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First Occultation Observations by a Small NEO, (3200) Phaethon

Presentation #412.01 in the session “Asteroids: NEOs Physical Properties 2”.

Published onOct 26, 2020
First Occultation Observations by a Small NEO, (3200) Phaethon

(3200) Phaethon is the source of the Geminid meteor shower, unusual as most meteor streams come from comets, not asteroids. Phaethon is also a 2025 flyby target of JAXA’s DESTINY+ mission. On 2019 July 29, an occultation of 7.3-mag. SAO 40261 by the small NEO (3200) Phaethon occurred across the southwestern USA. Timings of the occultation might tell us more about Phaethon and would provide a precise astrometric position. A large campaign, organized by the Southwest Research Institute (SwRI) and the International Occultation Timing Association (IOTA), deployed 62 stations, almost half being unattended pre-pointed small non-tracking telescopes automated with video cameras and recorders. The JPL path prediction, using Gaia and 2017 radar data, proved to be quite accurate. All but 10 of the stations recorded the star, with 6 of them having an occultation, validating the radar size (5.8 km) and shape, apparently a rotating rubble pile similar to Ryugu and Bennu. The smaller size from IR data was not confirmed. Two months later, Phaethon occulted a much fainter 12th-mag. star in the California desert. The Dunhams pre-pointed two 25cm telescopes, as well as some 12cm telescopes that couldn’t record the star well. In addition, R. Jones and G. Lyzenga recorded with 20cm scopes. Jones had no occultation, but the two 25cm scopes nearby recorded short events, establishing the southern limit of the path to within a kilometer. This provided a 2nd precise astrometric point that allowed determination of Phaethon’s orbit such that 4 more occultations in October, observed from 2-3 stations each near the predicted path centers at locations in Virginia, France, Japan, Italy, and Algeria, all recorded nearly central occultations. Before the effort for the July 29th occultation, a small comet-like non-gravitational term in Phaethon’s motion had been determined to about 2 sigma accuracy. After including the results from the July and Sept. events, the non-gravitational term was improved to 9 sigma. Before the DPS meeting, readers can find out more about these efforts from a presentation for IOTA’s annual meeting available at this URL, 4th from the bottom of that page.

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