Posted on 04/12/2002 6:52:15 PM PDT by blam
Scientists say asteroids' bigger belt is nothing to worry about
Astronomers who found there are twice as many asteroids near Mars as previously thought say there is still little chance of one colliding with Earth.
The latest estimates show there are up to 1.9 million asteroids within the main asteroid belt.
Scientists haven't worked out the orbit of every rock but believe they rarely change direction and so are unlikely to hit us.
Astronomers used the European Space Agency's Infrared Space Observatory to look at part of the asteroid belt and then estimated the total number there.
Edward Tedesco of TerraSystems and Francois-Xavier Désert of the Astrophysical Laboratory in Grenoble, France, say there are between 1.1 million and 1.9 million.
Nature reports it will take years before astronomers can discover the precise pathway for each asteroid but they say any which have been in Earth-crossing orbits will already have hit us.
Story filed: 09:27 Friday 12th April 2002
What happens when a large comet 'rips' through the asteroid belt. Billards?
Earth rips space rocks asunder
By Dr David Whitehouse
BBC News Online science editor
Binary asteroids - two rocky objects orbiting one another - appear to be common in Earth-crossing orbits, say astronomers.
Using the world's two most powerful astronomical radar telescopes, astronomers say that these double asteroid systems are formed when single asteroids are torn apart during encounters with the Earth.
Writing in Science, the researchers estimate that about 16% of so-called near-Earth asteroids (NEAs) larger than 200 metres (656 feet) in diameter are likely to be double systems.
To date, five such binary systems have been identified by radar.
Jean-Luc Margot of the California Institute of Technology says that theoretical studies suggest that the binary asteroids appear to be formed extremely close to Earth - within a distance equal to a few times our planet's radius.
Nudged towards Earth
NEAs form in the asteroid belt, between the orbits of Mars and Jupiter, and are nudged by the gravity of nearby planets, largely Jupiter, into orbits that take them to the Earth's neighbourhood.
In 2000, Margot and colleagues, using measurements from the Goldstone radar telescope, discovered that a small, roughly 800-metre-diameter (half-mile) asteroid, 2000 DP107, was a binary system.
Observations made last October with the much more sensitive Arecibo radar telescope established the physical characteristics of DP107's two asteroids as well as their orbit about each other.
The smaller object is about 300 metres (1,000 feet) in diameter and is orbiting the larger asteroid every 42 hours at a distance of 2.6 kilometres (1.6 miles). The two asteroids appear to be locked in synchronous rotation, with the smaller always with the same face turned towards to the larger.
Since DP107 was discovered, four more binary NEAs have been found, all in Earth-crossing orbits and each with a main asteroid significantly larger than the smaller body.
Double impact
Astronomers have long suspected that there could be binary NEAs. Of about 28 known terrestrial impact craters with diameters greater than 20 kilometres, at least three are double craters formed by impacts of objects about the same size as the newly discovered binaries.
Also, astronomers have noted the changes in the brightness of some NEAs, indicating a double system was causing an eclipse or occultation of one by the other.
NEAs, basically piles of rubble held together by gravity, are on trajectories that bring them within a few thousand kilometres of the planets, where tidal forces can increase the spin rate of the asteroid, causing it to fly apart. The ejected rubble then reforms in orbit around the larger asteroid.
There is an important reason for studying binary asteroids, says Nasa's Steve Ostro: their potential for colliding with Earth.
He says that knowing the density of potentially hazardous asteroids, "is an extremely important input to any mitigation plans".
"Getting NEA densities from radar is dirt cheap compared with getting a density with a spacecraft."
Jean-Luc Margot says: "Radar gives us very precise measurements of the size of the objects and their shape.
"From this we can obtain the mass of each of the objects allowing, for the first time, measurements of NEA densities, a very important indicator of their composition and internal structure."
What incredible potential - if we could develop it economically.
Well, that's a comfort.
Unlikely. They aren't as dense as is often shown in movies. They hardly ever collide.
Two things I find interesting:
1. The orbital patterns of the small asteroids around the larger will allow a more precise measurement of the mass of the central body. Kepler's third law is used this way for binary stars and galaxies orbiting larger galaxies. The size estimates (and a density of ~5) give of 1.4 x 10^11 metric tons for the larger and 2.2 x 10^8 metric tons for the smaller. So we could work out a typical orbit...
2. The multiple impact craters (in proximity) on Earth are a real eye-opener. I saw Shoemaker-Lever hit Jupiter in 26+- pieces (NASA photos)... and just imagine a binary asteroid system hitting Earth.
The plumes from the Shoemaker-Levy impacts extended (to everyones suprise) 2,000 miles back into space.
Generally nothing. Comets are small. Astreoids (even in the belt) are rare; the belt is mostly open space.
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