from now, all star formation will cease, ending the Stelliferous Era that’s be running since not long after our universe first formed.
How did we discover dark matter? What is dark matter made of? How is dark matter different than dark energy?Astronomy’s free downloadable eBook, The Science Behind Dark Matter, contains everything you need to know about the elusive and invisible substance.
Much later, in the so-called Degenerate Era, galaxies will be gone, too. Stellar remnants will fall apart. And all remaining matter will be locked up inside black holes.
In fact, black holes will be the last surviving sentinels of the universe as we know it. In the Black Hole Era, they’ll be the only “normal” matter left. But eventually, even these titans will disappear, too.
Stephen Hawking predicted that black holes slowly evaporate by releasing their particles into the universe. First, the smaller, solar-mass black holes will vanish. And by a googol years into the future (a 1 followed by 100 zeroes), Hawking radiation will have killed off even the supermassive black holes.
No normal matter will remain in this final “Dark Era” of the universe, which will last far longer than everything that came before it. And the second law of thermodynamics tells us that in this time frame, all energy will ultimately be evenly distributed. The cosmos will settle at its final resting temperature, just above absolute zero, the coldest temperature possible.
If this future seems dark and depressing, take comfort in knowing that every earthling will have died long before we have to worry about it. In fact, on this timescale of trillions of years, even the existence of our entire species registers as but a brief ray of sunlight before an infinite winter of darkness.
In the EHT image of Sagittarius A*, what are the brighter areas in the accretion disk?
Paul Kerns Indianapolis, Indiana
Although we’re confident about the size and width of the ring, we think the bright spots could just be artifacts of our very difficult image-reconstruction techniques, combined with challenges in imaging the source.
The main problem is that the plasma around Sagittarius A* (abbreviated Sgr A*) moves around very quickly while we try to take its image. We do expect some blobs due to random fluctuations in the turbulent accretion flow of the plasma around the black hole, but we’re not quite sure that’s what we see in the image.
The other black hole imaged by the Event Horizon Telescope (EHT), M87*, evolves on much longer timescales and is easier to image. We’re confident that the brightness asymmetry in that image is due to a phenomenon called Doppler beaming, whereby material moving toward us near the speed of light becomes brighter. Combined with a known large-scale jet direction, this allowed us to figure out which way M87* is rotating: clockwise from our point of view.
Ongoing improvements to the EHT will allow us to be more confident in these kinds of detailed features in our images of Sgr A*. We’re adding new stations around the world, upgrading the technology at existing sites, and working on our imaging algorithms. Stay tuned!
Angelo Ricarte Postdoctoral Fellow, Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts
For many observers, their first view of Saturn is one they will never forget. The sixth planet is responsible for recruiting many people to both amateur and professional astronomy with its majestic rings, bright colors, and tantalizing detail on display in the eyepiece. Here are some features to look for to get the most out of observing Saturn during this apparition.
Rings
Saturn’s most prominent and popular feature is its massive rings. You can see them with as little as 25x magnification — a pair of handheld binoculars will do. Rather than one continuous band, the rings are divided into sections with space between, which you can sometimes see on nights with a steady atmosphere. The most prominent gap is the 3,000-mile-wide (4,800 kilometers) Cassini Division between the two most obvious rings, A and B. With larger apertures (at least 6 or 8 inches) and good seeing conditions, the narrower Encke Gap reveals itself near the outer edge of the A ring.
The rings themselves, while broad, are very thin — ranging from a mere 33 feet (10 meters) to 0.6 mile (1 km) in width — and consist primarily of chunks of water ice spanning a range of sizes from meters to smaller than the width of a human hair. Saturn has a distinctly three-dimensional look to it, in part from limb darkening on the edges of the sphere, as well as from the shadow it casts on its rings. I have heard many people remark on how much it looks like I hung a picture in front of my 8-inch Schmidt-Cassegrain at outreach events! At opposition, when Saturn is directly opposite the Sun from Earth, a special brightening effect known as the Seeliger effect occurs due to the ring particles’ shadows “hiding” behind themselves. This causes the rings to appear much brighter than the planet’s surface for a few days before and after opposition. Opposition occurs Aug. 27, 2023; it next occurs Sept. 8, 2024.
The ringed behemoth has an axial tilt of 26.7°, meaning that, like Earth, Saturn experiences seasons. As it swings around its 29.4-Earth-year orbit, our view of Saturn changes from looking more toward its north pole to more toward its south pole. Its last maximum tilt away from us took place in March 2003, followed by maximum tilt toward us in 2016. In March 2025, the rings will appear edge-on for observers, and in 2033, it will reach maximum tilt away from us once again. Watch how the appearance of Saturn’s rings changes over the coming years as a result.
Surface features
While not as bright and apparent as Jupiter’s cloud bands, Saturn also sports belts of varying brightness and color. Occasionally light and dark spots appear, making Saturn’s 10-hour-and-14-minute rotation more obvious (10 hours 38 minutes at the slower-moving high latitudes). Rarely, a large storm will erupt, caused by an upwelling of warmer gas that can be thousands of miles wide. Revisit Saturn often to spot changes over time, and try out some color filters to discern features.
Moons
Despite Saturn’s great distance, several of its 100-plus moons are visible under the telescope’s gaze. The easiest to spot is hazy and mysterious Titan, the second-largest moon in the solar system, larger even than Mercury and the dwarf planets. Reaching as bright as magnitude 8.4, it is visible in binoculars and easy to spy in a telescope. The upcoming NASA Dragonfly mission will explore the methane lakes and cryovolcanoes of this frigid world.
You can sweep up Rhea with a 3-inch refractor as it orbits Saturn once every 4.5 days. On dark nights, you can also catch Saturn-hugging Dione and Tethys in as little as a 3-inch aperture, glowing at magnitudes 10.4 and 10.3 with 2.7-day and 1.9-day orbits, respectively. With a 6-inch aperture or larger, three more moons may reveal themselves: Enceladus; Iapetus; and under dark skies and excellent conditions, magnitude 12.9 Mimas, which I personally have yet to spot. You can decipher which is which using a planetarium app on your smartphone or computer.
Saturn is a popular target for a reason — its beauty and fascinating features make it worth returning to again and again.
Tiny metallic particles dredged up from the seafloor in the South Pacific could be the first substantial bits of material ever found from outside the solar system, according to test results reported in a scientific paper submitted last week by Harvard University astrophysicist Avi Loeb and his team. It is a dramatic claim of a major finding that could help theorists better understand planetary formation processes. But other scientists say that these preliminary results are not sufficient to prove these pinhead-sized spheres are really interstellar, and that further lab tests are needed to settle the matter.
But virtually everyone agrees that it’s possible to settle the question definitively, and that the claim is important enough that every effort should be made to nail down the answer. And many applaud Loeb’s risky and difficult mission to do the hard work necessary to find and recover the material needed to make such an answer possible.
Searching the seafloor
The material, presently being analyzed in a Harvard lab, was scooped up off the Pacific seafloor near Papua New Guinea in June. The search team prepared a sled covered with powerful magnets and dragged it along the ocean bottom behind a boat that Loeb had chartered for the project. The search was aiming to find remnants of a meteorite whose reentry was detected by US government sensors as a bright fireball on Jan. 8, 2014, and which Loeb and his student Amir Siraj had determined to have come from outside the solar system, based on the speed of its entry into Earth’s atmosphere. More than 700 spherules, mostly less than one millimeter across — about the size of a pinhead — were meticulously recovered from the mass of material picked up by the sled and have been undergoing analysis ever since the team returned to the US.
Loeb and his co-authors have written a paper on their initial test results and submitted it to the journal Ocean Sciences for peer review. They also posted it to the online arXiv preprint server, as is common practice in the physical sciences, on Aug. 29. The paper cites results from a mass spectrometer, an instrument that can determine the relative abundances of the elements within the spherules. The team says that of 57 spherules tested so far, five of them show a distinctive and quite anomalous composition.
Spherules can form when a fragment of a meteoroid is ablated as it enters the atmosphere. This image of one of the recovered spherules was taken with an electron microprobe in Stein Jacobsen’s laboratory at Harvard University. Credit: Avi Loeb
By comparison with typical meteoritic material, these five spherules have highly enhanced levels of several elements, especially beryllium, lanthanum, and uranium, with concentrations as much as a thousand times greater than in typical meteorites. Loeb and co-author Stein Jacobsen, a Harvard cosmochemist who did the mass spectroscopy measurements, dubbed this composition BeLaU after those elements.
After running the tests on those samples, “Jacobsen came to me and said, ‘This is something new, I haven’t seen it before’,” Loeb says. It was a composition that Jacobsen had never seen before in either meteorites or terrestrial sources. The unusual abundances, Loeb says, could be accounted for if the material came from a planet with a magma ocean — a planet so hot that its outer layers consisted of molten rock, and fragments of this material were dispersed through space.
Alternatively, it could be material produced in a supernova explosion, or possibly a collision of two neutron stars, either of which could theoretically produce such an unusual elementary enhancement. Or, Loeb and his co-authors suggest in the paper, it could be remnants of a technological object produced by intelligent aliens — a possibility, however remote, that was one of Loeb’s major motivations in searching for the material in the first place.
An extraordinary claim
Even hinting at that possibility is enough to create a backlash from some other astronomers, who feel that Loeb and his colleagues are going beyond what their evidence can so far support. “I object mightily to that language,” says Andrew Westphal, associate director of the Space Sciences Laboratory at the University of California, Berkeley. “There is absolutely nothing in any of these data that give any hint that this is some artifact technological object. … It does their credibility no good to even talk about this.”
But Westphal is also skeptical of the claim that the samples come outside the solar system at all. He says his main objection is that their comparison is based on known meteorites and material brought back by a few space missions. The assumption is that this is representative of the entire solar system, and “that just is not true.” Meteorites, he says, are heavily dominated by inner solar system material, and a few from Mars and the Moon. “We have no samples, for example, from Venus or Mercury, that we know of,” he says. So the claim that it must be extrasolar material because we’ve never seen anything like it has “a fundamental problem, because there can always be some body in the solar system from which this originated that we don’t have any samples of in the laboratory,” he says.
Others are more open to the possibilities, and welcome the fact that Loeb, Jacobsen, and their team are continuing their work and that answers will be forthcoming. Loeb says he welcomes comments and criticism that are based on the science itself.
That’s why, he says, he wanted to put out these results now, even though less than 10 percent of the recovered spherules have been tested so far. “My rationale was, let’s put the 10 percent analysis out, so that we can get feedback from the community, because maybe we are missing something.” Because the technique used for their mass spectrometry tests destroys most of the material tested, “if we were to analyze a major fraction of these spherules, we would lose them for further analysis,” he says. In the meantime, “I want to know if there are any comments or recommendations or questions the community has, and then we can correct our analysis accordingly.”
Loeb says that with much of their collection of material still intact, “we can still share some spherules with others so that they can check that they get the same answer. I just want to do it correctly.”
Isotopes — a potential smoking gun?
Westphal offers one suggestion: Measure the ratios of oxygen isotopes — forms of the same element but with varying numbers of neutrons — in the suspected extrasolar spherules. “Their claim that they’re extrasolar is really easy to test,” he says. Oxygen isotope ratios are remarkably uniform across all bodies in the solar system, varying by only parts per thousand, but dramatically different in material outside it, with variations a hundred times greater. The tiny samples collected by Loeb’s team are “giant” compared to microscopic materials that he routinely deals with, he says, so such measurements would be easy to do. If the spherules really are extrasolar, he says, “that would really be the smoking gun.”
The spherules appearing here under a microscope were obtained from a run along what Loeb and Siraj have calculated to be the most likely path of the meteor. Credit: Avi Loeb
Don Brownlee, an astronomer at the University of Washington in Seattle who specializes in analysis of extraterrestrial materials, has a similar suggestion. Pointing out that one of the unusually enhanced components of the spherules was uranium, he says, “the simple check would be, if it’s uranium from outside the solar system, it will definitely have a different ratio of U-235 to U-238, because these two isotopes have different half-lives … This is something people measure all the time because they’re trying to detect differences between natural uranium and something that’s been enriched for making bombs and so forth.”
Such isotopic analyses are routine, Brownlee says, and “if they can find some significant isotopic anomalies, they should be able to absolutely prove that this is from outside the solar system.”
And if the material really is extrasolar, he says, “there’s two possibilities. One is that they’re stuff like comets and asteroids that got ejected from other planetary systems, or they’re spacecraft that somehow failed and ended up crashing in the solar system.” Either way, it should be possible to tell the difference, he says.
Brownlee, widely known for his somewhat contrarian view that advanced life is extremely rare in the universe, concedes that he could easily be proved wrong. “If you found something somewhere in the solar system that you could prove was extraterrestrial, and it was an exotic material that could be used to make spacecraft, then I think that would be a convincing thing.” For example, materials like refined aluminum or titanium do not occur naturally but are widely used in our spacecraft — and such materials “will last forever” in space, he says.
Ben Weiss, a professor at MIT who specializes in analyzing meteoritic material, says he admires Loeb’s persistence in carrying out this difficult recovery of material. “What’s cool about what he’s doing is he’s trying to find the fragments of a meteor. People don’t do that very often. If you could even find meteor fragments at the bottom of the ocean from any observed meteor, that’s a major advance in our ability to recover extraterrestrial material, and a big challenge. … That’s already a pretty big thing to try, an ambitious thing, whether they’re extrasolar or not.”
More to come
In addition to the ongoing debate over the recovered samples, there are also lingering questions over the nature of the meteor. Some scientists are skeptical that the object that entered the atmosphere in 2014 originated from outside of the solar system in the first place. Loeb and Siraj’s analysis rests on velocity data from a US government network of sensors designed to detect missiles. The U.S. military sent a letter supporting the claimed velocity from the network in 2022. But some scientists say that the fact that the underlying data remains classified undermines the claim that the object had an interstellar velocity and make its actual path uncertain.
6/ “I had the pleasure of signing a memo with @ussfspoc’s Chief Scientist, Dr. Mozer, to confirm that a previously-detected interstellar object was indeed an interstellar object, a confirmation that assisted the broader astronomical community.” pic.twitter.com/PGlIOnCSrW
Loeb and his team point to the fact that spherules on the seafloor were more heavily concentrated within the expected landing zone of the observed meteor than in other nearby regions as additional evidence that they came from the meteorite in question, but Weiss says “it would be nice to have some more statistical tests here, some quantitative measures” to document the significance of the increased concentration.
Weiss says that he’d prefer to wait until the paper passes peer review before commenting in detail, because analyzing the paper’s claims as if he were a reviewer would take days of detailed study. “I’m expecting that this is just the beginning” of a long process of testing and debate, he says, as Loeb and his team continue their analysis.
George Flynn, an astrobiophysicist at the State University of New York at Plattsburgh, says that “if they are interstellar, they’re the first macro samples of interstellar material available on earth for study, and that would be a tremendous claim.” But, he says, “they need isotopic evidence. That’s the one thing everybody will accept. … This is obviously going to be a controversy that’ll continue for a while, but I really think if they can get some convincing isotopic data, they’ll answer the question one way or the other.” He adds, “we certainly should be open-minded about it, but their evidence isn’t there yet.”
Peak viewing season for the giant planets continues. Saturn is visible all night, at its best in the late evening. Jupiter rises later and dominates the early morning. Neptune reaches opposition near a 5th-magnitude star — grab binoculars to catch the best view of 2023. Uranus hovers between the Pleiades and Jupiter, offering good opportunities to catch this distant giant. Venus grows to greatest brilliancy before dawn — you can’t miss it — and Mercury comes up to join it later in September.
Mars is slowly approaching its November solar conjunction. Shining at magnitude 1.7, it’s a challenging object low in the western sky after sunset. Mars stands 3° high 30 minutes after sunset and drops to half that elevation less than 15 minutes later, so the observing window is very narrow. Look for Spica, which is brighter, located 12° east of Mars by midmonth.
The crescent Moon passes in front of Mars during a daylight occultation Sept. 16, visible across the U.S. The timing of the event varies with location. It’s midafternoon for the Eastern Seaboard (in Miami, Mars disappears around 3:25 p.m. EDT), and early afternoon for Midwestern states (12:26 p.m. MDT in Denver). Mars misses the southern limb of the Moon for observers along the West Coast except in the extreme northwest (e.g., Portland).
By evening, you can spot Mars nearly 2° west of the very thin, two-day-old Moon. The Red Planet soon dips out of view as it heads for the Sun.
Iapetus sits far west of Saturn early in September. But near the end of the month, the moon lies just 17″ south of the disk. Credit: Astronomy: Roen Kelly
Saturn is at its finest in early September. It just passed opposition Aug. 27 and is visible all night in Aquarius. The ringed planet shines at magnitude 0.4 and dims 0.1 magnitude by midmonth. You’ll find it low in the southeastern sky after sunset; it climbs steadily to its highest due south around local midnight on Sept. 1. By the end of September, Saturn reaches this point two hours earlier.
A bright gibbous Moon lies about 3° below Saturn Sept. 26. With increasing darkness as the autumn Sun sets ever earlier, conditions are favorable for late-evening viewing.
Through a telescope, the rings’ northern side is sunlit and tilted toward us by 9° in early September. The angle increases to 10° by the 30th. The dusky appearance of the outer A ring contrasts nicely with the brighter B ring. The two are separated by the dark Cassini Division. The inner C ring is lighter and transparent.
Saturn’s yellowish disk spans 19″ and the rings extend 42″. We’re viewing the world from about 8.8 astronomical units (818 million miles) away; this distance slowly increases throughout the month. (One astronomical unit, or AU, is the average Earth-Sun distance.)
Saturn’s brightest moon is magnitude 8.5 Titan. Its 16-day period carries it roughly north of the planet from Sept. 7 to 8 and 23 to 24. It appears roughly south Sept. 15 and 16. Look also for three fainter, 10th-magnitude moons — Tethys, Dione, and Rhea — orbiting closer in. Rhea is a shade brighter than the other two. Tethys and Dione orbit in 1.9 and 2.7 days, respectively, while Rhea takes 4.5 days. Occasionally, bright field stars wander into view, so take care not to mistake these for moons. From Sept. 8 to 10, for instance, Saturn moves between an 8th- and 9th-magnitude stellar pair.
Enceladus is very faint at magnitude 12. It lies nearer to the rings, so Saturn’s brilliance makes it a challenge, but spotting it is a thrill.
Iapetus reaches western elongation Sept. 10, shining at 10th magnitude. It stands 9′ west of the planet, far beyond the other moons, making it trickier to identify. On Sept. 12, it lies between two 10th-magnitude stars. On Sept. 20, Iapetus appears double, sitting next to a magnitude 10.6 star. Watch for a few minutes and the moon will reveal itself by moving relative to the star. The following night, Titan sits beside the same star.
Iapetus moves toward Saturn, approaching superior conjunction on the 29th. Don’t confuse it for the 10th-magnitude star due west of Saturn on Sept. 26. Iapetus, fading as its darker hemisphere turns earthward, is closer to the planet. The night of Sept. 29, Iapetus is 17″ south of Saturn.
Look on Sept. 13 just before 11 p.m. EDT: Tethys begins to transit the planet, followed a few minutes later by its shadow. Good seeing conditions are needed to observe the event; the best way to record it is to capture high-speed video and process later. The transit lasts about 90 minutes.
The solar system’s most distant planet reaches opposition this month, shortly after passing 5th-magnitude 20 Piscium. Credit: Astronomy: Roen Kelly
This is the best month of the year to spot Neptune. It has a fine appulse with 5th-magnitude 20 Piscium shortly before reaching opposition, which makes spotting the magnitude 7.7 world much easier.
On Sept. 1, grab a pair of binoculars to find Neptune 16′ northeast of the star. Watch each night as Neptune moves southwestward, reaching a point 4′ due north of 20 Psc on Sept. 10. The star itself is a binary with 10″ separation, while a 10th-magnitude star stands 3′ to its west.
Neptune reaches opposition Sept. 19, now 13′ west of 20 Psc. The distant planet lies 28.9 AU from Earth. Through a telescope, Neptune spans 2″ and glows with a bluish hue. By the end of September, the planet is 31′ southwest of 20 Psc.
Jupiter rises around 10 p.m. in early September and stands 20° high in the east at the same time on Sept. 30. It starts the month at magnitude –2.6 — an unmistakable object in the faint constellation Aries. The best views of the giant planet are in the early-morning hours, when Jupiter stands more than 60° high in the southern sky.
Binoculars will reveal some of its moons as well as the 6th-magnitude star Sigma (σ) Arietis. The star stands due north of Jupiter on the 18th.
Jupiter is a favorite of observers because of the wealth of detail in its atmosphere: dark equatorial belts, the Great Red Spot, and an ever-changing view as new features regularly appear, given its fast rotation period of less than 10 hours.
The Galilean moons orbit with periods from about two to 17 days. When they pass in front of Jupiter, they cast their shadows on the cloud tops. Io undergoes regular transits across Jupiter’s disk. Events occur overnight on Sept. 3/4, 12/13, 19/20, and 26/27. The shadow of Io is first to appear, followed by the moon. In early September the moon trails its shadow by 74 minutes; this shrinks to 54 minutes by the 26th, due to the changing relative position of Earth with respect to Jupiter and the Sun.
Ganymede, Jupiter’s largest moon, casts its big shadow on the planet’s southern polar regions Sept. 6/7, starting at 1:51 a.m. EDT on the 7th (still the 6th in western time zones) and ending 3:40 a.m. EDT. The following night (Sept. 7/8), Europa’s shadow transits Jupiter, beginning at 12:30 a.m. EDT on the 8th. The event is already underway for the western U.S. as Jupiter rises. The shadow exits the disk at 2:49 a.m. EDT. Europa begins a transit eight minutes after its shadow leaves.
Ganymede also undergoes an occultation behind the planet’s northern limb Sept. 17/18. This is best viewed from the eastern half of U.S. due to its lower altitude in the west. Ganymede disappears slowly due to the shallow angle of the limb near the pole. It begins at 12:28 a.m. EDT on Sept. 18 (note this is still late on the 17th in the Central time zone), but you’ll see the moon blend with the limb of Jupiter a bit earlier. It reappears at 1:22 a.m. EDT.
Uranus stands about 8.5° southwest of the Pleiades star cluster (M45) and 7.5° northeast of Jupiter. With binoculars, scan southeast of M45 until you come across an arc-shaped trio of 5th-magnitude stars led by Tau (τ) Arietis. Drop 3° south of this grouping to find Uranus. It glows at magnitude 5.7.
Uranus has started its retrograde path westward. It moves slowly early in the month and picks up the pace later. During September it moves less than 0.5°. While you’ve got Jupiter in your telescope, remember to swing east just a few degrees to view this distant planet. Its greenish-hued, 4″-wide disk is evocative, standing just over 19 AU from Earth. When William Herschel spotted it in 1781, it was the first planet ever discovered with a telescope.
On Sept. 11, brilliant Venus hangs beneath a crescent Moon. The pair sits near the picturesque Beehive Cluster (M44). Credit: Astronomy: Roen Kelly
Venus dominates the morning sky, shining brilliantly in the hour before dawn. On Sept. 1 it is magnitude –4.6; it brightens to its greatest brilliancy of –4.8 by the 9th. Venus rises on Sept. 1 just before 5 a.m. local daylight time, around the same time astronomical twilight begins. The planet is in southern Cancer, hanging more than 20° below Castor and Pollux, the brightest stars in Gemini.
On Sept. 11, a lovely crescent Moon hangs 11° due north of Venus. Scan the sky around them with binoculars to catch a view of M44, the Beehive Cluster, about 4° southwest of the Moon.
On the 1st, a telescope shows Venus’ 50″-wide crescent is 11 percent lit. By the 11th, it’s a 21-percent-lit crescent spanning 43″ — a product of its increasing distance from Earth and growing angular separation from the Sun. The planet continues shrinking, reaching 32″ on the 30th, while its crescent is 36 percent lit. Venus crosses into Leo Sept. 25 and ends the month 7.8° west of Regulus, the Lion’s brightest star.
The best morning view of Mercury in 2023 occurs this month. The tiny planet hops into the morning sky after its Sept. 6 inferior conjunction. It’s already magnitude 1 by the 16th, standing 8° below Regulus. Brightening quickly, it’s magnitude 0 four days later on the 20th, rising 90 minutes before the Sun. It reaches greatest western elongation of 18° on the 22nd, shining at magnitude –0.3. Mercury continues to brighten, reaching magnitude –1 on the 29th. The world is a dramatic object 9° high 30 minutes before sunrise as Venus, high above, watches over it.
Sept. 23 marks the autumnal equinox (2:50 a.m. EDT), when the Sun appears above the celestial equator, moving southward.
Note: Moon phases in the calendar vary in size due to the distance from Earth and are shown at 0h Universal Time. Credit: Astronomy: Roen Kelly
Rising Moon: Mythological muscle men
You can use the Harvest Moon effect twice this month. After Full phase, the waning gibbous Moon still rises in the early evening. Without having to stay up late, it’s a splendid opportunity to see reversed light on Luna. Here on Earth, the play of light and shadows in your neighborhood is different at sunrise and sunset; likewise on the Moon. It’s not simply the same scene running backward in time!
Look to the northern third of our satellite on the evening of the 2nd and 3rd (both this month and next) to see the neatly paired muscle men of mythology, Atlas and Hercules. Atlas is closer to the limb. Hercules, 43 miles across, sports a floor of lava punctured by a sharp-edged crater of more modest size. The older Atlas has wrinkles and a jumble of central peaks typical of larger craters. At sunset on the 3rd, Atlas’ central features drop into darkness, while Hercules’ inner crater will soon succumb to shadow, but hasn’t yet.
Under “normal” (waxing) lighting just after midmonth, the pair are emerging into sunlight and their shadows appear reversed: The outer rims are lit on the eastern side, while the inner crater walls are now lit to the west. On the 18th, the dynamic duo makes quite the standout feature on the crescent. If you get a streak of good weather, see if you can notice night by night how the lava seems to get darker and produce better contrast. At sunrise or sunset, all surfaces scatter light in a similar fashion, but at higher Sun angles, the differences become striking.
Meteor Watch: Ecliptic glow
There are no major meteor showers in September. As the bright planets beckon early-morning observers, be on the lookout for the zodiacal light in the predawn sky. It’s visible only from very dark locations. The distinctive cone-shaped brightening lights up the steeply inclined ecliptic with a glow similar to that of the Milky Way. It extends from Leo through Cancer and into Gemini. This glow is from meteoritic dust that pervades the inner solar system.
Watch for the zodiacal light in the third week of September around New Moon. Venus will be embedded in the faint light and can help guide your eye along the ecliptic. Watch for Mercury rising later as well. Twilight will be a lower, wider glow along the eastern horizon that quickly drowns out the zodiacal light.
This month, Venus (bright object near the horizon) is embedded in the cone-shaped glow of the zodiacal light. Credit: Alan Dyer
Comet Search: Reliable returner
Initiating a year-long feast of comets brighter than 10th magnitude is 103P/Hartley (Hartley 2). Get under some dark skies with at least a 4-inch scope throughout this apparition as it is unlikely to break 8th magnitude, coming closest to Earth Sept. 25/26. Wait until late evening for the comet to climb up in the northeast.
Imagers should try a 135mm lens to frame the green glow with the California Nebula (NGC 1499) before the second week of the month, and Auriga’s Messier clusters and Flame Nebula (IC 405) during the third. Few photos from two orbits ago show a white dust tail, though we know that Hartley puts out dust — dramatically revealed by a spacecraft visit in 2010.
Readers in the southern U.S. can glimpse C/2021 T4 (Lemmon) masquerading as a ghost of Zubenelgenubi from the 9th to the 14th right as evening twilight fades. If you want to get the periodic 2P/Encke on your lifetime list, the New Moon weekend of the 16th and 17th is a must while it is up near Castor. Otherwise, Encke quickly drops into morning twilight and our next decent view is more than a decade away. From Sept. 23 to 25, it skirts 3° north of Leo’s big spiral galaxy NGC 2903.
Comet Hartley 2 covers a wide swath of sky, moving from Perseus into Auriga. This chart shows the comet’s path for the first two weeks of September; visit our website for additional charts covering the second half of the month. Credit: Astronomy: Roen Kelly
Locating Asteroids: Between the rings
Aquarius is a bit toughto see from the suburbs. Set your sights on Saturn in the southeast and drop 7° — about a finder field — to land close to main-belt asteroid 8 Flora.
Start a little more south of Saturn, using the 5th-magnitude star Upsilon (υ) Aquarii as an anchor. Take along a nebula filter to catch the rather large, famous smoke ring of the Helix Nebula (NGC 7293).
The somewhat-sparse star fields here can be helpful for checking position and orientation, since their patterns won’t be confused by hordes of background suns. Measuring a respectable 90 miles across, 8th-magnitude Flora forms a nearly straight line with a pair of brighter and fainter widely separated stars from the 7th to the 9th. Mid-month, Flora scootches past a small, hockey-stick-shaped asterism, making it super easy in both cases to make a field drawing in a logbook to plot its passage.
Wait past 10 p.m. for Flora to climb above rooftops and trees, and give it a rest when the Moon is nearby from the 25th to the 27th.
Asteroid Flora floats through a sparse region of Aquarius, aiding in its identification. The position of Saturn is shown on Sept. 15. Credit: Astronomy: Roen Kelly
Venus seems like the last place you’d ever think about looking for life. It features searing-hot temperatures; a thick, hazy atmosphere raining droplet of sulfuric acid; and potentially active volcanoes spewing hot lava and corrosive gases. It’s often been compared to visions of hell, a landscape of fire and brimstone.
And yet, scientists have long proposed that regions of Venus’ atmosphere, high above the surface, might provide a habitat for life. These ideas have been greeted skeptically but over the last few years, bits and pieces of evidence have begun to pile up, suggesting there really might be a habitable zone there. More recently, a group of scientists claimed to have detected what may be evidence of living organisms there now, perhaps in the form of highly adapted microbes floating in droplets high in the thick, cloudy air.
The signs of possible life so far have all been indirect and scant, but together add up to a picture that some researchers say merits further investigation. And instead of waiting for politicians and administrators to approve a mission, they have obtained private funding to send a probe to Venus in 2025 — with two more missions in the planning stages.
Hints of life
The medium-term Morning Star Missions to Venus will study the planet’s clouds for indicators of habitability. Shown in an artist’s rendering, a parachute-borne probe might stay aloft for 30 to 60 minutes to measure characteristics such as cloud composition. Credit: Weston Buchanan
David Grinspoon, an astrobiologist at the Planetary Science Institute, says he began to wonder about possible life on Venus in the 1990s. “For me, it had to do with the Magellan findings,” he says, referring to the spacecraft that returned reams of radar data and revolutionized our image of the planet. Magellan revealed a young surface that had been drastically reshaped by volcanism. In fact, “there were all kinds of hints that it’s probably currently geologically active,” he says. In May of this year, new evidence from those old radar images showed clear changes in the shape of a volcanic vent. “Seeing this very convincing example of an apparently volcanic feature that changed over such a short timescale really adds substantially to the case that Venus today is quite active,” Grinspoon says.
Magellan data also confirmed the existence of a layer in the clouds that showed “a region that is not only habitable, but has energy sources and nutrients,” Grinspoon says. There have been signs of puzzling chemical anomalies in the atmosphere, he adds, although “none of them are solid enough to be considered evidence for life.” For years, his suggestions of the possibility of life there made him “a bit of a lonely voice in the wilderness,” he says.
Then, in 2020, the idea was thrust into the spotlight when a team of researchers led by Jane Greaves of Cardiff University in Wales reported they had detected phosphine gas in Venus’ clouds. On Earth, phosphine is a byproduct of living organisms, produced by microbes in swamps and inside the guts of animals.
The team explored every mechanism they could think of that is capable of making phosphine, and concluded that none could account for the amounts they reported — except life. “[It] is a gas that doesn’t belong in the context of its environment” on Venus, says Sara Seager, a study co-author and planetary scientist at MIT. “It really shouldn’t be there. It doesn’t appear to be made by any chemical or physical process that we can think of. … It’s only associated with life, or with some other chemistry that we do not currently understand.”
The claim remains controversial. Many scientists question the detection as a weak signal amid a noisy background, or suspect that it may be a different molecule that absorbs at a similar wavelength. Nathalie Cabrol, director of the Carl Sagan Center for the Study of Life in the Universe at the SETI Institute in California, who was not part of the study, notes the detection was a difficult one to make. There were several independent teams that sought to verify the findings “and some of them couldn’t find the phosphine,” she says.
Seager candidly acknowledges the controversy and her colleagues’ skepticism: “It’s still kind of going back and forth, as science should,” she says.
This timeline illustrates how a balloon mission to Venus would enter the atmosphere (left), slow using a parachute before inflating (middle), and later deploy a set of atmospheric miniprobes (right). The current mission concept calls for the balloon to operate for one week at an altitude of 32 miles (52 km). Credit: Astronomy: Roen Kelly, After Seager, S.; Petkowski, J.J.; Carr, C.E.; Grinspoon, D.H.; Ehlmann, B.L.; Saikia, S.J.; Agrawal, R.; Buchanan, W.P.; Weber, M.U.; French, R.; Et Al. Venus Life Finder Missions Motivation and Summary. Aerospace 2022, 9, 385. Https://Doi.Org/10.3390/ Aerospace9070385 (Cc By 4.0)
Getting answers
Seager and her colleagues used Earth-based radio telescopes to observe Venus and make their detection. But resolving the questions they’ve raised will probably require a close-up view.
Seager has put together a team of more than a dozen scientists, including Grinspoon. They have done a detailed study that concludes the overall evidence is now suggestive enough, and the possibility of finding alien life in our closest neighbor planet is so important, that it’s time to launch new missions to Venus to answer these questions once and for all.
The group, originally the Venus Life Finder Mission Team and now called Morning Star Missions to Venus, has published several reports outlining the evidence so far, along with the types of instruments that could provide definitive answers about the compounds in the planet’s atmosphere and their origins. They also discuss the missions needed to deliver those instruments to Venus and allow them to survive long enough to return useful data. To that end, Seager says, her team has settled on three mission concepts. Designated the Morning Star Missions, the first is already funded and under construction. It will use a privately developed rocket system from the company Rocket Lab. Set to launch from New Zealand in January 2025, it will take a few months to get to Venus, where it will release an instrument package to plunge through the atmosphere and collect data as it falls.
“This will be the first probe in nearly four decades that goes into the atmosphere” of Venus, Seager says, beating NASA’s DAVINCI mission, scheduled to launch in 2029. “It won’t have a parachute, so it’ll take about an hour before it crash-lands on the surface, and we have five precious minutes [to collect data] in the cloud layers.”
The Rocket Lab Mission to Venus, scheduled for launch in early 2025 and depicted approaching
its destination in this artist’s concept, will carry a small probe that will drop through the Venusian atmosphere.
Credit: French, R.; Mandy, C.; Hunter, R.; Mosleh, E.; Sinclair, D.; Beck, P.; Seager, S.; Petkowski, J.J.; Carr, C.E.; Grinspoon, D.H.; Et Al. Rocket Lab Mission to Venus. Aerospace2022,9,445. Https:// Doi.Org/10.3390/
The Rocket Lab Mission to Venus will be the first mission ever developed to search for organic matter in the clouds. Its instrument, the Autofluorescence Nephelometer (AFN), will shine a laser onto cloud particles, causing any complex organic molecules within to fluoresce. The AFN will also measure laser light reflected back from the droplets to determine their overall size and shape.
The second proposed mission will carry a much larger payload and include either a probe dropped with a parachute, or a balloon that could hover in the atmospheric layers of interest for a longer period. One key question the team hopes this mission will address is the acidity of the atmosphere. Current measurements show the acidity is far too high for any known organism to survive. But those observations are of the bulk atmosphere from afar. Within the clouds, the acidity may vary greatly. And some droplets in the clouds may be relatively hospitable, comparable to acidic environments on Earth that support highly adapted life-forms called extremophiles. The team is developing an acidity sensor that can handle the extreme conditions of Venus to find out if such low-acidity droplets exist.
Another question is how much water is in the upper cloud layers, where the atmospheric pressure and temperature resemble those of Earth, potentially providing a refuge for life high above the extreme heat and pressure at the surface. All known life depends on water, but the amount of this life-giving substance detected so far on Venus is extremely low. To sustain life, there would have to be areas where water is more concentrated. Detectors on this probe will aim to determine whether such concentrations are present.
The mission will also aim to take more precise measurements of compounds such as phosphine in the clouds. The team is working on securing funding for this much more ambitious multistage mission, with launch opportunities between 2026 and 2031.
“I think balloons are the right way to explore Venus,” says Robert Zubrin, an aerospace engineer and founder of the Mars Society. “They have enormous advantages. It’s a very easy place to fly balloons because of the thick CO2 atmosphere.” He says it would not be difficult to design a balloon that could hover in selected levels of the Venusian atmosphere for days or weeks. The Mars Society has already begun testing its own design of such a balloon on Earth.
The third mission in the Morning Star series is by far the most ambitious. It also remains the least defined but aims to return a sample from the clouds to Earth for analysis, much as soil and rocks are being collected on Mars for a future sample return mission. The current concept calls for a spacecraft to orbit Venus and deploy a balloon into the atmosphere to collect liquid and solid material from the cloud layers as it descends. Then, a small rocket containing the samples would be launched from the balloon to rendezvous with the orbiter, which would return to Earth for reentry and collection.
The most ambitious and furthestoff Venus samplereturn mission will make a round trip between our planets, retrieving a sample of cloud droplets via a balloon before rocketing back to meet an orbiting spacecraft that will return to Earth with its precious cargo. Credit: Astronomy: Roen Kelly, After Seager, S.; Petkowski, J.J.; Carr, C.E.; Grinspoon, D.H.; Ehlmann, B.L.; Saikia, S.J.; Agrawal, R.; Buchanan, W.P.; Weber, M.U.; French, R.; Et Al. Venus Life Finder Missions Motivation and Summary. Aerospace 2022, 9, 385. Https:// Doi.Org/10.3390/Aerospace9070385 (Cc By 4.0)
A scientific bonanza
These proposals are not the only plans for a return to Venus: There are already two NASA missions (including DAVINCI) and one ESA mission planned for the coming decade, and India and Russia have contemplated endeavors as well. But none of these would include instruments aimed at answering the key questions related to the possibility of life or the habitability of Venus’ clouds.
“Of the three very expensive government missions going to Venus, none of them are probing the cloud particles directly,” Seager says. “This idea of life on Venus, it’s too taboo for large government agencies.”
Many scientists remain skeptical of the possibility of life on Venus. Cabrol says that when looking at the whole picture, a geological explanation for the observed anomalies — including phosphine — “checks a lot more boxes, and a lot more easily, than the biological explanation.”
But mysteries keep piling up. In addition to phosphine, astronomers have made possible detections of ammonia, which is also a gas that shouldn’t exist on Venus without some biological production mechanism. Other anomalies include tiny unexplained amounts of oxygen, less sulfur dioxide and water vapor than models predict, and indications that the cloud drops are not spherical — which they should be if they are composed of pure sulfuric acid. The droplets also show a different refractive index than that of pure sulfuric acid. This means there must be some other compound within the droplets, Seager says — perhaps even a slushy substance. Finally, there is some unknown process that absorbs half of all the ultraviolet sunlight hitting the planet.
Taken all together, Seager says, the simplest explanation for all these anomalies seems to be that some kind of life-form is making ammonia gas, as some microbes do on Earth. This single possibility would produce a “cascade effect that solves all these other problems,” she says.
Seager explains that if microbes in the Venus atmosphere are producing ammonia, that ammonia would be absorbed by droplets of sulfuric acid, reducing their extreme acidity to a level at which some Earth organisms can survive. The added ammonia would also cause the droplets to absorb more sulfur dioxide and water vapor, which would explain the depletion of those substances in the clouds. In addition, “by changing the chemistry of the droplet, it’s probably not spherical anymore because it’s kind of a salt slurry,” she says. Also, ammonia production, if it is biological, could create oxygen as a byproduct, “so we would just tie all those [anomalous observations] together” to fit into one consistent picture involving high-flying microbes in the venusian air.
The most complex proposed Morning Star Mission to Venus will collect cloud samples via a canister carried aloft on a balloon, imagined here. These samples would then be returned to Earth for detailed study in terrestrial labs. Credit: Weston Buchanan
Cabrol says she can accept the idea of a habitable zone on Venus, but she finds it unlikely that it is actually inhabited. While there are living organisms in Earth’s atmosphere, she points out, none of them truly reside there. They are simply being transported from one place to another. That they could survive and create an ecosystem that is entirely airborne seems unlikely, she says, because that environment would simply be far too unstable.
Nevertheless, Cabrol supports the Morning Star team’s mission proposals because the questions are too important to leave unanswered. “What I like is that they have science objectives that are squarely aligned with a science hypothesis, and there are clear goals, with the kinds of things you expect to see, and what it means if you don’t see them,” she says. “This is good stuff. … I think they’re taking a very scientific, reasoned approach.”
“We definitely have evidence of intricate chemistry in the atmosphere that’s beyond what we can piece together right now,” Seager says. “Whether or not there’s life, there’s definitely something interesting there that we don’t understand.” And the only way to find out, she says, is to go there.
The results will be a scientific bonanza. If there really are microbes wafting through the venusian sky, finding them would rank as one of the most significant discoveries in human history — the first detection of alien life. And if there aren’t, then there must be some unique and unknown geochemical processes occurring there to generate these unexpected chemical signatures. Discovering these processes could be important for understanding how rocky planets form and evolve not only in our solar system, but across the galaxy.
Either way, the results of these ambitious missions will be significant and profound.
