The report goes on to say that with sufficient warning four types of mitigation could meet the threat from all NEOs, except what it called the most energetic ones:

• Civil defense (evacuation, sheltering in place, providing emergency

• infrastructure) is a cost-effective mitigation measure for saving lives from the smallest comet or asteroid hit and is a necessary part of mitigation for larger events.

• “Slow push” or “slow pull” methods use a spacecraft to exert force on the target object to gradually change its orbit to avoid collision with the Earth. This technique is practical only for small NEOs (tens of meters to roughly 100 meters in diameter) or possibly for medium-sized objects (hundreds of meters), but would likely require decades of warning. Of the slow push/pull techniques, the gravity tractor appears to be by far the closest to technological readiness.

• Kinetic methods, which fly a spacecraft into the NEO to change its orbit, could defend against moderately sized objects (many hundreds of meters to 1 kilometer in diameter), but also may require decades of warning time.