Is Earth on a collision path with a major asteroid?

March 2, 2007

Even if you think the only life in the universe is on Earth, we're still not alone. In addition to the other planets and moons in our solar system, there are countless tons of space dust, millions of meteors, asteroids, comets and various types and sizes of debris (including the trash we've left up there ourselves) flying around at incredible speeds and in all sorts of orbits. Earth gets hit by stuff every day -- it just doesn't show up on the common man's radar because the impact isn't newsworthy. Space dust doesn't hurt us. Most large asteroids that hit Earth are the size of a basketball by the time they make it through the burning conditions of Earth's atmosphere, and those hit about once a week. It's pretty much just the astronomers that notice. It would take something massive -- space-terms massive -- to make the rest of us notice. And the last time the rest of us really noticed was in 1908, when an asteroid about the size of a football field exploded in Earth's atmosphere with the force of a 15-megaton bomb, leveling an 800 square-mile (2,000-sq-km) area of Siberia. The nuclear bomb the United States dropped on Hiroshima had a 15-kiloton yield.


So imagine the furrowed brows when NASA announced that it was expanding its Near Earth Object Program, which identifies and tracks asteroids, and the European Space Agency is launching a mission to test a potential asteroid-deflection method. As it turns out, statistically speaking, an asteroid the size of a football field should hit Earth about every hundred years. So -- statistically speaking -- we're due. But the asteroid that has scientists huddled together, holding conferences and releasing carefully worded statements is at least twice the size of a football field, and "nuke it" is actually being whispered as a last resort. It's called Apophis, and it's about 250 meters across. The 45-million-ton rock is orbiting the Sun at 28,000 miles per hour (45,000 kph). If it hits Earth, it could easily level a large city.

According to all sources, the chances of it hitting are slim, about one in 45,000, and getting slimmer. In 2005, scientists calculated that Apophis had a one in 5,500 chance of colliding with Earth, and they predict that the chance of a hit will continue to decrease. Using calculations based on the relative positions of Earth and Apophis in 2007, the asteroid will be within 24,000 miles (39,000 km) of Earth in 2029. That's very, very close, much closer to Earth than the moon is, and we'll be able to see it with the naked eye in both day and night-time. But that's not the close call astronomers are worried about. It could come even closer to Earth in 2036, and there are some algorithms that predict a collision, but most experts say it won't hit us. Still, preparations are underway.

The idea is to plan early for the best avoidance strategy. With 20 years to go, we could probably make sure Apophis won't hit us even if it wants to. Most scientists think that blowing it up with a nuclear bomb is a bad idea -- that we'd just end up with a bunch of large asteroids hitting Earth instead of one really large one. Others says if we blow it up early enough, there would be enough time for the trajectory of the pieces to shift out of the danger zone. At the moment, though, the method of choice for saving Earth from Apophis is deflection.

There are a few big ideas out there. One has several spacecraft landing on Apophis, drilling through the surface and pumping out what's inside. NASA has actually done something like this successfully with its Deep Impact mission, which crashed an impactor into a comet with the purpose of revealing the comet's composition. With Apophis, the point would be to pump the material out into space with enough force to push Apophis in the opposite direction, throwing it off course. Scientists are also talking about sending a spacecraft into the asteroid's orbit to fly next to it. This "gravity tractor" spaceship would essentially alter the gravity equation that keeps Apophis on its path, pulling on the asteroid until its position no longer threatens Earth.

But according to Donald Yeomans of NASA's Near Earth Object Project, the simplest way to deflect Apophis is to send a spacecraft up there to just slam into it, knocking it out of the way.

Forty-five million tons of speeding asteroid aside, it could be that all the fuss about Apophis is about something at once both more and less threatening. NASA estimates that there are 100,000 asteroids orbiting dangerously close to Earth right now that are big enough to cause a problem, and the agency is only tracking 4,000 of them. We may need to solidify a plan of attack. On the other hand, there are 100,000 asteroids orbiting dangerously close to Earth, right now. And right now. And ... right now.

Still here? For more information on Apophis and related topics, check out the following links:


  • Gray, Richard. "Hollywood got it wrong, this is how you stop an apocalyptic asteroid." Telegraph UK. Feb. 25, 2007. 2007/02/25/waster25.xml
  • Hainey, Raymond. "It may hit Earth ... but don't worry, we've got a plan." News. Feb. 19, 2007.
  • O'Keefe, Mary. "Local Scientist Advises How to Deflect an Asteroid." La CaƱada Valley Sun. Mar. 1, 2007. news/lnws-deathfromabove0301.txt
  • Wild, Kendall. "Keeping watch on god of destruction." The Rutland Herald. Feb. 28, 2007. article?AID=/20070228/OPINION03/702280343/1039/OPINION03