The folks at the B612 Foundation have hired the Jet Propulsion Laboratory to use their advanced computer simulations to determine if a gravity tractor can significantly alter the course of a medium size asteroid enough so that it misses the "keyhole" -the "small region slightly further from the Earth than the resonance line per se, which would, should the asteroid pass through it result in an impact at the time of the resonant return."
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Summary Statement by B612 Foundation regarding its contract with JPL to conduct a detailed performance analysis of a transponder equipped Gravity Tractor spacecraft.
A successful NEO deflection campaign will involve several key functional elements, including the ability to, in situ, precisely determine the orbit of a threatening NEO prior to and post deflection, and to precisely adjust the NEO’s orbit to assure its successful passage between return keyholes at the time of its closest approach to Earth. B612’s contract with JPL called on it to quantify these two critical capabilities. The analysis verified the viability of the transponder-Gravity Tractor (t-GT) spacecraft to perform these critical deflection functions. A full report of this work is now available on the B612 website at http://www.b612foundation.org/press/press.html, #18.
The full report from JPL is available here, and here are some highlights from their conclusions:
1.5 Study Conclusions
1. This study has shown that a relatively simple and robust thrust control law can keep a gravity tractor spacecraft in close proximity to the station-keeping location required to effectively tow an irregularly shaped, rotating near-Earth asteroid. For our test case, the spacecraft could be kept within a 20 x 50 x 50 meter box at a nominal distance of 155 meters from the asteroid’s center-of-mass for a total translational ΔV monthly cost of 40 - 45 m/s with a corresponding monthly fuel consumption of only ~ 1.4 kg.
16. A gravity tractor could be useful for the possible case in which a primary deflection technique such as a kinetic impactor happens to move the asteroid trajectory into a keyhole; the gravity tractor could shift the asteroid’s trajectory enough to miss a secondary impact keyhole. At the same time, tracking of the gravity tractor spacecraft could provide precision orbit information for the asteroid before and after the primary deflection attempt and after the gravity tractor trim maneuver.
17. While the gravity tractor in our simulation example was a viable method for towing the asteroid away from the 2049 keyhole, and hence avoiding a 2054 Earth collision, there might be other impacting scenarios for which it would not be viable. Each threat scenario would have to be analyzed individually to determine whether a gravity tractor could be used to move an asteroid trajectory away from a keyhole.
Additional general study 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.
• The primary deflection techniques (e.g., kinetic energy impactor) provide relatively uncertain amounts of deflection (e.g., the momentum multiplier β is unknown)
• Secondary impact possibilities (keyholes) must be carefully examined for each specific case.
• Determining potential keyholes during Earth encounters and determining optimal times for tractoring to avoid a keyhole passage requires fully perturbed, non-linear numerical analysis (two-body analyses do not suffice).
• 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 an asteroid’s orbit. It is not 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.
• A close flyby, such as the one that occurs in 2046 for this scenario can magnify the asteroid’s position uncertainty for subsequent flybys by a large factor.
In review, it appears that we have the technology available to prevent a catastrophic impact provided we have the time to track and analyze the threat years before potential impact. What this should emphasize more is the reality that the kinetic energy that will be necessary to prevent an immediate impact -say, a threat that will hit in a year or less- is not available or even being considered at the present time. And more disturbing is the fact that this is not a bigger part of the budget at NASA.
I'm afraid that when this does become an issue, the time we've lost engineering a solution to this problem will become insurmountable. JPL and the B612 Foundation make me a little less concerned, but we still need to be aware of our shortcomings.