Astronomy and Astrophysics – Astronomy
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004sass...23..174b&link_type=abstract
The Society for Astronomical Sciences 23rd Annual Symposium on Telescope Science, held May 26-27, 2004, at Big Bear, CA. Publish
Astronomy and Astrophysics
Astronomy
Scientific paper
Amateur astronomers can support radar observations of near-Earth asteroids (NEAs) principally by obtaining astrometry for objects with poorly-determined orbits and by obtaining lightcurves to estimate rotation periods and pole directions. The number of NEAs observed by radar has accelerated sharply in the last few years and the need for support has increased significantly. Optical astrometry is necessary for radar targets when the 3-sigma plane-of- sky pointing uncertainty is larger than about 15 arcseconds because the Are- cibo and Goldstone radar telescopes have narrow beam widths. Astrometry is particularly important for newly-discovered targets-of-opportunity, which of- ten have large plane-of-sky, Doppler, and range uncertainties.
Photometric observations assist radar observations of asteroids in several important ways:
1. The rotation period and pole direction are very helpful for planning radar observations. We use the spin vector to estimate signal-to-noise ratios and to compute longitude and latitude coverage, which help justify requests for telescope time. 2. If the spin vector is available, it greatly facilitates inverting delay- Doppler radar data to construct an asteroid's three-dimensional shape. This is probably the most important way that photometry can support radar observations. 3. Lightcurves can be used with radar data (and independently) to reconstruct asteroid shapes and spin states. 4. Lightcurve observations can discover and characterize the orbital and ro- tation periods of binary NEAs and can complement sparse radar observa- tions. Combined radar + lightcurve observations can yield binary NEA orb- ital parameters, masses, and bulk densities. 5. For asteroids with irregular shapes and well-determined spin states, photometric and radar observations during future close approaches may re- veal changes in the spin state due to thermal torques caused by absorption and and re-emission of sunlight (the "YORP" effect). If detected, the magnitude of the change can constrain the object's mass, density, and thermal conductivity. 6. For Doppler-only datasets, if the rotation period becomes available, the object's Doppler roadening (bandwidth) and rotation period yield its pole- on dimensions, which constrain the shape, radar and optical albedos, and composition.
To date, 147 near-Earth asteroids have been detected by radar at Arecibo and/or Goldstone. Rotation periods have been reported for only about one-half of those objects and pole directions are available for an even smaller per- centage. If more rotation periods and pole directions were available, they would enable us to reconstruct more asteroid shapes and they would greatly increase the value of many unpublished radar observations. The need for photometric observations is great and we strongly encourage observations by amateur astronomers.
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