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Studies of Hickson Compact Groups

Asteroid Trail Picture

Asteroid Trail Description

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Black and white close-ups of each of these galaxies. (The page is 64 Kb.)

detailed views of HCG 87: 64 Kb

Studies of Hickson Compact Groups

table of characteristics for the HCG 87 group

Asteroid Trail

During the Hubble Heritage team's observing run, an asteroid crossed the Hubble Space Telescope's field of view. (For the sake of simplicity, we removed the trail from our release image.)

Below is a description by Matt McMaster (STScI) of the asteroid in this image, including an explanation of why the trail is curved!

Also visit our asteroid supplemental pages to see a movie of an asteroid trail.

HCG 87 with asteroid

The Hickson Compact Group 87 Asteroid Story by Matt McMaster

The small streaks you see in the image are actually an asteroid moving across the field of view.

At the time that HST was observing this group of galaxies, the asteroid was about 246 million kilometers (154 million miles) from the Earth and a little over 1.5 times that from the sun. This places it in the asteroid belt, a band of several thousand asteroids between the orbits of Mars and Jupiter. The asteroid is hurtling along at about 64,500 kilometers/hr (40,380 miles/hr).

As you can see, there are three separate streaks in the image but this does not mean that HST was seeing three different objects at the same time. In order to bring out the rich detail of the Hickson group of galaxies, HST had to take several images in different filters; the filters allow only selected colors of light to pass through. While the filters were being changed, HST was not observing the group of galaxies but the asteroid continued to move. Because of this, there was a small gap in the asteroid trail. The larger gap is caused not by the filters being changed, but by the view being blocked by the Earth. Because HST orbits the Earth, its view can be blocked when its path puts the planet between it and the object it is trying to observe, in this case the Hickson group of galaxies. Because it takes much longer for the telescope to move so that the Earth no longer blocks its view than it does to change a filter, the gap in the asteroid trail is larger.

The curved shape of the trails is also unique to HST. If an astronomer on Earth were also observing the group of galaxies at the same time that HST was, he or she would see the asteroid moving in a fairly straight line. But since HST is orbiting the Earth every 90 minutes, the asteroid trail has a bend to it. This bend is very useful for determining the asteroid's position and speed. While it can take an observer on the ground several nights to get a good idea of where an asteroid is and how fast it it moving, HST can do it in less than one orbit.

This is not the first trail of an asteroid that HST has seen, it has observed well over a hundred asteroids, with assorted shapes and sizes. Not all of the trails seen by HST have bends in them and this depends on where the telescope is in its orbit. The trails are bent most when HST is at its northernmost or southernmost part of its orbit. While the trails are straight, or nearly so, when the telescope is near the equator. This makes determining where an asteroid is and how fast its moving nearly impossible. Occasionally, there will be a sharp 'hook' at one of the ends of a trail. This is where HST moves from being in front of the Earth (as seen from the area of the sky it is observing) to where HST is alongside or even moving behind the Earth (again, as seen from the area of the sky that the telescope is observing).

The length of a trail depends on how close the object is to Earth and how long HST was looking at the area of the sky the asteroid was in (the exposure time). An asteroid between Earth an Mars would have a longer trail than one between Mars and Jupiter with the same exposure time for both. For a given asteroid, the longer the exposure time, the longer the trail. If you look closely at the trails in the image, you might be able to see that the trails are not all the same length.

Finally, if HST were looking at the asteroid rather than the Hickson group of galaxies, the telescope would be moved so that it kept pace with the asteroid. The asteroid would appear as a small dot, while the galaxies would have trails. Can you figure out why?