Apophis 2029: Decadal Opportunity for the Science of Planetary Defense

Many advancements in planetary science are achieved by seizing upon the opportunity created by rare natural events (e.g. comet Shoemaker-Levy 9 Jupiter impact; 1994) and by planned physical interactions (e.g. Deep Impact into comet Tempel 1; 2005). A forthcoming opportunity to seize upon a natural physical “experiment” occurs on April 13, 2029 with the unprecedentedly close Earth encounter by the large 340m asteroid (99942) Apophis. On that date, nature is performing the “experiment” of subjecting the physical body of Apophis to Earth’s tidal torques as it approaches to within 31,000 km of Earth’s surface, a distance that is closer than orbiting geosynchronous satellites. A potentially hazardous asteroid (PHA) as large as Apophis encountering Earth this closely (within 0.1 lunar distances) is, on average, a once-per-thousand year event. (In context, Apophis is 7 times larger and 350 times more massive than the Tunguska 1908 impactor; 5000 times more massive than Chelyabinsk 2013.) Because of this event’s incredible rarity, knowledge gained through measurements and outcomes of the Apophis 2029 “natural experiment” are clearly a decadal, if not millennial, opportunity for planetary science. Most specifically, this knowledge opportunity is for the science supporting planetary defense. Further, on April 13, 2029 all of Earth will be watching: Apophis will be visible to the naked eye speeding across the evening sky for an estimated 2 billion people spanning western Europe and northern Africa. In this White Paper we outline our current best understanding, and uncertainties, for scientific advances in the physical study of potentially hazardous asteroids that may be achievable by measuring physical changes of Apophis’ spin, surface structure, and/or shape configuration in response to Earth’s tidal torques. If tidal torques themselves, or surface configuration changes induce any measurable seismic vibration signal inside Apophis, a new field of asteroid seismology has the potential to be born. Over six decades of planetary science, seismology has been achieved beyond Earth for only two planetary worlds: Moon and Mars. With this White Paper, we take no position on how to implement specific investigations capable of achieving the science advances offered by the Apophis 2029 opportunity. Instead we advocate that competitive selection of investigations, be they theoretical or observational (ground-based, space-based, or in situ), be executed under the direction of NASA’s Planetary Defense Coordination Office, with a projected and perhaps necessarily augmented budget capable of supporting them. International collaboration is strongly encouraged. Thus, summarizing in four specific points, we urge the framers of the Decadal Survey to: • Recognize the decadal, if not millennial, opportunity for the science of planetary defense presented by the Apophis 2029 once-per-thousand-year “natural experiment.” • Prioritize as a top-level planetary defense science goal modeling and measuring the physical outcome on Apophis exerted by Earth’s tidal torques so as to achieve the greatest possible new insights into the physical nature, including the internal structure, of PHAs. • Recognize that time is of the essence for defining and implementing investigations of physical effects on Apophis, particularly if in situ measurements are to be considered. An Apophis 2029 Science Definition Team may be prescribed. • Recognize that the achievable knowledge of PHAs presented by the Apophis 2029 opportunity could have immeasurable benefits to the future of humanity, in the highly unlikely, but not impossible necessity to mitigate a future impact threat