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Beginner's Guide

2.   What do asteroids look like in the sky?

Given a single image of a starfield, it would be virtually impossible to distinguish an asteroid from the stars. Indeed, the word asteroid basically means star-like. In addition, most asteroids are far enough away and therefore faint enough, that they are not obvious to observers, requiring telescopes and multiple observations to be seen.

Fig 1: 4 Vesta, taken by Tony Hoffman, 21 Jan 2006

So how can you tell there is an asteroid in the picture? Typically, asteroid observers will either take one long exposure -- tracking on the stars so the stars are still pinpoints, but any asteroids will trail -- or take multiple exposures with sufficient time in between. The images can then be stacked into one showing the multiple positions of the comet or combined into an animation that shows the motion of the comet.

Fig 2: 2004 XP14, taken by Johnny Horne, 3 July 2006
This asteroid moved enough during the single short exposure to leave a streak.

Fig 3: Taken by Elizabeth Warner, 1 Dec 2003
Object: 4201 Orosz
exposure: Two 5-min images taken XX minutes apart
camera setup: ST-7 (22'x15')
telescope setup: 10" LX200 (f/4.2)
Here, two images are blinked to show the changing position of the asteroid at the center of the frame.

Sketch of Vesta
Fig 4: Sketch of Vesta over a week. The first sketch was made on 8 Feb 1999 using an Orion 80mm Shorttube and 32mm Plössl eyepiece. The position of Vesta was marked and several field stars were also sketched. Observations were then made on subsequent nights. As Vesta moved out of the original field of view, stars were added for context.
Sketch made by Elizabeth Warner while working at USC's Melton Memorial Observatory.

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Updated: 30-Jul-2013