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
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
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?