Thursday, October 08, 2009

Two impacts that will happen, one that won't

NASA's LCROSS spacecraft and its Centaur booster rocket
Image Credit: NASA

Early tomorrow morning, the moon will be hit by two fast-moving bits of space debris.  In 2036, the Earth will almost certainly not be hit by a 250 yard-wide rock.  All three of these pieces of news make me happy.

First, the moon.  The two bits of "space debris" are NASA's LCROSS probe and its spent Centaur booster rocket.  The rocket, weighing in at about 2.5 tons, will hit first, impacting the crater Cabeus (near the moon's south pole) at 7:31 am (and 19 seconds) EDT tomorrow, while moving at a speed of 9000 kilometers per hour.  This hit should gouge a crater about 20 meters wide and 5 meters deep, and may toss as much as 385 tons of lunar soil and rock into the air.

NASA wants this plume of debris to be tossed up, because the LCROSS spacecraft is going to fly through the plume, taking pictures, analyzing the chemical makeup of the debris, and sending that information back to Earth before it collides with the moon itself 4 and a half minutes later.  That impact will create a second plume of debris.  Earth-based telescopes on the night-side of the Earth will be staring at and analyzing the impact site, too, assuming the weather is good.  (The weather at the observatories in Hawaii is looking chancy).

The whole point of these two collisions is to look for one specific chemical: water.   Satellites have found indirect evidence of water on the moon, including a recent claim of a few-molecule-thin layer over large parts of the moon.  The idea here is that the floor of the crater Cabeus never sees the sun, so if any water is brought into the moon by comets, it may freeze out and stay in these craters.

Finding water on the moon has two important implications.  First, there is the functional implication that if there is water on the moon, any lunar bases we humans build might be able to use it instead of bringing water from Earth.  Water is heavy, and every pound of material we send to the moon costs thousands of dollars.  Given that a pound of water is only about a half liter (a pint's a pound the world around), a bottle of Earth water would cost ten thousand dollars.  Granted, it could (and would) be recycled, but if you thought Perrier was expensive... (Do you think a moon base vending machine would take ten-thousand-dollar bills?  Do you think any astronauts would recognize Salmon Chase?)

The scientific side of lunar water is also quite interesting.  Many models of the early solar system claim that water should not have been able to exist where the Earth formed, yet we are positively swimming in the stuff.  Other models say that we may have originally had water, but big collisions with protoplanets when the Earth formed should have dried the Earth out.  Yet they didn't.  Or perhaps the very early Earth was dry, and comets brought water to the Earth.  By studying the detailed composition of any moon water, planetary scientists can hopefully determine where that water came from (comets?  Original to the moon?).

If you are interested in watching the LCROSS impact, your best bet is to tune in to NASA TV starting at 6:15am EDT (3:15am Pacific) to watch the live broadcast.  NASA TV will show pictures from the LCROSS satellite as they come in, plus any pictures and video from telescopes observing the impact.  If you live west of the Mississippi River, you may have a chance of seeing the impact in the pre-dawn sky, but you'll need a telescope of at least 10-inches in diameter.  Universe Today has detailed instructions for anyone wanting to see it themselves.

So much for the impacts that will happen.  Now, onto the one that won't.  In late 2004, a major stir was caused by the announcement that the 250-meter wide asteroid Apophis had a 2% chance of hitting the Earth in 2029.  As astronomers gathered more data on this asteroid, we were able to better calculate its orbit and determine that it will not hit the Earth in 2029, though it will pass very close, only about 20,000 miles away.  This is inside the orbit of geosynchronous satellites!  There also was a chance (1-in-45,000) that, if Apophis passed through a tiny kidney-shaped region of space during its 2029 passage of Earth, it would be put on an orbit that would collide with the Earth in 2036.

Yesterday, new calculations based on combining new data with the older observations of the asteroid gave a much more accurate prediction of the asteroid's path.  The result lowered the chance of a collision in 2036 to a very tiny chance: 1-in-4 million.  Now, that chance is still not zero, but it is so incredibly small we can breathe easy.  Now, we astronomers will keep looking and recalculating its orbit.  Because this asteroid comes so close to Earth, we're going to get some excellent data on it in the next twenty years.  These data will tell us a lot about the asteroid, where it came from, what it is made out of, what its structure is.  These data will also allow us to determine its orbit even more precisely, and to plan out a strategy should we find that the asteroid will hit Earth in the even more distant future.

If you want to know more about near-Earth asteroids and the likelihood of the Earth being hit any time soon, check out the Asteroid Watch website for the most current news and science.

One last note.  Some news reports have said that NASA is "bombing" the Moon with LCROSS.  That's wrong.  I don't know who used the phrase first (I actually think I heard it from NASA), but it's misleading.  There are no explosives on the satellite or the Centaur booster.   Explosives wouldn't do much, and would add additional chemicals to the analysis that would be hard to take out.  We're just crashing the probes into the Moon.  Also, some people think we are damaging/trashing the Moon with this impact.  Meteors this size routinely hit the Moon (I don't know the exact frequency, but couple-ton fireballs burn up in  the Earth's atmosphere several times a year, such as over Canada a few weeks ago).  We're just adding one event to a common occurrence.


  1. I saw the headline and said whhhattt?? then I saw the story and said oh....more money waisted...damn

  2. I think it is insane. I agree!!!