Explanatory text by Russell Schweickart and Edward Lu
Deflecting an asteroid (more generally NEO) that is thought to be headed for an impact with Earth is a complex undertaking. Because of the variables and tracking uncertainty more than one mission may be required. For this reason we refer to it as a deflection campaign. In all cases one wants to first deploy a spacecraft (s/c) to rendezvous with the NEO and make a precise determination of its orbit. While tracking from the ground may be extensive, a nearby spacecraft with an active radio transponder on board will be able to dramatically reduce the residual uncertainties in the NEO orbit based on ground tracking. With a precise orbit determination the threatened impact can be confirmed and, if a "direct impact", the precise impact location calculated. If the NEO threat is a "keyhole impact" this potential can also be determined or the uncertainty dramatically reduced.
A "direct impact" is one in which the NEO is headed for an Earth impact and in the interval to impact its orbit will not be gravitationally modified by any other object. The deflection "target" in a direct impact is the Earth, 12,400 km in diameter.
A "keyhole impact" is one in which the NEO will impact the Earth only if it passes through a very narrow "window" or "keyhole" close to the Earth several years prior to that impact. It is the gravitational change in the NEO orbit during this close pass which sets up the subsequent Earth impact several years later. The deflection "target"in a keyhole impact is the keyhole, usually a few km or smaller in size. If the NEO does not pass through the keyhole it will not hit the Earth.
All NEO deflections are accomplished by slightly changing the velocity of the NEO several years prior to an impact. A sufficient change in velocity will result in the NEO arriving at the intersection with the Earth either before the EArth has arrived or after the Earth has passed through the intersection, depending on whether the deflection increases or decreases the NEO velocity.
A direct impact requires a robust deflection, i.e. a significant increase in the velocity of the NEO. This can be accomplished in the vast majority of cases by a kinetic impact, i.e. running a s/c into the NEO, either from behind or in front. In rare instances (e.g. once per 100,000 years or more) a more robust deflection may be required and a nuclear explosion may have to be considered. In either case the deflection, while robust, will be very imprecise. Because of this the possibility exists that while sucessfully missing the Earth, the NEO may now pass through a return keyhole and return for an impact a few years hence. I.e. a successful direct impact deflection may (~ 1-3% of cases) result in a subsequent "keyhole impact".
A keyhole impact requires a very weak deflection, i.e. a very small change in the NEO velocity. However since keyholes are distributed both in front and behind the Earth a precise change in velocity is required to assure that a deflection results in the NEO passing between all these keyholes. If the original threat is a keyhole impact only this precise deflection is required since the target is very small. similarly, if a direct impact is deflected using a robust, but imprecise deflection technique, then a subsequent precise deflection may be required to assure that the NEO not only misses the Earth but also passes safely between all keyholes.
For precision deflections the technique available using current technology is a gravity tractor (GT). A gravity tractor is simply a spacecraft that "hovers" in place either immediately in front of or immediately behind the asteroid to be deflected. By maintaining its position in immediate proximity to the asteroid the very weak gravitational attraction between the two objects pulls the asteroid toward the GT. Generally a pair (or matching pairs) of very low thrust ion engines, canted outward to avoid direct impingement on the NEO, supply the force necessary for the GT to hover in place. Since the GT s/c will also be equipped with a transponder it can be continuously monitored from Earth and over time (weeks or months) the change in the asteroid velocity can be precisely measured and the towing operation terminated when the desired velocity change is achieved.
The overall logic for a deflection campaign is therefore:
1) When telescopic tracking determines that a NEO has reached an unacceptable threat level a GT spacecraft would be launched to rendezvous with the NEO and precisely determine its orbit. This is the orbit determination phase (OD) of the campaign.
2) At the completion of the OD phase (several days would usually suffice) a determination would be made of whether a direct impact or a keyhole impact is in fact indicated. If the threat is a direct impact the OD phase will generally be able to pinpoint the impact zone. If the threat is a keyhole impact, the OD phase will significantly reduce the uncertainty in the future path of the NEO and allow a determination to be made of whether a keyhole deflection is required or not.
At this point the logic splits; direct impacts continue with step 3; keyhole impacts pick up again at step 5.
3) For direct impacts a kinetic impact mission would be launched to impact the asteroid and provide the primary deflection velocity change. The GT s/c would stand off to the side and monitor the KI impact to confirm its success.
4) Subsequent to the primary deflection the GT s/c would again conduct a precision OD and precisely determine the post-deflection NEO orbit. The success of the primary deflection would be confirmed (or not) and a determination would be made as to whether or not the NEO, as it passes by the Earth, will (or will not) now also pass between keyholes at the time of the original impact. If a keyhole passage is not threatened then the mission is confirmed to be fully successful. If a keyhole passage is now threatened a small adjustment in the velocity is made by the GT.
5) the GT s/c now moved into towing position to adjust the NEO velocity to assure that it will pass successfully between all return keyholes. On completion of this maneuver and a final precise OD check the mission is confirmed to be successful.
In all cases the NEO, in a slightly modified orbit, will still make a very close pass by the Earth at the time of the original impact, but it will no longer impact the Earth and it will pass between all return keyholes which would threaten an immediate subsequent impact.
This map shows the positions of all known asteroids in the solar system.