Apophis is coming! On April 13, 2029, the 350-meter asteroid Apophis will pass by Earth with a miss distance of six Earth radii — about one-tenth the lunar distance — a miss distance closer than geosynchronous satellites. Such a close encounter by an object this large is on average a once-per-thousand-year occurrence.

The workshop “Apophis T–9 Years:  Knowledge Opportunities for the Science of Planetary Defense” was held to explore scientific options for making the most of this once-in-a-millennium “natural scientific experiment.” The 1994 Comet Shoemaker-Levy 9 impact was frequently referenced as a rare natural opportunity to advance planetary science. The Apophis workshop was initially planned as an in-person meeting to be held in April 2020; the workshop was recast as a virtual meeting conducted November 4–6, 2020. Over 3 days, 60 presentations were made to a consistent online audience of approximately 150 attendees, with a total registration of 300. Presentations were a mix of invited talks, contributed talks, and lightning talks introducing electronic posters. Ample discussion time was given to each talk, and open discussion periods were held at the end of each day. Questions were actively fielded by the session chairs using the chat feature of the virtual meeting platform.

Attendees were unanimous in agreeing that “time is of the essence” for formulating and implementing plans, especially if in situ exploration is warranted. The highest-priority science opportunity presented by the flyby is the potential for Earth’s tidal forces to induce “seismic shaking” in the body of Apophis, thereby producing the measurement opportunity to map the internal structure of a potentially hazardous asteroid. Participants noted that in six decades of planetary exploration, natural seismicity measurements have only been achieved for two bodies: the Moon and Mars. Thus Apophis clearly presents a decadal science opportunity.

The measurability of any seismic signal in Apophis remains a significant unknown both in its magnitude and in the methodology. Workshop attendees encourage ongoing analysis of the opportunity, including a Science Definition Study for minimum requirements for any in situ investigation.

For more information, including links to the program and abstracts, visit the meeting website at https://www.hou.usra.edu/meetings/apophis2020/.

— Text provided by Richard Binzel (Massachusetts Institute of Technology)