The principal program of NEO science the Project ASTEROID telescope will pursue is unfiltered photometry of NEO's to obtain a light curves to determine the rotation rates of the bodies. If a body's size is known (e.g., from color filter photometry and an assumed albedo typical of such bodies) and the rotation rate exceeds a certain calculated value, then the body most likely is not a rubble pile, and is probably a solid body. On the other hand, if a certain rotation rate is never exceeded, e.g., about once every 2 hours, then perhaps smaller bodies are all rubble piles. Thus determining NEO rotation rates provides information on body material strengths, which yields clues to structural composition. Since NEO's are often quite faint, with current surveys reporting objects with V magnitudes approaching 22 (PanSTARRS plans to reach R-band magnitude of 24), to obtain the required photometric accuracy for a rotation light curve (a few percent), one needs as much photon flux as possible. Such observations are not usually referenced to a standard photometric system when published, so they are made unfiltered to optimize the limiting magnitude and signal-to-noise ratio (SNR).
Since the bodies can be rotating at rates in the range from an hour to a day or more, images of a target should be taken at approximately 10-minute intervals over at least a two-hour period (if the target is bright enough for the resulting short integrations), and at half-hour periods throughout multiple nights. Exposures should be long enough to yield an SNR of 50, if possible. For faint NEO's moving at relatively high rates, this might require tracking on the target instead of at sidereal rates. The Project now has available for routine data reduction software that can perform photometry on field reference stars that are smeared through tracking on moving targets, and by stacking multiple images to enhance the SNR of the target. The field of view required is whatever would be sufficient to contain both the target and a sufficient number of catalogued field stars of known magnitude. A minimum field to ensure there are enough photometric comparison stars in the field for accurate photometry in average fields would be 20 arc minutes, but in sparsely populated fields, such as those far away from the galactic plane, a field of at least 30 arc minutes would be needed to ensure enough photometric comparison stars, and 40 arc minutes would be better. This would help to link the photometry from one image to another (through the appearance of the same photometric standard stars or comparison stars on multiple images) as the target moves throughout the entire observation period, which could be a few days.
Examples of NEO light curves were provided for display
on this Web site by Brian Warner of
Palmer Divide Observatory. Please click on the
following links for a "Smaller" (640 x 480) image or a
"Larger" (884 x 768) image.
Example NEO Light Curves
Number
Designation
Name
640 x 480 Link
884 x 768 Link
25143
N/A
Itokawa
Small
Large
52387
1993OM7
N/A
Small
Large
65803
1996GT
N/A
Small
Large
69230
N/A
Hermes
Small
Large
N/A
2000DP107
N/A
Small
Large
Last modified: January 3, 2008.
