(99942) Apophis is one of the most important near-Earth asteroids ever discovered. It will make a very close Earth approach on Friday, April 13, 2029 when it will pass within only a few Earth radii above Earth's surface. This will be the closest approach by something this large currently known. Apophis approached within 0.1 au in January of 2013, and we organized an extensive radar campaign at Goldstone and Arecibo between December 2012 and March 2013. Our primary objective was to obtain ranging measurements to improve the orbit, characterize its physical properties, and facilitate detection of the Yarkovsky effect leading to estimation of the mass and bulk density. We obtained frequency-only data, and 150 m and 75 m ranging data. The signal-to-noise ratios (SNRs) were strong enough so that coarsely resolved delay-Doppler images revealed aspects of Apophis’ shape. The visible extents varied from ~0.2 km to ~0.4 km and the bandwidths varied from 0.9 Hz to1.4 Hz. The object is clearly elongated, which is consistent with the large lightcurve amplitude of ~0.9 mag reported by Behrend et al. ( http://obswww.unige.ch/~behrend/r099942a.png) and Pravec et al. (Icarus 233, 48-60, 2014). The radar images suggest that Apophis could be a contact binary.Pravec et al. have estimated the shape and spin of Apophis from lightcurves that were collected at similar times as our radar data. The radar data were not strong enough for 3D shape modeling, but we used the Shape software (Hudson, Remote Sens. Rev. 8, 195-203, 1993 and Magri et al., Icarus 186, 152-177, 2007) to scale the convex lightcurve-derived shape model to fit the radar data. We find that the best χ2 value corresponds to the model that has 0.43×0.30×0.26 (±0.04, ±0.03, ±0.03) km for long, intermediate, and short axis, and a dynamically equivalent, equal volume ellipsoid of 0.31±0.03 km.
AAS/Division for Planetary Sciences Meeting Abstracts #47
- Pub Date:
- November 2015