Mapping the Motion of Asteroids
Imagine watching the still night sky. Imagine
a fast moving rock streaking through your field
of view. It reflects the sunlight into your eyes,
leaving a bright trail
marking it's orbit about the sun. Blink quickly
and look ...it
is still there. Blink again and look.
Something distracts you and you look away. When
you look back it is no longer there.
The image above is the average of 3 separate HST
exposures of a piece of sky. Each exposure serendipitously
imaged the trail of the same asteroid. The first
exposure has been colored cyan.
Blink. The next exposure green.
Blink. Another exposure magenta.
Another way to show
the motion of the asteroid across a field of view
is to show each black and white exposure as a frame
and dislay one frame after another. This is called
blinking and it is rather like a movie.
Select
Another Hubble Heritage release with an asteroid
trail is Hickson Compact Group 87. Notes and a picture
of this trail can be found here.
A description of the trail can be found at this
link.
A press release by
Evans and Stapelfeldt describes other asteroids
which they have detected mainly using the blinking
technique. Notice that the marks created
during the exposure by high energy atomic nuclei
or electrons, called cosmic rays, were not removed
in the images of the previous release.
Image on the left is
with Cosmic Rays while the image on
the right is without
Cosmic Rays
However there was only one exposure showing the
trail for the asteroid above. So how was it detected
and its distance measured? The answers below are
from Evans
et. al. 1998 published in Icarus.
-
Why is the asteroid trail curved?
It is immediately obvious that asteroid trails
in HST are very different from those seen at
ground-based observatories. HST asteroid trails
are usually curved. This curvature is not due
to motion of the asteroid itself, but rather
to the
parallax caused by HST's orbital motion
around the Earth during the exposures. During
a typical 15 min exposure, the position of HST
will shift by about 1 Earth radius.
Also see Matt McMaster's description of
the asteroid
trail in HCG 87.
-
How does one know this mark is an asteroid?
This distinctive curavture also helps to distinguish
moving objects from cosmic ray
tracks, which are usually straight.
Also a moving object trail is easily recognizable
by eye amidst the back-ground noise by the continuity
of its brightness and motion across 2 or more
images. It also appears slightly broadened by
the telescope point spread function, a feature
not shared by cosmic rays. Faint cosmic ray
trails are rare, so moving object trails which
are too dim to show the telescope point spread
function can also be recognized. Trails that
appear on only a single frame are also readily
identified. It was difficult to conceive of
an automated computer recognition algorithm
which would reliably identify real trails near
the noise level, and all trails reported in
this paper were discovered by eye.
-
Is this curve of the trail useful for scientific
measurements?
The
parallax induced by the orbital motion of
HST provides a unique opportunity to determine
the distance to each asteroid from a single
resolved trail. To do this we must compare the
observed trail to predicted trails appropriate
to the specific circumstances of an HST observation,
and seek a best-fit solution by varying the
geocentric distance.
The distance solution for asteroid u2805m01t4
is shown below. This plots data (open and filled
circles) and model (solid lines) trails. Fits
are shown for Earth distances of 0.4, 0.55,
0.93, and 2.5 AU. The fit for 0.93 AU is not
labeled but goes through the points. This is
a potential Mars crosser found at a heliocentric
distance of 1.68 AU, identical to Mars' aphelion
distance. The fit is excellent because of the
length and curvature of the trail.
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