Near Earth Objects or NEOs are extraterrestrial objects, i.e., comets, asteroids, and meteoroids, that orbit close to the Earth. The usual interpretation of an object being “close” is one that has a perihelion distance of less than 1.3 AU, wherein one AU (astronomical unit) is the average distance of the Earth from the Sun over one complete revolution.
Interest in NEOs is driven by the concern that an impact from a big one can be devastating. The extinction of the dinosaurs 65 million years ago is believed to have been caused by such an impact.
These scenarios have already been depicted by Hollywood through films like Deep Impact and Armageddon. On both films, the heroes landed on the NEOs and did their heroic acts there. Skeptics think this is impossible. Now, you might be amazed but in reality, real spacecrafts (although unmanned) have already made close contact with a few NEOs.
One of them was with 433 Eros, a near-Earth asteroid with dimensions approximately equal to 34.4 x 11.2 x 11.2 km. NASA’s Near Earth Asteroid Rendezvous (NEAR) Shoemaker probe successfully landed on the asteroid in 2001.
A NASA image of asteroid Eros (left) and Robert Gaskell's shape model of the asteroid (right). Credit: PSI
As a matter of fact, an Apollo named 4581 Ascepius did get uncomfortably near the Earth on March 23, 1989. On that date, this 300-m asteroid passed through the exact spot where the Earth was about 6 hours earlier. An impact would have been devastating – even more devastating than the most powerful nuclear bomb that man has ever detonated.
Despite all the sophisticated equipment in NASA and all the other space agencies worldwide combined, it can be very difficult to monitor the entire sky. After all, it is a very large sky to monitor. NASA, under a congressional mandate, was tasked in 1992 to locate at least 90% of large near-Earth asteroids within one decade.
As a result, some organizations have been funded by NASA to aid them in this project, which is named Spaceguard. Most organizations and other initiatives throughout the world that focus on monitoring NEOs all associate themselves directly or indirectly (since some of them are not really funded by NASA) to Spaceguard.
The Oort cloud is a hypothetical region in the outskirts of our Solar System where comets are believed to reside. Want to read more about the Oort cloud? Here are some articles from Universe Today:
Oort Cloud
Where do comets come from?
Tired eyes? Listen to some episodes at Astronomy Cast. Here are two that might interest you:
Stellar Roche Limits, Seeing Black Holes, and Water on Mars
The Search for Extraterrestrial Intelligence
References:
NASA Near Earth Object Program
Wikipedia
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