Using a Gravity Tractor to Help Mitigate Asteroid Collisions with Earth
Yeomans, D.K.1; Bhaskaran, S.1; Broschart, S.R.1; Chesley, S.R.1; Chodas, P.W.1; Jones, M.A.1; Sweetser, T.H.1; Lu, E.T.2; Schweickart, R.L.2
1JPL/Caltech; 2B612 Foundation

Under B612 Foundation sponsorship, a study was undertaken to determine the feasibility of using a gravity tractor as a mitigation measure when a Near-Earth Asteroid (NEA) has been identified as a serious Earth impact threat [1]. In such a circumstance, a gravity tractor spacecraft with a transponder (t-GT), in residence with the threatening NEA, could provide accurate NEA tracking to verify that a deflection was needed. If a separate impulsive (e.g., kinetic energy impact) deflection is carried out, it would be necessary to verify afterwards that the primary impact threat had been eliminated and a subsequent impact had not been enabled by the NEA being deflected into a resonant Earth return keyhole that would allow a later Earth impact; in this latter case, the t-GT spacecraft could be used to provide a trim maneuver by towing the NEA to avoid this secondary impact keyhole and the spacecraft transponder could be used to verify that the tractoring was successfully carried out.

The report is available on-line [2]. The general conclusions include the following:

  • The most threatening NEAs are those on Earth similar orbits.
  • Simulations show that most actual Earth impactor discoveries surpass 99% impact probability very early in the second optical apparition - or after optical and radar data are obtained during the discovery apparition.
  • Impulsive primary deflection techniques (e.g., kinetic energy impactor) provide relatively uncertain amounts of deflection (e.g., the ejecta blow back momentum multiplier β is unknown).
  • Secondary impact possibilities (keyholes) must be carefully examined for each specific case.
  • An asteroid close flyby of a planet could magnify, by a large factor, the asteroid's position uncertainty for subsequent flybys and Earth approaches.
  • NEA planetary encounters can also dramatically affect the efficiency of earlier tractoring. For example, the optimal time for tractoring may not be as soon as possible and some tractoring start times can cause counter productive results.
  • Identifying potential keyholes during Earth encounters and determining optimal times for tractoring to avoid a keyhole passage require fully perturbed, non-linear numerical analysis.
  • A relatively simple and robust thrust control law could keep a gravity tractor spacecraft in close proximity to the station-keeping location required to effectively tow an irregularly shaped, rotating near-Earth asteroid. Only modest fuel consumption is required.
  • The combination of radiometric tracking of a nearby spacecraft with optical imaging of the asteroid from the spacecraft is sufficient to significantly improve knowledge of the asteroid's orbit. It would not be necessary to place a transponder on the surface of the asteroid to achieve precise asteroid tracking.
  • The asteroid orbit accuracy improvements provided by the spacecraft range from factors of 2 to 5 over the knowledge which can be obtained using only Earth-based observations of the asteroid. The size of the improvement is dependent on the relative viewing geometry and hence the time period over which the spacecraft is tracked.
  • The amount of time it takes to realize these improvements in the knowledge of the asteroid's ephemeris is measured in days to weeks. A spacecraft need not be in place for months or years for the improvements to take place.
  • While the gravity tractor in our simulation example was a viable method for towing the asteroid away from a keyhole, and hence avoiding a subsequent Earth impact, there might be other impacting scenarios for which it would not be viable. Each threat scenario has to be analyzed individually to determine whether a keyhole is a concern and whether a gravity tractor could be used to move an asteroid trajectory away from it.

    References:
    [1] Lu E.T. and Love S.G. (2005) Nature, 438, 177-178.
    [2] http://neo.jpl.nasa.gov/neo/b612_report.html