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G. Black (U. Virginia), D. B. Campbell (Cornell), R. Treacy (NRAO), M. C. Nolan (Arecibo/NAIC)
We've developed a technique to use a radio interferometer to image near earth objects (NEOs) during their close Earth approach when they can be illuminated by a ground-based radar system. There is great potential for this technique to yield detailed information that is complementary to other observational methods. We are using the NAIC's Arecibo Observatory's 1~MW 13~cm radar transmitter with the NRAO's Very Long Baseline Array (VLBA) as the receiving instrument. The VLBA, with antenna spacings of several thousands of kilometers, has a potential resolution on the order of milli-arcseconds; a couple of orders of magnitude smaller than typical ground-based telescopic observations, and sufficient to determine the gross shapes and orientations of spin vectors. Milli-arcsecond astrometry of these quickly moving objects can greatly improve their orbits and extend the span over which future Earth encounters can be predicted.
The VLBA hardware correlator limits the frequency resolution and complicates incorporating a model of the near-field geometry. Typical target bandwidths are ~1~Hz while the correlator's narrowest resolution is 120~Hz. To avoid these difficulties a specialized computer interface was designed to transfer the raw data to commercial PCs. We can now use this system to obtain the individual antenna data streams and subsequently correlate them in software, bypassing the hardware correlator entirely. Software processing permits synthesis of narrower frequency bins, plus easier access for iterations to improve the near field model or correct a poor ephemeris a posteriori. This system could also be used to achieve high time resolution on strong sources.
We have recently used this system to observe near Earth asteroid (25143) Itokawa, a sub-kilometer sized object that passed within 0.013~AU of the Earth and is the target of the Japanese Hayabusa mission.
The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the NSF.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.