Updated 12/30/03


The largest impact feature on the Long Duration Exposure Facility was about 5 millimeters across. This surface was aluminum covered with Teflon.

Courtesy NASA.


a trash can


It looks like a crater on the moon, but this tiny crater actually was made on a satellite designed to measure impacts. Less than a centimeter across, this impact produced lots of tiny paint fragments that may some day hit other satellites.

Courtesy NASA.

Space: The junkman cometh

Microscopic menace
Space may seem empty, until you whack up against a tiny particle that makes "faster than a speeding bullet" seem pokier than a trudging tortoise. Meteoroids -- hunks of rock and ice. Bits of spacecraft. Flecks of paint. The odd glove or lens cap. A rocket motor or so. A crater, magnified.

Add it up, and you have a zoo of weird crud that's moving rather quickly through space in low earth orbit (up to 600 miles high). We often hear 17,000 miles an hour -- that's roughly orbital speed and equal to about 8 kilometers per second -- but in fact the range of velocities is much greater. Rare bits of interplanetary stuff that have been whipped around by the sun's gravity may be travelling 80 or 90 kilometers per second!

Space junk ain't glamorous, but it's important -- even deadly. The U.S. Space Command tracks about 7,000 objects bigger than a baseball or so, and warns crewed satellites about the danger.

The threat comes from meteoroids -- anything larger than a molecule and smaller than an asteroid -- and space junk. Most of this stuff is small -- and hard to detect. To learn about space junk, NASA launched the Long Duration Exposure Facility (LDEF) satellite in 1984 and returned it to Earth in 1990. The satellite -- studded with panels, doors and other space equipment -- spent almost six years as an expensive dart board for space junk.

Down to Earth
Once back on Earth, the various test panels were "deintegrated," (that's NASA-ese for "taken apart"). In almost 6 years of exposure, there was one hit big enough to penetrate a 2.5 millimeter satellite wall on every seven square meters of target.

(BTW, those hits could cause "mission degradation," NASA-ese for "the durn thing don't work so hot anymore," or the dreaded "mission denial," meaning, "It's plumb busted!") Most of the 19,000 craters that were studied were small, although one was half a centimeter across. Roughly one billion craters larger than 10 billionths of a meter were also evident. While damage by particles was expected, some was caused, surprisingly, by atomic oxygen in space.

Black crater on metal surface. According to LDEF, about 40 percent of the junk in low Earth orbit is man-made. That proportion could be rising because space junk is apparently proliferating like cast-off clothing in a dusty attic: Each collision can form new particles that become new space junk. When microparticles strike stainless steel bolts on spacecraft, they release bits of stainless steel. LDEF was hammered by bits of stainless steel, and also with gold, which probably came from protective coatings from spacecraft.

Much space junk can be blamed on the "toss-and-forget" attitude of early space projects. "People have treated space pretty much the way they have treated this planet," says Lawrence Murr, chair of the department of metallurgical and materials engineering at the University of Texas at El Paso. "This is such a great, vast place... a little bit of junk won't make much difference." a winter boot

While LDEF attributed about 60 percent of the stuff in low Earth Orbit to natural sources, a University of Chicago space dust experiment could change that number. The experiment, consisting of small squares of plastic that looks like Saran wrap, was launched in February, 1999. The plastic detectors can determine the mass, velocity and sometimes the origin of incoming particles.

Very preliminary data indicates that 80 to 90 percent of the particles were made by people, says Tony Tuzzolini, a scientist at the University of Chicago. As predicted, most of the particles are smaller than 0.1 millimeter in diameter.

When it comes to impacts, could fast be worse than slow?

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The Why Files
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