Michelle Thaller
A Mysterious Stranger in the Dark
At the beginning of November, the International Astronomical Union convened a committee to address a potentially lethal threat from outer space. On September 29th, two astronomers noticed a peculiar, dim object passing by Earth at a distance of five million miles. We saw it just as it slipped away into space, before anyone had a chance to get a better look. Five million miles may seem like a long way out, but that's only about 20 times farther away than the moon, and whenever anything from space gets that close, astronomers start to get a little nervous.
The mysterious interloper was most likely a small asteroid, a chunk of rock and iron from the floating junkyard we call the Asteroid Belt. Between the planets Mars and Jupiter is a drifting field of debris left over from the formation of our solar system. Many astronomers think the asteroid belt is where a planet tried to form, but was pulled apart before it could solidify, caught between the strong opposing tugs of Jupiter and the sun's gravity.
Ceres, the largest known asteroid, is more than five hundred miles across, but most asteroids are less than a mile in size. Every once in a while, the orbit of an asteroid is disrupted by some close encounter with Jupiter or Mars, and it falls in toward the inner solar system. Some of these manage to settle into stable orbits around the sun; we know of dozens that cross the Earth's path. And when the orbits of two objects intersect, the outcome is inevitable: someday they will collide.
What astronomers wanted to know, and quickly, was whether the mysterious object posed any serious threat of a collision in the near future.
Astronomers checked their archives for past observations of the dark rock, now labeled 2000 SG344, and found it to be in an orbit that will take it even closer to Earth during its next passage, in the year 2030. But (and I didn't want to hold you in suspense for too long), it will not collide with Earth. At least not that time. But each orbit seems to bring it a little closer, and now there is a one in a thousand chance that it will hit Earth in September 2071.
What does this mean for us? At this point, we don't know. It depends hugely on how big SG344 really is, and that's not very easy to find out. Small asteroids are incredibly dim, so all astronomers can really do is make an educated guess about how large they are. Some asteroids are slightly more reflective than others, making them brighter (and therefore seemingly larger) in the dim sunlight of outer space.
Estimates of SG344's size currently range from about 30-70 meters across. That's large enough to cause some mischief (like obliterating a city unfortunate enough to be in the way), but too small to cause any kind of global catastrophe. But don't breathe that sigh of relief too quickly. We know that Earth has gotten walloped by giant rocks from space in the past, with huge consequences. The question is not if it will happen again, but when.
Really spectacular collisions are incredibly rare, and that's a good thing. Remember Comet Shoemaker-Levy 9, which slammed into Jupiter in 1994? Fireballs larger than the Earth were visible above the clouds of Jupiter as fragments of comet hit the atmosphere and exploded. We were really happy the comet didn't hit here.
What would have happened if it did? Scientists ranging from geologists to atomic bomb specialists are trying to find out. The last really major impact seems to have happened 65 million years ago. Yup, that's the extinction of the dinosaurs. Scientists think the Earth was whacked by an object (either a comet or an asteroid) about ten miles across. The resulting explosion carved out the biggest crater the Earth has sustained in the last billion years (located in the Yucatan Peninsula), and almost wiped out life on Earth entirely.
The catastrophe was almost unthinkable. A monstrous fireball and resulting superheated winds wiped out all life for thousands of miles, but that was merely the destroyer-comet's calling card. Burning debris was flung halfway across the Earth, heating the atmosphere enough to ignite planet-wide forest fires. Subsequent smoke and dust became thick enough to blot out the Sun for years, effectively killing all plant life on the Earth's surface. Life in the sea wasn't safe either; billions of tons of sulphur-laden dust, thrown up into the air during the explosion, settled back to Earth in the form of scorchingly toxic acid rain. Even the ozone layer was destroyed, allowing the sun's radiation to sterilize the soil. It's amazing to think that any life survived that, but you and I are proof that some did.
Such planet-resurfacing impacts occur on time-scales of tens of millions of years. It will happen again, but the chance of you (or even humanity as we know it) having to deal with it is vanishingly small. What we might have to deal with someday, however, is the impact of a smaller body, say about half a mile across.
Statistically, we should pick up one of those objects every 300,000 years or so. An object that size wouldn't destroy life as we know it, but it would put a huge strain on world civilization. A supercomputer at the United States Department of Energy has recently been commissioned to calculate just how devastating such an impact would be. Since the 75% of the Earth's surface is covered with water, scientists are trying to find out what would happen in the (more likely) event that an asteroid hits in the ocean.
The results are not heartening -- the damage is likely to be twice as severe with a water impact. Any asteroid or comet smashing into the ocean would create monster tsunamis (tidal waves) that would wipe out most coastal regions (home to over half the Earth's population). In the case of the United States, the giant waves would overrun the entire Eastern seaboard, stopping only at the foothills of the Appalachian Mountains. Even a comparatively tiny rock (1,000 feet across) would create waves 300 feet high!
Stuff like this always leaves me feeling pretty insecure. But remember, these events are extremely unlikely. And as for the mysterious object SG344, there's a hopeful (and somewhat humorous) end to the story. When astronomers were rushing to find out what SG344's orbit looked like, they discovered that it was strangely similar to Earth's orbit, almost identical, in fact.
Not many objects have orbits like that, unless they came from Earth originally. How's that? Interestingly, one of SG344's "close encounters" with Earth took place back in 1971, at the height of NASA's Apollo program. The giant Saturn V rockets used to reach the moon had a booster stage called the S-IVB, which was jettisoned after use. Most of these rocket stages crashed (as planned) onto the moon, but one of them, from Apollo 12, seems to have gotten itself into an orbit around the Sun.
And the estimated size of the object is about right too, considering that a rocket's metallic skin is more reflective than a dark, rocky asteroid. Is this mysterious stranger in space actually a relic of the moon missions, returning home? We can't know either way for sure, as SG344's orbit is taking it into the glare of the Sun, making it impossible to observe. But when it slips back around again, we'll be ready, and waiting.