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The Shallow Sky

How About a Shallow Sky Challenge?

Akkana Peck

This month we'll talk about some unusual topics in shallow sky observing. Ever feel annoyed when your deep sky observer friends make fun of you because planets aren't challenging to locate or difficult to see? Well, that just shows how little they know about planetary observing. Our solar system is full of targets much more challenging than the Virgo cluster, so let's talk about some of them and how you can observe them this April.

Sketch of Chiron by the author.
Most of us who have been involved in astronomy for more than a few years have seen comets, thanks to the excellent visitations a few years ago by Comets Hyakutake, Hale-Bopp, and the somewhat less bright but still good Linear S4. What we usually observe are comets which leave their remote homes and venture into the solar system, where solar radiation sweeps their ice and dust into tails which make them easily visible. But how many of us have seen comets which haven't been diverted from their usual abode? Why not try observing comets while they're still in the Oort cloud?

The Oort cloud is a spherical shell of proto-comets and debris encircling the solar system far beyond the orbit of Pluto, at a distance of about 50,000 astronomical units from the sun (for comparison, Pluto sits at about 30AU). It is thought to contain as many as a trillion comets -the ambitious planetary observer looking for challenges won't run out of targets here! Better still, since the Oort cloud isn't restricted to the plane of the ecliptic, its objects are distributed about the sky - you don't have to worry about that pesky far southern ecliptic, the way you do with most planets. If your skies are darker to the north, look for some Oort objects there!

Ida and Dactyl. Sketch by the author.
If your skies aren't quite dark enough to catch any of these Oort cloud objects, try something a little easier: try for objects in the Kuiper belt, only 30 to 100 AU out from the sun, and the trans-Neptunian "Centaur" asteroids, ranging between the orbits of Jupiter and Neptune. These objects, also mostly comet-like collections of rock and ice, range in size up to Centaur 2060 Chiron, about 170km in diameter, unless (as some astronomers do) you count Pluto (diameter 2274 km) and its moon Charon (1172 km) as Kuiper belt objects rather than a major planet.

As the biggest Centaur object, Chiron makes a good starting point. It's big enough that upon discovery there was talk of whether it should be considered the tenth planet!

Chiron ranges about 11-13 AU from the sun, and ranges between magnitudes 16-18, so it should be a nice challenge for people with large telescopes or CCD imaging equipment. If you have steady seeing, try for the coma: it is a comet, and it does have an observable coma, which extends about 30".

Eugenia and its moon. (Note that east-west are reversed in this refractor view.) Sketch by the author.
If your skies aren't dark enough for these far-away objects, but you still want an observing challenge, try something closer to home: observing an asteroid's moon! Asteroid 243 Ida, orbiting in the asteroid belt between Mars and Jupiter, was the first asteroid found to have a moon, which has been named Dactyl. At least three other asteroids, 3671 Dionysius, 45 Eugenia, and 762 Pulcova, have since been found to have moons. The brightest of these, Eugenia, runs about magnitude 11.9, itself an easy target for a medium sized telescope; give yourself a real challenge and try to see Eugenia's moon.

Don't forget to call your deep-sky friends over for a look at these challenging shallow sky targets. They may be surprised to find that planetary observing isn't as easy as they had previously thought. And keep in mind, the best time to see these objects is near the very beginning of this month!

Evidence that aliens visited the moon might be found in this sketch by Jane Houston Jones.

Mail to: Akkana Peck
Copyright © 2001 San Jose Astronomical Association
Last updated: July 19, 2007

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