Our planet is surrounded by a swarm of alien invaders. They fly through space faster than a speeding bullet and they're nearly impossible to track with conventional ground-based radars.
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 millimetre 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.
So what can a beleaguered satellite operator do?
Not much, says Rob Suggs, the Space Environments team leader at the Marshall Space Flight Centre 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 minimise cross section or point sensitive components away from the incoming particles. They can also turn off high voltage power supplies, and avoid complex manoeuvres, 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 Centre 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.
The radar records about 200 echoes per hour on an average day, says Suggs. Most are weak reflections from meteor trails too dim to see with the unaided eye. We're sensitive to meteors down to visual magnitude 7.5, noted Bill Cooke. That corresponds to meteoroids with masses greater than about 10-5 gm.
Nowadays satellites are well-enough armoured 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 - rivalling the surface temperature of the Sun. Usually the entire meteoroid is vaporised 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 metres 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 bs 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.
Keeping track of space dust, which is spread throughout the inner solar system, is a big job. The radar in Huntsville can only detect meteoroids that crash into Earth's atmosphere above the southeastern United States.
The Marshall facility, which Suggs says is working beautifully, costs less than 500 US Dollars (plus a PC to control the receiver and analyse 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.