Accurate NEO Orbits from Occultation Observations

Precise orbit determination is very important for planetary defense against Potentially Hazardous Asteroids (PHAs). Orbits of NEOs are generally best improved by radar observations, but these are only possible when the NEO is relatively close to the Earth. A threatening NEO can be out of radar range for decades, during which its orbit might evolve from YORP perturbations and other small non-gravitational forces. Now that we have precise stellar data from ESA's Gaia mission, observations of occultations of stars by NEOs can provide astrometric points at least as accurate as the size of the NEO as seen from the Earth, normally about a milliarcsecond, or less. Well-observed occultations can also give information about the size, shape, and multiplicity of the object.

On 2019 July 29, an occultation of 7.3-mag. SAO 40261 by (3200) Phaethon occurred across the southwestern USA. Phaethon, the source of the strong annual Geminids meteor shower, is also a flyby target of JAXA's DESTINY+ mission. A few months before the occultation, Dr. Tomoko Arai, PI of DESTINY+, contacted NASA and the International Occultation Timing Association (IOTA), asking if they could organize a campaign to observe the rare event. The Southwest Research Institute (SwRI) and IOTA managed to deploy 52 telescopes across the path, and its wide uncertainty, that all recorded the star with accurate timing. Six of the stations recorded the occultation, with the chords fit well with Phaethon's shape model determined from 2017 Arecibo radar observations. The accurate astrometric point determined from the occultation allowed predicting and observing an occultation of a 12th-mag. star two months later, then 4 more during 2019 October, and another on 2020 October 5. The resulting precise astrometric points improved the accuracy of determination of the A2 non-gravitational term of Phaethon's orbit by a factor of 3, from 3 sigma to 10 sigma. A presentation about this was given at the 2020 October DPS meeting, with a press release pointing to much more information about the Phaethon occultations at http://iota.jhuapl.edu/PhaethonLinksAndGraphics.htm

Predictions for some more occultations by Phaethon, later this year, are at http://iota.jhuapl.edu/2020-2022Phaethon.htm. We will soon extend those predictions for a few years. With the Phaethon occultation successes, we are predicting occultations by other NEOs. Some first predictions are at http://iota.jhuapl.edu/Apophis2021.htm . Astronomers in Algeria have computed some occultations by the PHAs (3122) Florence and (159402) 1999 AP10, and tried to observe them, but without success.

We will participate in the 2021 PDC exercise, seeing if there might be any occultations that could be used to refine its orbit, and possibly give some characterization of the object. The success of that will depend heavily on finding any precovery observations.