The success of movies such as Armageddon and Deep Impact taps into the public worry that a strike by an asteroid major or other “Near Earth Object” could be devastating to the Earth. The impact of a relatively small asteroid above Chelyabinsk, Russia in February, causing close to 1,500 injuries, has reinforced that the space objects have the potential to wreak havoc on Earth. In this post, we will explore the questions of: what is the extent of this threat, what are we doing to identify specific objects that pose a danger to Earth, and what are our options if we identify an object on collision course with Earth.
The vast majority of the Near-Earth Objects (NEOs) that are possible threats to the Earth are asteroids, which are rocky bodies that orbit around the sun. The orbit of some of these asteroids intersects the orbit of the Earth, creating the potential for a collision. For example, the dinosaurs and up to three quarters of Earth’s plant and animal species went extinct when a meteorite (or comet) struck what is now the Gulf of Mexico. This event has been referred to as the Cretaceous-Palogene extinction event. This asteroid was about 10 miles wide, and the collision released more than a billion times as much energy as the bombs dropped on Hiroshima and Nagasaki.
(Artist rendition of a meteoroid impact- Wikipedia)
The danger of an asteroid collision is directly related to the size of impacting asteroid. Asteroids range in size from just tens of feet to more than 500 miles wide. However, the vast majority of objects are on the smaller side. The graph after this paragraph shows that the vast majority of discovered objects that conceivably threaten the Earth are smaller than a mile across. The Chelyabinsk meteoroid was about 50 feet in size, and all of its damage occurred from a shockwave it created, not its impact. A 200 foot asteroid could flatten a major city, but such a collision happens only about once a millennium, and rarely in populated areas. An asteroid of about a mile in size could be regional or global in devastation, but only occurs once in many millions of years.
To prevent NEOs catching us by surprise and give governments the time to coordinate and create a response, many programs to monitor NEOs have been put in place. In 1998, NASA put in place a goal to detect 90% of large asteroids with threatening orbital properties. For the purposes of this survey, a threatening asteroid is one kilometer (about 0.6 miles) across. There is no one single program for finding these steroids, but many different programs are on the lookout. Continual discoveries of large asteroids have pointed to the fact that this work is far from complete.
An important development has been the development of the Torino scale, which grades the seriousness of an asteroid’s threat to Earth from 0 to 10, with different recommended levels of attention based on the threat level. The Torino score is based on 1) how likely an impact is in the next 100 years and 2) how powerful the collision would be should it happen. While some objects have received a Torino score as high as 4, currently there is only one object with a score of 1 (with the rest being 0). More information and a description of the Torino levels can be found at http://en.wikipedia.org/wiki/Torino_Scale
. While other scales have been development, the straightforwardness of the Torino scale has made it popular.
For the most part, planning for asteroid impact has concentrated on increased surveillance and tracking of asteroids. If an asteroid were confirmed to be on track to hit Earth, there are some ideas about what to do, but we are not yet ready to take on that challenge. On June 18, NASA issued a “Grand Challenge”, asking for ideas on how to stop an asteroid on track to hit the Earth.
Some of the suggested methods are:
1) Explode a large bomb next to it and try to knock it off course.
2) Attach a motor to it to drive it off course
3) Use the power of the sun to slightly divert its course- perhaps by painting part of it white!
4) Break it up using weapons, hope that the pieces are so small that they burn up upon entry to Earth.