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Mapping the Motion of Asteroids

asteroid u2ov2j02t3

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 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.

asteroid u2805m01t4
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.

    asteroid trail distance plot