In the News: What if an asteroid were headed toward earth?

 Asteroid 2004 BL86 (large object) has a small moon (top, small bright object)

Asteroid 2004 BL86 (large object) has a small moon (top, small bright object)

On Monday January 26, an asteroid, 2004 BL86, came close enough to earth for us to see. To add to the excitement, this asteroid has its own moon. (NASA reports that they have found 150 asteroids with moons, or even double moons.)

Now, at 745,000 miles away, 3.1 times the distance to the moon, 2004 BL86 is not dangerously close. However, it got us curious about NASA’s Near Earth program, which closely tracks asteroids that enter Earth's "neighborhood" and can predict their path years in advance.

Perhaps some readers remember that in 2004 an asteroid named 99942 Apophis was evaluated as having a 2.7% chance of hitting earth in the year 2029. Refined measurements of it's movements showed that we were in the clear for 2029, but that a very small chance of impact remained for the year 2036. A 1 in 45,000 chance to be precise. Even with a 0.0022% chance of an asteroid hitting the earth, there was serious discussion about how to deal with this risk, which shows us that NASA takes it's job seriously. Then in 2013, it was announced that the chance of the asteroid hitting the earth in 2036 was effectively zero. But what if the risk had remained? What actions could be taken?

There have been several proposals for deflecting asteroids, and in this case the necessary deflection to keep us safe was deemed small.  One study found a “130 x 130 ft. patch with a lightweight reflective surface" would change the energy absorption of Apophis by 0.5% which would be enough it influence it's trajectory so it would certainly bypass Earth, IF the procedure was done by the year 2018.

Another report had more extensive suggestions. (A side note: This report also estimated the potential damage of the asteroid hitting earth at 400 billion and the cost of a deflection mission at 400 million. It suggested the course of action be taken based on financial concerns and ignored the loss of human life and suffering (ahem)).

The first two suggestions of this proposal would need to be accomplished prior to the year 2029. This is because the year 2029 is where the 0.0022% danger comes from. We knew that in this year the asteroid would come relatively close to us and if asteroid came at exactly 18,893 miles from the earth it would fly through a "gravitational keyhole" which would alter its trajectory enough to set it on a course where it would hit the planet. To miss the keyhole, the asteroid would only need to be deflected about a mile. However, if it were to pass through the keyhole, it would be to be deflected 5000 miles. The latter obviously a much harder task, and would perhaps (but probably not) be impossible.

Apophis was discovered in 2004. Image credit: UH/IA

The first proposed method was “kinetic deflection” which would consist of hitting the asteroid with a large spacecraft to knock into a slightly new trajectory. That's it. Oh, of course there are angles, masses and speeds to work out but the basic concept is pretty simple.

The second method promoted in this report for pre-2029 use is the “gravitational tractor” which is much like what it sounds. Remember, all masses in space create a gravitational pull which effects other nearby masses. Here a rocket-propelled vehicle would essentially tow the asteroid onto a safer orbit through it's gravitational pull.

If the asteroid entered the keyhole in 2029, however, and was scheduled to hit earth, neither of the above methods would be powerful enough to keep us safe. In this case a more extreme courses of action would be needed such as "buried bombs" or nuclear weapons detonated near the planet. These are the methods NASA considers most viable for moving asteroids off course. However, bombs (as well as the kinetic deflection method) would send smaller pieces of the asteroid into orbit in an uncontrolled (by us) manner, and some of these could very well end up landing on Earth and still cause substantial destruction.

Other proposals have also floated around such using a laser or giant mirror to "boil" off part of the asteroid.

Luckily, Earth seems safe for quite a while. Jupiter, on the other hand, with its massive size and gravitational pull, has taken the hit from several asteroids and comets. We can be grateful to this large planet as it is thought to be, quite literally, taking hits for us.  

Bill Nye has a nice simple video explaining these concepts if you'd like to check it out. If any of our readers have expertise in this area we'd love to hear from you!

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