"By now, we have all heard about a handful of asteroids that are big enough to level a city or two and have a small but non-negligible chance of hitting Earth. Should we find one heading straight at Earth, what can we do about it, if anything at all?

That is the question addressed by Carusi and colleagues in a study published in the April issue of Icarus, a leading international journal in the planetary sciences. They conducted case studies of two near-Earth asteroids (NEAs) known as 99942 Apophis and 2004 VD17, whose initial orbit estimates indicated measurable probabilities of hitting Earth in 2036 and 2102, respectively. Although refinements to their orbital calculations through intensive follow-up observations have substantially lowered their chances of collisions with Earth, the authors treated the asteroids' initial orbital estimates as full-blown drills to study how such asteroids can be deflected, and to build realistic strategies to prepare ourselves for such events.

The report presents computer simulations that calculate the minimum orbital velocity change we must impart on the asteroids to deflect them away from Earth. A larger velocity change requires a stronger force, and thus imposes a greater technological and financial challenge. To make the exercise realistic, the authors considered performing their deflection maneuvers only when the asteroids cross the orbit of Earth—as the asteroids under consideration are NEAs, they have repeated Earth orbit crossings leading up to the predicted impact dates.

As expected, in general, the authors' calculations show that greater speed changes are needed as the hypothesized impact date comes closer. However, a careful examination also reveals that there are windows of opportunity in which deflection becomes considerably easier largely due to the relative orbital geometry of the asteroids and Earth. For example, in the case of 99942 Apophis, estimated to be a 400 meter chunk of rock, an impactor with 300 kg mass can deflect the asteroid to safety with a carefully angled interception on January 27th, 2020, about 16 years before impact. The authors note that such a deflection maneuver is already achievable with currently existing technologies. However, their study illustrates that things are not always that easy.

The other asteroid they considered, 2004 VD17, has an orbit closely overlapping that of Earth's over a longer span than 99942 Apophis does, and such orbital characteristics makes its deflection much more tricky. Still, the scientists found windows of opportunity such as one in 2021, 81 years before its hypothesized collision with Earth, in which an impactor weighing about a ton could deflect the asteroid away from Earth.

The authors' findings also come with a bit of bad news. While it may be technologically feasible to exert a force large enough to deflect 2004 VD17, their calculations also reveal that the impactor could shatter the asteroid, which is equivalent to converting an approaching rifle bullet into a shotgun round, with consequences that are unpredictable at best. 99942 Apophis, in contrast, should survive the relatively modest forces required to deflect it.

This study by Carusi et al. shows that deflecting real asteroids is within reach of currently existing technologies, given enough time and planning. By definition, NEAs orbit near Earth, so any that threaten us are expected to have a few close encounters with Earth, during which they are easy to find, before the final collision. Therefore, the long planning period considered in this study is realistic.

The current study's strategy will not, however, work well for deflecting objects with highly elliptical orbits such as long period comets; nevertheless, most objects that impose significant threats to Earth are NEAs since their orbits bring them so close to here. The study highlights the importance of efforts such as the SpaceWatch project hosted by the University of Arizona—its goal is to find and track all objects with chances of impacting Earth. It may well turn out that spotting an asteroid heading our way before it is too late is far more difficult than developing technologies to deflect them. "

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"99942 Apophis (pronounced /əˈpɒfɪs/, previously known by its provisional designation 2004 MN4) is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a small probability (up to 2.7%) that it would strike the Earth in 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029. However, a possibility remains that during the 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole, a precise region in space no more than about 600 meters across, that would set up a future impact on April 13, 2036. This possibility kept the asteroid at Level 1 on the Torino impact hazard scale until August 2006. It broke the record for the highest level on the Torino Scale, being, for only a short time, a level 4, before it was lowered.5

Additional observations of the trajectory of Apophis revealed the keyhole would likely be missed and on August 5, 2006 Apophis was lowered to a Level 0 on the Torino Scale. As of October 7, 2009, the impact probability for April 13, 2036, is calculated as 1 in 250,000.6 An additional impact date in 2037 was also identified; the impact probability for that encounter was calculated as 1 in 12.3 million"

Also, read http://neo.jpl.nasa.gov/apophis/

Regards!

An impact by a asteroid is likely eventually. I definitely think we should be working on some type of energy sourse to deflect it. I think astrophysicists and engineers could make it given enough funds for R &D.

The problem is that it is extremely expensive and difficult to get large amounts of mass off the Earth and far enough into space to be able to do much along the lines of rocket motors, etc.

High power lasers are not feasible in the near future at least, and would also have the problem of either being deployed in orbit (concerns of mass to orbit as well as weaponization of space), or shooting up through the atmosphere, which would disperse much of its energy, reducing effectiveness, and heating the atmosphere to boot.

Using solar power for asymmetric heating is one possibility. Impacts using a moderately massive satellite moving at high relative velocity is easier. Using a nuclear explosion is problematic as it again weaponizes spaces (a political issue) and that it could change a single body into a shower of smaller, but still deadly projectiles. Changing the courses of many such smaller bodies becomes logistically impossible.

About the sonic blast comment Dave... Sonic blasts require air or something to move through. Space is a vacuum. You won't get sonic anything in space.