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Track an asteroid pair

Pallas and Iris come within reach of small telescopes these October nights.
ChapleGlenn
Over the years, I’ve taken attendees at public star parties on telescopic voyages to all corners of the universe. We’ve viewed the Sun during daytime (with an over-the-front filter or by solar projection, of course) and explored virtually everything the night sky has to offer, except for one class of objects: asteroids.

Most observers can recognize the Sun and the Moon, planets and comets, and deep-sky objects from double stars to nebulae, clusters, and galaxies at first glance. But an asteroid? As the name implies — aster is the Latin word for “star” — it looks like a star, so it doesn’t stand out when real stars clutter the same field. Things would get more interesting if the viewer were to return the next night and notice that one of the “stars” had moved. But star parties typically are one-night deals, so asteroids aren’t on the menu.

Before I continue, let me explain why I use asteroid rather than the broader astronomical term minor planet. Simply put, I like “asteroid” better. Maybe it’s the negative connotation of “minor planet.” Asteroids aren’t minor at all. Just ask the dinosaurs.

Asteroids may not be jaw-dropping telescopic sights, but it’s fun to track their motion over several evenings. I enjoy taking out a finder chart for a particular asteroid, star-hopping with my telescope to the appropriate location, and looking for a “star” in the position where the asteroid is supposed to be. I make a sketch that plots my suspect and the surrounding stars, being sure to include those near where the asteroid’s path will take it on subsequent evenings.

If a follow-up observation confirms that my suspect is no longer there and a new “star” appears where I saw none earlier, I carve another notch in my telescope tube. Well, actually, I record the asteroid name and date of sighting in a logbook. Through my little 3-inch Edmund Scientific reflector, I’ve seen more than 100 asteroids. If I’d notched the tube each time I captured one, I’d have whittled the poor thing down to nothing!

All eyes on Iris

This month, two of the first 10 asteroids discovered — 2 Pallas and 7 Iris — will be within reach of small telescopes and even binoculars. Because I gear this column to the novice, let’s set our sights on Iris, which is brighter and conveniently located near some bright guide stars.
ScreenShot20170905at11.00.00AM
The brightest asteroid this autumn is 7th-magnitude Iris, which conveniently lies near the brightest stars in Aries.
Astronomy: Roen Kelly
English astronomer John Hind discovered Iris on the evening of August 13, 1847. This was Hind’s first asteroid find; over the next seven years, he would add nine more. Hind is perhaps better known for discovering the striking red-hued carbon star R Leporis, also known as Hind’s Crimson Star, and Hind’s Variable Nebula (NGC 1555), which the star T Tauri illuminates.

Like most asteroids, Iris orbits the Sun in the gap between Mars and Jupiter. Studies of its brightness indicate that this 130-mile-wide asteroid rotates once every 7.1 hours. Astronomers classify it as an S-type, or silicaceous. Most of these stony asteroids inhabit the inner part of the asteroid belt.

Iris reaches opposition and peak visibility October 29, when it lies about 2° south of the 2nd-magnitude star Hamal (Alpha [α] Arietis). At a typical opposition, Iris glows at 8th or 9th magnitude, but this time around, opposition occurs when the asteroid is about as close to Earth as it can get, just 79 million miles away, and thus it peaks at magnitude 6.9.

If you’ve never seen an asteroid, this is one you’ll want to check out. Not only is it bright enough to be picked up through binoculars, but it also spends autumn close to the three bright stars that form the head of Aries the Ram. To capture Iris and put a notch in your telescope tube (or, preferably, a note in your logbook), use the process I described earlier. If you’re successful, go ahead and try your luck with October’s other bright asteroid, Pallas. It’s a bit more than a magnitude fainter than Iris and drifts across a rather barren region near the border between Eridanus and Fornax, but you can do it.

Although asteroids might not be high on the list of showpiece celestial sights, asteroid hunting is a fun and relaxing way to test your observing skills.

Questions, comments, or suggestions? Email me at gchaple@hotmail.com. Next month: touring Andromeda the Princess with a small telescope. Clear skies!
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