Observing Asteroids

The asteroids orbit mainly outside the orbit of Earth and so they behave much the same way as Mars does. The best time to view an asteroid then is when it is at opposition to the Sun. For each of the big four, this occurs each sixteen to twenty months. Since the asteroids are very faint, you will always need optical aid to find them, except for Vesta, which can reach naked-eye visibility at very favorable oppositions. Each of the Big Four can be seen with binoculars.

Ceres is not only the largest asteroid, but has the orbit most like that of the planets. It stays fairly close to the ecliptic and has the lowest eccentricity. Pallas and Juno stray a considerable distance from the ecliptic with orbits inclined 34 and 13 degrees, respectively, so it is possible to find them wandering through constellations not normally associated with planetary visitors. Vesta's orbit is more like that of Ceres, inclined only about 8 degrees but somewhat closer to the Sun than is Ceres.

It is interesting to track the movement of the asteroids as the come around to opposition. Because they pass often so far north or south of Earth's orbital plane, they will often trace interesting tracks in the sky. Ceres and Vesta tend to behave more like Mars usually tracing flat oval-shaped tracks. Asteroids with higher eccentricity orbits tend to make patterns that are more exaggerated in shape. Asteroids near the upper or lower culmination of their orbits will make broad ovals that are almost circular in shape. Asteroids that come to opposition near their node will make broad "Z" shape patterns.

Identifying asteroids in your telescope requires some discipline. No asteroid grows large enough in angular size to be visible as a disk in a telescope so what you are looking for is the extra star in a given area. There are then three ways to identify an asteroid. The first is to know a star field well enough that any extra star

Figure 9.2. Position of Ceres on Sept. 26, 2005. Graphic by author made with Redshift 4.



Figure 9.3. Position of Ceres on Oct. 2, 2005. Graphic by author made with Redshift 4.

in the area will be apparent to you. You can also study a given an area over a few days and watch for the star that moves. In either case, you need a good star chart to pick out the extra star in a given area. The third option is to use long-exposure photography. Over the course of time, an asteroid on the move will leave a short streak on your film as we illustrated in Observing Project 7C. If the asteroid is slow moving, it may not leave such a streak but if you take images over successive nights you can chronicle the movement of any faint background object.

An amateur activity that has great potential for scientific value is timing asteroid occulations of background stars. Each year, Sky & Telescope and other major astronomy publications list all predicted asteroid occulations of background stars for the coming year. Timing and track locations are very important to astronomers. By timing occultation duration, we get a much more precise idea about how large that pinpoint of light really is. By combining that information with a spectro-graphic analysis, we can determine how massive that point of light is. Gaining precise ground track locations will tell us more precisely about the orbit of that asteroid. Gaining this precise data will teach us a great deal about the rock that one day in the future might have Earth in its sights. In this chapter's Observing Projects, we'll talk about how to time asteroid occulations.

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