Feb. 10, 2000 — A Valentine’s Day “rendezvous” with an asteroid named for the Greek god of love? We’re sure this is coincidence. But the yearlong study of Eros is no accident. Asteroids have a lot to teach us about where we came from.
In the huge gap between the orbits of Mars and Jupiter, about 220 million to 300 million miles from where you are reading this right now, are thousands of asteroids — in a feature of the solar system known, appropriately enough, as The Asteroid Belt.
Astronomers believe that as solid material condensed out of the gaseous early universe, little bits were drawn together either by gentle collisions or by gravity and grew into the moons and planets of our solar system, about 4.6 billion years ago. Asteroids formed at the same time, from the same stellar gas. They are medium-sized chunks that never joined together.
They are “stuck” in the asteroid belt, circling the sun just as the planets do, unable to glom together into a planet of their own because of the gravitational pull of giant Jupiter, the next farthest planet out in the solar system. Jupiter’s so big, with gravity so strong, that it has sped up the asteroids and altered their orbits, so now when they collide they’re going too fast to stick together. Instead, they mostly whack each other into smaller bits.
Asteroid Factoid 1: Many of them act like mini-planets, rotating on a single axis as they orbit the sun at about 36,000 miles per hour, or 57,600 kilometers per hour. (In comparison, Earth orbits the sun at 64,800 mph, or 103680 kph.)
Brian Marsden, who runs the Smithsonian Astrophysical Observatory’s registry for near-Earth objects (NEOs are things flying around in space that could conceivably whack Earth someday), says asteroids are rocks, made of mostly the same kinds of things rocks here on Earth are made of: silica, carbon, iron, nickel and so on.
But asteroids haven’t been altered by the dynamic forces affecting rocks on Earth, like weathering and erosion, underground heat and pressure. So learning more about asteroids could help us know more about how our own planet formed.
Asteroid Factoid 2: Asteroids range in size from pebbles up to beefy Ceres, 500 miles across.
So if asteroids are stuck way out there, why do some come closer to us? Sometimes Jupiter tugs a space rock into an exaggerated elliptical orbit that brings it closer to Earth. (Eros, the one we’re supposed to start dating on Valentine’s Day, comes a convenient 12 million miles from Earth.)
This creates two opportunities: First, science can study it more closely. Second, Hollywood can make movies scaring the bejabbers out of us by hinting that a monster asteroid might one day smack us into oblivion.
Marsden says “The Big One” hasn’t been spotted. Yet. We are pestered only by the occasional little asteroid flying through our atmosphere, appearing as a “shooting star” as friction burns it up.
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Asteroid Factoid 3: Most meteors are pieces of asteroids. They are meteoroids when they are out in space. They are meteors when we see them flying through the sky as shooting stars. They are meteorites when they survive their fiery trip down to the surface of the earth. (Meteorite factite: Only the outer inch or so of the ones that get to the surface are melted by the heat of atmospheric flight. The insides are pretty much unaltered space rock.)
We can study meteorites in our labs. We know that they, and their asteroid parents, are made of iron, nickel, silicates, carbon and other minerals common on Earth. But astronomers studying asteroids out in space examine the various light waves they give off as a way of telling what they’re made of. They have found that those spectra are close to, but don’t exactly match, the mineralogical composition of meteorites studied in the lab. Nobody understands why they don’t match.
What unique forces are affecting asteroids in space? Understanding that could help us figure out how those forces affected the formation of the earth.
So we want to study an asteroid in space close up and personal. We’ve tried before. The Deep Space 1 probe flew past the asteroid Braille last year but sent back only a few fuzzy pictures. The Galileo spacecraft got some great information about the large asteroids Ida and Gaspra as it passed through the asteroid belt on its way to studying Jupiter. And the NEAR satellite got some good pix of Mathilde, passing 753 miles from that asteroid before heading on to Eros.
We were supposed to begin dating Eros last year, but a rocket didn’t fire as planned. As a result, the NEAR satellite had to make a very wide U-turn and head back for this year’s try. NASA says the balky rocket is fine now. They do not say if NEAR is carrying roses to present to Eros for the Valentine’s Day rendezvous.
David Ropeik is a longtime science journalist and currently serves as Director of Risk Communication at the Harvard Center for Risk Analysis.
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