From the June 2015 issue

Web Extra: 25 asteroids to spot through binoculars

These targets are all brighter than 10th magnitude, so you can observe them easily with both eyes.
By | Published: June 29, 2015
Ceres and Vesta appeared near each other in the sky last July, allowing observers to create sketches of the asteroid pair.
Ceres and Vesta appeared near each other in the sky last July, allowing observers to create sketches of the asteroid pair.
Michael Rosolina
The August 2015 issue of Astronomy contains the story, “Observe asteroids through binoculars.” In the piece, I concentrate on describing techniques I use, and that left little space for describing the asteroids you can spot. So, I’ve done it here. The following list contains brief descriptions of 25 asteroids brighter than 10th magnitude that you can observe through 10×50 (or larger) binoculars.

4 Vesta is one of the largest asteroids in the solar system, with a mean diameter of 326 miles (525 kilometers). It was discovered by Heinrich Wilhelm Olbers in 1807 and is named after Vesta, the virgin goddess of home and hearth from Roman mythology. Vesta is the second most massive asteroid after the dwarf planet Ceres. The less massive Pallas is slightly larger, making Vesta third in volume. Vesta is the last remaining rocky protoplanet of the kind that formed the terrestrial planets. It lost some 1 percent of its mass less than a billion years ago in a collision that left an enormous crater occupying much of its southern hemisphere. Vesta is the brightest asteroid visible from Earth. Its maximum distance from the Sun is slightly farther than the minimum distance of Ceres from the Sun, though its orbit lies entirely within that of Ceres.

1 Ceres is the largest asteroid and the only dwarf planet in the inner solar system. It is a rock-ice body 590 miles (950km) in diameter and the smallest identified dwarf planet. Discovered in 1801 by Giuseppe Piazzi, it was the first asteroid to be identified, though it was classified as a planet at the time. It is named after Ceres, the Roman goddess of growing plants, the harvest, and motherly love. Ceres’ surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clays. It appears to be differentiated into a rocky core and icy mantle and may harbor an ocean of liquid water under its surface. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, and hence even at its brightest, it is still too dim to be seen with the naked eye except under extremely dark skies.

7 Iris is a large main-belt asteroid orbiting the Sun between Mars and Jupiter. It is the fourth-brightest object in the asteroid belt and has a stony composition. Iris was discovered in 1847 by J. R. Hind. It was Hind’s first asteroid discovery and the seventh asteroid to be discovered overall. Iris is named after the rainbow goddess in Greek mythology, who was a messenger to the gods.

2 Pallas is the second asteroid discovered and one of the largest in the solar system. Its diameter of 338 miles (544km) is slightly larger than that of 4 Vesta. It is, however, 10 to30 percent less massive than Vesta, placing it third among the asteroids. It is possibly the largest irregularly shaped body in the solar system and a remnant protoplanet. When Pallas was discovered by astronomer Heinrich Wilhelm Matthäus Olbers in 1802, it was counted as a planet, as were other asteroids in the early 19th century. The discovery of many more asteroids after 1845 eventually led to their reclassification. Pallas’ surface appears to be a silicate material; the surface spectrum and estimated density resemble carbonaceous chondrite meteorites. The asteroid’s orbit, at 34.8°, is unusually highly inclined to the plane of the asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.

3 Juno was the third asteroid discovered and is one of the larger main-belt asteroids, being one of the two largest stony (S-type) asteroids, along with 15 Eunomia. Juno was discovered in 1804 by German astronomer Karl L. Harding. It is the second most massive S-type asteroid after 15 Eunomia. Even so, Juno has only 3 percent the mass of Ceres. Among S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties. This high albedo explains its relatively high apparent magnitude for a small object not near the inner edge of the asteroid belt. Juno can reach magnitude 7.5 at a favourable opposition, which is brighter than Neptune or Titan, and is the reason for it being discovered before the larger asteroids.

6 Hebe is a large main-belt asteroid containing around half a percent of the mass of the asteroid belt. Its apparently high bulk density (greater than that of the Earth’s Moon or even Mars), however, means that by volume it does not rank among the top 20 asteroids. This high bulk density suggests an extremely solid body that has not been impacted by collisions, which is not typical of asteroids of its size — they tend to be loosely bound rubble piles. In brightness, Hebe is the fifth-brightest object in the asteroid belt after Vesta, Ceres, Iris, and Pallas. It has a mean opposition magnitude of 8.3, about equal to the mean brightness of Titan and can reach magnitude 7.5 at an opposition near perihelion. Hebe is probably the parent body of the H chondrite meteorites, which account for a remarkable 40 percent of all meteorites striking the Earth.

18 Melpomene is a large, bright main-belt asteroid that was discovered by J. R. Hind in 1852 and named after the Muse of tragedy in Greek mythology. It is classified as an S-type asteroid and is composed of silicates and metals. Melpomene occulted the star SAO 114159 on December 11, 1978, allowing scientists to detect a possible satellite with a diameter of at least 23 miles (37km). The satellite candidate received a provisional designation S/1978 (18) 1. In 1988, a search for satellites or dust orbiting this asteroid was performed using the UH88 telescope at the Mauna Kea Observatories, but the effort came up empty. The Hubble Space Telescope observed Melpomene in 1993, resolving the asteroid’s slightly elongated shape, but no satellites were detected. Melpomene has also been studied by radar.

15 Eunomia is a large asteroid in the inner asteroid belt. It is the largest of the stony (S-type) asteroids and somewhere between the 8th- and 12th-largest main-belt asteroid overall (uncertainty in diameters causes uncertainty in its ranking). It is the largest Eunomian asteroid and is estimated to contain 1 percent of the mass of the asteroid belt. Eunomia was discovered by Annibale de Gasparis in 1851 and is named after Eunomia, a personification of order and law in Greek mythology. As the largest S-type asteroid (with 3 Juno being a very close second), Eunomia has attracted a moderate amount of scientific attention. It appears to be an elongated but fairly regularly shaped body, with what seem to be four sides of differing curvature and noticeably different average compositions. Its elongation led to the suggestion that Eunomia may be a binary object, but this has been refuted. Like other true members of the family, its surface is composed of silicates and some nickel-iron and is quite bright.

8 Flora is a large, bright main-belt asteroid. It is the innermost large asteroid: No asteroid closer to the Sun has a diameter above 15.5 miles (25km) or two-elevenths that of Flora itself, and not until the tiny 149 Medusa was discovered was a single asteroid orbiting at a closer mean distance known. It is the seventh-brightest asteroid with a mean opposition magnitude of 8.7. Flora can reach magnitude 7.9 at a favorable opposition near perihelion, such as occurred in November 2007. Flora may be the residual core of an intensely heated, thermally evolved, and magmatically differentiated planetesimal that was subsequently disrupted. Flora was discovered by J. R. Hind in 1847. It was his second asteroid discovery after 7 Iris.The name Flora was proposed by John Herschel, from the Latin goddess of flowers and gardens.

324 Bamberga is one of the largest asteroids in the asteroid belt. It was discovered by Johann Palisa in 1892 in Vienna, making it one of the last large (diameter over 125 miles [200km]) asteroids discovered. Apart from the near-Earth asteroid Eros, it was the last asteroid easily visible with binoculars to be discovered. Although its very high orbital eccentricity means its opposition magnitude varies greatly, at a rare opposition near perihelion Bamberga can reach magnitude 8.0, which is as bright as Saturn’s moon Titan. Such near-perihelion oppositions occur on a regular cycle every 22 years, with the last occurring in 2013. Its brightness at these rare near-perihelion oppositions makes Bamberga the brightest C-type asteroid, roughly 1 magnitude brighter than 10 Hygieas maximum brightness of around 9.1. At such an opposition, Bamberga can in fact be closer to Earth than any main-belt asteroid with a brightness above magnitude 9.5, getting as close as 78 percent of the Earth-Sun distance.

9 Metis is one of the larger main-belt asteroids. It is composed of silicates and metallic nickel-iron and may be the core remnant of a large asteroid that was destroyed by an ancient collision. Metis was discovered by Andrew Graham in 1848 at Markree Observatory in Ireland; it was his only asteroid discovery. It also has been the only asteroid to have been discovered as a result of observations from Ireland until 2008, when, 160 years later, Dave McDonald from observatory J65 discovered 2008 TM9. The asteroid’s name comes from the mythological Metis, a Titaness and Oceanid, daughter of Tethys and Oceanus.

192 Nausikaa is a large main-belt S-type asteroid. It was discovered by Johann Palisa in 1879. The name derives from Nausicaä, a princess in Homer’s Odyssey. The sidereal rotation period is 13.6217 hours. Based on the light-curve data obtained from Nausikaa, a possible satellite was reported in 1985. However, this has not been confirmed. A shape model of Nausikaa has been constructed, also based on the light-curve data. It indicates a roughly cut, but not very elongated body. Nausikaa’s orbital period is 3.72 years and its distance from the Sun varies between 1.81 and 2.99 astronomical units (1 AU is the average Earth-Sun distance). Nausikaa brightened to magnitude 8.3 at a quite favorable opposition September 2, 2011, when it was 1.875 AU from the Sun and 0.866 AU from Earth.

20 Massalia is a large and fairly bright main-belt asteroid. It is also the largest member of the Massalia family of asteroids. Its name is Greek for Marseille, the city from which one of the two independent co-discovers, Jean Chacornac, first sighted it. Massalia has an above average density for S-type asteroids, similar to the density of silicate rocks. As such, it appears to be a solid unfractured body, a rarity among asteroids of its size. Apart from the few largest bodies over 250 miles (400km) in diameter, such as 1 Ceres and 4 Vesta, most asteroids appear to have been significantly fractured or are even rubble piles.

27 Euterpe is a large main-belt asteroid. It was discovered by J. R. Hind in 1853 and named after Euterpē, the Muse of music in Greek mythology. Euterpe is one of the brightest asteroids in the night sky. On December 25, 2015, during a perihelic opposition, it will shine with an apparent magnitude of 8.3.

12 Victoria is a large main-belt asteroid discovered by J. R. Hind in 1850. Victoria is officially named after the Roman goddess of victory, but the name also honors Queen Victoria. The coincidence with the name of the then reigning queen caused quite a controversy at the time, and B. A. Gould, editor of the prestigious The Astronomical Journal, adopted the alternate name Clio (now used by 84 Klio), proposed by the discoverer. However, W. C. Bond, of the Harvard College Observatory, then the highest authority on astronomy in America, held that the mythological condition was fulfilled and the name therefore acceptable, and his opinion eventually prevailed. Radar and speckle interferometry observations show that the shape of Victoria is elongated, and it is suspected of being a binary asteroid.

29 Amphitrite is one of the largest S-type asteroids, probably third in diameter after Eunomia and Juno, although Iris and Herculina are similar in size. Amphitrite was discovered by Albert Marth in 1854 at the private South Villa Observatory in London. It was Marth’s only asteroid discovery. Its name was chosen by George Bishop, the owner of the observatory, who named it after Amphitrite, a sea goddess in Greek mythology. Amphirite’s orbit is less eccentric and inclined than those of its larger cousins; indeed, it is the most circular of any asteroid discovered up to that point. As a consequence, it never becomes as bright as Iris or Hebe, especially as it is much farther from the Sun than those asteroids. It can reach magnitudes of around 8.6 at a favorable opposition, but usually is around the binocular limit of magnitude 9.5.

11 Parthenope is a large, bright main-belt asteroid that was discovered by Annibale de Gasparis in 1850, the second of his nine asteroid discoveries. It was named after Parthenopē, one of the Sirens in Greek mythology, said to have founded the city of Naples.

5 Astraea is a large main-belt asteroid whose surface is highly reflective. Its composition is probably a mixture of nickel-iron with magnesium- and iron-silicates. Astraea was the fifth asteroid discovered in 1845, by K. L. Hencke and named for Astræa, a goddess of justice. It was his first of two asteroid discoveries. The second was 6 Hebe. An amateur astronomer and post office employee, Hencke was looking for 4 Vesta when he stumbled on Astraea. The King of Prussia awarded him an annual pension of 1,200 marks for the discovery. Astraea is physically unremarkable but notable mainly because for 38 years (after the discovery of Vesta in 1807) it had been thought that there were only four asteroids. In terms of maximum brightness, it is indeed only the seventeenth-brightest main-belt asteroid, being fainter than 192 Nausikaa and even, at rare near-perihelion oppositions, the highly eccentric carbonaceous 324 Bamberga. It will be at magnitude 8.7 on a favorable opposition February 15, 2016.

43 Ariadne is a fairly large and bright main-belt asteroid. It is the second-largest member of the Flora asteroid family. It was discovered by N. R. Pogson in 1857 and named after the Greek heroine Ariadne. Ariadne is very elongated (almost twice as long as its smallest dimension) and probably bilobed or at least very angular. The maximum apparent size of Ariadne is equivalent to the maximum apparent size of Pluto.

89 Julia is a large main-belt asteroid that was discovered by French astronomer Édouard Stephan in 1866. This was first of his two asteroid discoveries; the other was 91 Aegina. 89 Julia is believed to be named after Saint Julia of Corsica. The spectrum of 89 Julia shows the signature of silicate-rich minerals with possible indications of an abundant calcic clinopyroxene component. It is classified as an S-type asteroid.

39 Laetitia is a large main-belt asteroid that was discovered by French astronomer Jean Chacornac in 1856 and named after Laetitia, a minor Roman goddess of gaiety. Photometric observations of this asteroid gathered between 1968 and 1974 were used to build a light curve that provided shape and rotation information. It has the general shape of an elongated triaxial ellipsoid with ratios between the lengths of the axes equal to 15:9:5. Major surface features are on a scale of 6 miles (10km), and the surface color does not vary significantly across the surface.

44 Nysa is a large and bright main-belt asteroid as well as the brightest member of the Nysian asteroid family. It is classified as a rare class E asteroid and is probably the largest of this type (though 55 Pandora is only slightly smaller). It was discovered by Hermann Goldschmidt in 1857 and named after the mythical land of Nysa in Greek mythology. In 2002, scientists used 63 light curves from the Uppsala Asteroid Photometric Catalog (UAPC) to construct a shape model of 44 Nysa. The shape model is conical, which they interpreted as indicating the asteroid may actually be a contact binary.

19 Fortuna is one of the largest main-belt asteroids with a composition similar to 1 Ceres: a darkly colored surface that is heavily space-weathered with the composition of primitive organic compounds. Fortuna is 140 miles (225km) in diameter and has one of the darkest known geometric albedos for an asteroid over 90 miles (150km) in diameter. The Hubble Space Telescope observed Fortuna in 1993, finding it shape to be nearly spherical. It was discovered by J. R. Hind on August 22, 1852, and named after Fortuna, the Roman goddess of luck.

10 Hygiea is the fourth-largest asteroid in the solar system by volume and mass and is located in the asteroid belt. With somewhat oblong diameters of 220–310 miles (350–500km), it is the largest of the class of dark C-type asteroids with a carbonaceous surface. Despite its size, it appears very dim when observed from Earth. This is due to its dark surface and larger than average distance from the Sun. For this reason, several smaller asteroids were observed before Annibale de Gasparis discovered Hygiea in 1849.

14 Irene is a large main-belt asteroid discovered by John Russell Hind in 1851. 14 Irene was named after a personification of peace in Greek mythology. Irene’s fairly flat light curves indicate somewhat spherical proportions.