Issues That Drive Near-Earth Object Mitigation Responses

Details Issues That Drive Near-Earth Object Mitigation Responses Issues That Drive Near-Earth Object Mitigation Responses

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmnh32a..02y&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #NH32A-02

Astronomy and Astrophysics

Astronomy

[6022] Planetary Sciences: Comets And Small Bodies / Impact Phenomena

Scientific paper

Find them early: The most important aspect for the mitigation of hazardous near-Earth objects (NEOs) is to find them early enough so that an appropriate and timely response can be carried out. Although about 84% of the near-Earth asteroids (NEAs) larger than one kilometer had been found by mid-2009, only about 5% of the NEAs 140 meters and larger that could cause regional devastation had been found. Because of their fewer numbers, comets represent only ~1% of the asteroid threat in the Earth’s neighborhood. Improve impact predictions: The vast majority of true impactors will reach very high probabilities of impact after their second apparition and that is usually within a few years of discovery. Simulations suggest that radar astrometry increases the average Earth encounter predictability from 80 to 400 years and increases the impact warning time an average of up to 4 years. In addition, radar imaging can often resolve the shapes of NEOs down to the few meter level. For NEOs that have been identified as potential threats, the radiometric tracking of a rendezvous spacecraft, coupled with spacecraft imaging of the NEO, could be used to dramatically improve the asteroid’s ephemeris. Ongoing activities to improve the orbit determination and ephemeris prediction process for NEOs include efforts to de-bias reference star positions and the modeling of the small asteroid accelerations introduced by the asymmetric re-radiation of sunlight (i.e., Yarkovsky effect). Study risk mitigation responses: For the deflection of hazardous NEOs on direct impacting trajectories, relatively mature techniques include the kinetic energy impacts by spacecraft for relatively small (< 300 meters) NEOs and either stand-off or surface nuclear explosions for the larger, but far less numerous, NEO impactors. For both cases, the ejecta blown back as a result of the collision or explosion would introduce a momentum enhancement that would be expected to augment the momentum delivered by the crater forming mechanism itself. Studies need to be carried out to understand this momentum enhancement factor since it can vary by more than an order of magnitude depending upon the NEO’s structure and composition. For the few percent of simulated Earth impactors that pass closely by the Earth prior to impact, the gravitational attraction between the threatening NEO and a nearby thrusting spacecraft (gravity tractor) could be used to drag the NEO out of harms way. For example, near-Earth asteroid Apophis has a small chance of passing through a 610-meter “keyhole” during its dramatic Earth close approach to within 0.1 lunar distances on April 13, 2029 - a keyhole passage that would then allow it to return seven years later for an Earth impact on April 13, 2036. A gravity tractor could be employed in the early 2020’s to ensure that Apophis would not enter this tiny keyhole. However, the Apophis situation, where a very close Earth approach a few years prior to a potential impact offers a tremendous amount of leverage for a deflection, is not typical. In the near term, the most likely scenarios would include impacts by relatively small NEOs with little or no warning so that plans for rapid evacuations also need to be considered.

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