Yes, the risk of impact is low. However, the U.S. government spends plenty of money guarding against the prospect of other low-probability events. Despite evidence that full-body scanners neither work as well, nor are as safe, as claimed, the TSA has spent hundreds of millions of dollars on them, against the low probability that someone would try and smuggle something nefarious onto an airplane that previous security measures would have missed. The Defense Threat Reduction Agency in the Pentagon, the National Nuclear Security Administration in the Department of Energy, and the Department of Homeland Security's Domestic Nuclear Detection Office, among other government agencies, collectively spend billions annually countering the very low-probability event of nuclear terrorism. None of these events, however horrific, have the potential to cause damage on the scale that a celestial body impacting Earth could. Only a full-scale nuclear war with Russia might approach the damage of a large asteroid.
It is far easier to quantify the risk from asteroids that have already been tracked than from something as nebulous as terrorism. The Near Earth Object Program at NASA's Jet Propulsion Laboratory is doing an admirable job of estimating impact risk of currently known PHAs. But there are also the unknown unknowns. A report last year from NASA estimated that only 20 to 30 percent of PHAs have been found. It is entirely possible that tomorrow, or next year, or five years from now, an astronomer will find an object on a collision course with Earth.
However, if we have learned anything in the past several decades of space operations, it is that it is extremely difficult to throw together something at the last minute. Developing all of the technological infrastructure for something like a gravitational tractor will take years. Even if we had one sitting in Florida ready to be launched, such a tractor would also take years to do its work. The earlier such a device could be sent up, the better it could alter the trajectory of an incoming asteroid.
By the time we see the next bullet coming, it might be too late.
Konstantin Kakaes is a fellow at the New America Foundation.