Nature's Tiniest Space Junk
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Is Earth in peril? Is it time to head for the community storm shelter?
Not this time, say scientists. The interlopers are simply meteoroids, ever-present specks of dust shed by comets and asteroids. And they pose no danger to Earthlings other than occasional headaches suffered by satellite operators.
Most meteoroids are tiny -- typical grains span just a few tenths of a millimeter and weigh less than 10-4 grams. Nevertheless, meteoroids pack a considerable punch as they fly by Earth at speeds ranging from ~10 to 70 km/s (22,000 to 157,000 mph). A 10-4 gm speck of dust racing along at 30 km/s carries about the same energy as a very high energy cosmic ray, ~3 x 1020 electron volts. Collisions with high-speed space dust can electrify satellites, scrambling software and triggering mistaken control procedures.
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"Not much," says Rob Suggs, the Space Environments team leader at the Marshall Space Flight Center Engineering Directorate. Meteoroids are so numerous they're impossible to avoid entirely. If ground controllers know when Earth is going to enter a dense swarm they can orient their craft to minimize cross section or point sensitive components away from the incoming particles. "They can also turn off high voltage power supplies, and avoid complex maneuvers," added Suggs. "This is what we recommend when we give satellite operators our meteoroid forecasts."
Recurring annual meteor showers like the Leonids and Perseids are predictable. But what about times when Earth passes through an uncharted dust stream? There are plenty of examples in recent years of meteor flurries that took sky watchers and satellite operators by surprise.
To provide an early warning system for such events, Rob Suggs and colleagues Bill Cooke and Jeff Anderson have built an experimental "forward-scatter radar" at the Marshall Space Flight Center in Huntsville, AL. It monitors near-Earth meteoroid activity around the clock.
"Our system is pretty simple," says Suggs. "We use a 6-element 'Yagi' antenna and a computer-controlled shortwave receiver to listen for 67 MHz signals from distant Channel 4 TV stations." The transmitters are over the horizon and normally out of range. But when a meteor streaks overhead the system records a brief ping -- the echo of a TV signal bouncing off the meteor's trail. [more information]
Right: Rob Suggs and Bill Cooke captured this low-light video image of a Leonid meteor on Nov. 18, 2000, as it streaked above Huntsville, AL. The movie's sound track contains a radar echo from the meteor's ionized trail. [1.5 MB avi]
Nowadays satellites are well-enough armored to withstand a smattering of meteoroid strikes. However, "mechanical damage isn't the main concern," explains Jeff Anderson. "The 'plasma effect' is more important."
When a meteoroid hits a satellite, it can heat the impact site (an area 3 to 5 times larger than the meteoroid itself) to thousands of degrees Kelvin -- rivaling the surface temperature of the Sun. Usually the entire meteoroid is vaporized along with a tiny bit of the spacecraft.
"Such an impact forms a plasma cloud consisting of electrons and ions," added Anderson. "These clouds start out small, but they can quickly spread a few meters across the surface of the spacecraft. Electrons, which are light and mobile, rush away from heavier ions in the expanding cloud -- that creates a charge separation." The electric fields that build up between positively-charged and negatively-charged areas within the cloud induce currents in the body of the spacecraft.
"In 1993 a Perseid meteoroid hit a solar panel on the European Olympus communications satellite," recalled Cooke. "The project manager [for that satellite] believes the hit caused a pulse that sent false signals to the control system. The satellite went into the wrong automated sequencing and, for a while, they lost control of the spacecraft. It took all the remaining propellant to regain control, and that was the end of that mission."
Solar sails and sun shields for orbiting telescopes are also vulnerable to space dust. "The bigger you are the more likely you are to get popped by something," noted Cooke. Solar sails have lots of surface area so they make big targets.
In the inner solar system, where the meteoroid population is greatest, sails will lose one or two percent of their total area to meteoroid strikes every ten years. "It's important to build sails out of materials that don't propagate rips," says Cooke. "Otherwise meteoroids could pose a big problem for such spacecraft."
Above: An interplanetary meteoroid about 10 microns across. [more]
The Marshall facility, which Suggs says is "working beautifully," costs less than $500 (plus a PC to control the receiver and analyze the echoes in real time). But going global might not be easy. "Our challenge now is to figure out a calibration scheme that will work everywhere, regardless of how many transmitters are nearby and how they're distributed. We're breaking new ground and we still have lots of work to do."
To learn more about the Space Environment Group's meteor radar, and to hear some echoes for yourself, please visit SpaceWeather.com's Meteor Radar page.
Editor's Note: Radar isn't the only way to detect meteoroids (although it is the best way to record small ones). In places with dark skies, observers can usually spot 2 or 3 bright sporadic meteors each hour. You can also see the solar system's swarm of meteoroids glowing in the night sky by means of reflected sunlight. Sky watchers call such glows the Zodiacal Light and the "Gegenschein." --Tony Phillips
Comet dust and meteoroids -- learn more about nature's tiniest space junk from JPL's STARDUST web site.
US Space Command -- Their primary mission is to watch for missiles, but they also monitor space junk in Earth orbit.
Orbital Debris and Micrometeoroids -- Find out what's orbiting our planet.
Junk in Space! -- from NASA's Windows to the Universe web site
Long Duration Exposure Facility (LDEF) -- Find out about NASA's space debris experimentation.
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