Shooting Stars Can Shoot Down Satellites

In August 1993, the European Space Agency’s Olympus 1 experimental satellite was in trouble. The annual Perseid meteor shower, especially fierce that year, was just getting started. Usually, given sufficient warning of intense meteor activity, satellites can engage their defenses: They might,for example, protect themselves against incoming projectiles by orienting their solar panels against attacks, like shields. But Olympus 1 was at a disadvantage. A previous mishap had disabled the satellite’s ability to shift its solar arrays, leaving it defenseless. In short order, an incoming Perseid particle knocked out Olympus’s gyroscope stability, which sent the satellite spinning wildly. Attempts to regain control used up most of the craft’s fuel, leaving the US $850 million satellite with barely enough to propel itself into a graveyard orbit, where it could neither hit other satellites nor do much of anything useful. Olympus 1 will likely remain there forever, cold and dead before its time.

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Illustration: mckibillo

Meteoroids have taken out more than a few spacecraft. In addition to Olympus and Landsat 5, other possible victims were the Small Expendable Deployer System (March 1994) and the Miniature Sensor Technology Integration (also March 1994). [cont'd]

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In 2009, Landsat 5, a satellite jointly operated by NASA and the U.S. Geological Service, also began to spin out of control, again during an August peak of the Perseid meteor shower. Far below, satellite trackers scrambled to find out what had happened. Had the satellite been hit by a stray rock from space? Was the culprit a solar storm or an impact with one of the thousands of pieces of orbiting debris shed from other spacecraft or booster rockets? Or was the cause more alarming: an attack from a hostile nation?

Satellite failures such as these have cost the governments of the world billions of dollars. So it might appear strange that there are still huge gaps in our understanding of what can go wrong in space. Space agencies all over the world often struggle to figure out what has happened when some piece of hardware in orbit goes haywire. How can they differentiate between a malfunction caused by a tiny rock hurtling in from interplanetary space and an errant screw left in orbit decades ago? And more important: How can engineers properly protect spacecraft from such projectiles—whatever their provenance?

We live in a time when sending people to Mars is beginning to look like a real possibility, China has announced its plans to set up a moon base by 2030, and soldiers in the field depend on GPS satellites. And that’s not to mention regular citizens, who are finding it hard to live without the things satellites provide: long-distance communications, much of their entertainment, and help navigating around town. Therefore, many of us in the space sciences community are renewing our efforts to understand the myriad natural and artificial dangers that spacecraft constantly face.

After 53 years of sending equipment into space, planet Earth has accumulated a thick mantle of space debris. We are able to track about 20 000 objects—although the estimates that account for objects under 10 centimeters in diameter put the total number closer to 600 000. When a satellite hits an object in this belt, the collision may cause the satellite to splinter into many fragments, which then add to the accumulating debris. And because the satellite has been destroyed or crippled, it becomes necessary to send a replacement, increasing the potential for more debris later on.

Satellite designers can fashion shields that guard reasonably well against impacts with small pieces of space debris. And satellite operators can usually track and avoid the larger chunks, although sometimes that process doesn’t go so smoothly. Within the past year, both the space shuttle Discovery and the International Space Station had to take rapid evasive action to dodge one especially treacherous object. And in February 2009, Russia’s defunct Cosmos 2251 satellite collided with an Iridium Communications satellite, unlikely to be the last accident of its kind. Such incidents have prompted spaceflight operators to coin the term ”space situational awareness.” It’s an acknowledgment that you need to see and understand what’s going on in space. The growing focus on space situational awareness comes partly from the sheer number of satellites now in orbit and partly from the vulnerabilities inherent in the crowded, busy lanes of near-Earth space.

Adding to these concerns, a 2007 Chinese antisatellite test showed that even this newcomer to the space race is capable of destroying a target in low orbit using a ground-based missile. Another worry is the growing popularity of small satellites, often dubbed microsats or nanosats—names that don’t reflect their real size—which typically measure a few centimeters. Because these objects are hard to track and maintain in their proper orbits, they pose headaches for the people whose job it is to catalog everything circling Earth.

In the United States, the Air Force and NASA bankroll most of the fundamental research on reducing the threat of collision in space. Both organizations have obvious interests in being able to travel in space or to place useful objects into orbit.

I have worked with Bill Cooke at NASA’s Marshall Space Flight Center and with researchers at the Air Force’s Office of Scientific Research throughout my career, first while I conducted studies at MIT Lincoln Laboratory and later at Los Alamos National Laboratory, where my team searched for ground-based electromagnetic pulses using LANL’s radio frequency sensors, and now as an assistant professor at Stanford University. All the while, my job has been to learn about the dangers to spacecraft. I try to understand the many complications that can make a satellite mission go awry, including run-ins with orbital debris, lightning, and solar flares. My primary focus, however, is meteoroids, which are solid extraterrestrial bodies smaller than a boulder but larger than a dust grain.

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Photo: NASA

Between 1992 and 2002, the space shuttles were assessed for meteoroid and debris damage 50 times.  [cont'd]

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Satellite engineers have done much to mitigate space hazards, creating among other things the Whipple bumper, a kind of hypervelocity impact shield that’s designed to be as light as possible. You might guess that the biggest threats are the relatively bulky objects that make up space debris, which in low Earth orbit are bigger and more numerous than typical meteoroids. But meteoroids make up for their small size with high velocities. They tend to travel in the neighborhood of 12 to 72 kilometers per second when entering Earth’s atmosphere and can penetrate shielding more easily than a comparatively sluggish piece of space debris can. A Whipple shield can protect a satellite from space junk hitting at speeds up to 18 km/s, but it’s no match for faster meteoroids.

Given the danger they represent, it’s downright shocking how much remains unknown about meteoroids. Ironically enough, what’s best understood about them is how they act once they plunge into the atmosphere and become visible, when they are no longer a threat to spacecraft.

Every day, more than 100 billion meteoroids larger than one microgram enter Earth’s atmosphere, traveling at more than 11 km/s. By the time these have made their way through the ionosphere (a plasma that extends between 70 and 1000 km above Earth’s surface, its height depending on factors like solar cycle and time of day), the vast majority of meteoroids have been scoured away to nothing by the friction of the thickening atmosphere around them. As the mass is removed, it fans out behind the nucleus, forming a long glowing tail of plasma. These quick-moving flashes of light are commonly called shooting stars, although the correct term is meteor.

Meteor showers take place when Earth passes through the orbit of cometary particles. A shower is named after the constellation from which it appears to come. The Perseids seem to emanate from the constellation Perseus, the Leonids from the constellation Leo. Satellite operators are very familiar with these periodic barrages, and for the most part, they know how to protect their valuable space-borne assets.

Although it would be impossible to shield a satellite from a good-size cobble speeding along at many kilometers per second, such things are too rare to cause significant concern. The threats are the really small meteoroids—under 0.05 millimeter in diameter—which exist in much greater numbers. These interplanetary flyspecks aren’t big enough to make flashes in the atmosphere visible to the naked eye; they’re so tiny that spacecraft designers have not generally considered them much of a threat. But for satellites, they can be lethal. The damage these tiny grains inflict comes in part directly from the holes they make. Although they have little mass, they can travel extraordinarily fast. So even infinitesimal meteoroids can pack quite a punch.

Exactly how fast these meteoroids travel has been a bit of a mystery. For a long time, scientists simply assumed that the dominant population of submilligram meteoroids travel at around 20 km/s. But recent data gathered at high-power, large-aperture radar facilities (such as the Arecibo Observatory, EISCAT Scientific Association, Jicamarca Radio Observatory, and ARPA Long-Range Tracking and Instrumentation Radar, or ALTAIR) suggest the typical speed for meteoroids smaller than 50 micrometers is closer to 60 km/s. That’s a pretty large correction.

We’ve also learned that the mass of a meteoroid, as well as its composition, depends in part on where it comes from. The prevailing wisdom has been that none of this material hails from beyond our own solar system, but I and others have recently done work that challenges this long-held belief. Although our conclusion courts controversy, we think that at least 4 percent may come from interstellar space, from the exploding stars that create pulsars and from other exotic locales, like the dust-enshrouded star Beta Pictoris, 63.4 light-years from our solar system. These are impressively distant origins for a little nub of matter that could easily blow a hole in a billion-dollar satellite. Interstellar meteoroids are faster than the fastest meteoroids from inside the solar system, entering Earth’s atmosphere at speeds far greater than even the 72.8 km/s that most scientists currently define as the limit for a meteoroid originating outside our solar system.

Click on the image for the full table view.

One way to study meteoroids is to observe how they shift the orientation of the satellites they strike. Scientists can use this simple method to detect meteoroid collisions, because the angular momentum of an object in orbit doesn’t change without a reason. If a satellite’s velocity shifts (and if other variables, like gravity, light pressure, and atmospheric drag are ruled out), the only logical conclusion is that something has hit it. It’s easy to apply this technique with ALTAIR, because this radar can determine velocities with extreme precision. But even so, it’s impossible to tell the difference between a satellite’s collision with a large piece of relatively slow-moving space debris and a collision with a small but speedy meteoroid. Both could impart exactly the same change in momentum.

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Illustration: ESA

The European Space Agency's experimental satellite collided with a Perseid meteoroid in August of 1993.  [cont'd]

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A better way to differentiate meteoroids from space debris—and to distinguish among the different kinds of meteoroids—is sorely needed. A technique that could do that would also help solve another, related problem: not being able to tell the difference between naturally occurring phenomena in our own ionosphere and man-made artifacts like launch vehicles or missiles. A good approach, I and many other space scientists believe, is to try to gauge the size and speed of all this space flotsam and jetsam by looking carefully at the plasma trails these objects create when they hit the atmosphere. With ALTAIR, that’s a fairly straightforward exercise because the plasma reflects radio waves so well. Of course, you need to model the object’s motions as it burns up to calculate how fast it was going in the first place. But if you do this right, you can also figure out its mass, density, and radius. This method is certainly better than just measuring the momentum transferred to satellites when they are struck. But the best strategy of all would be to observe meteoroid impacts up close from a vantage point in space.

To that end, I am collaborating with Andrew Kalman at Stanford to develop a satellite called MEDUSSA, which stands for Meteoroid and Energetics Detection for Understanding Space Situational Awareness. If it is built and flown as I hope, the MEDUSSA satellite will be able to study exactly what takes place when micrometeoroids and energetic particles slam into it.

With this information, engineers should be better able to design satellites to resist impact damage. Of course, there will still be run-ins with meteoroids and orbital debris that destroy spacecraft in mysterious ways, just as there are disasters with aircraft that defy explanation. But MEDUSSA would certainly help. And one day down the road, I anticipate that every satellite that gets lofted will contain a stand-alone unit to sense impacts and report their effects—even if they are extreme enough to disable the rest of the satellite. Think of it as a satellite ”black box.” Maybe then, we’ll get a good picture of just all the unexpected things that can go wrong in space.

This article would not have been possible without the invaluable contributions of Stan Green.

This article originally appeared in print as "Space Invaders."