If you've ever walked through a swarm of gnats at a picnic, you have some idea of what it's like to navigate the mass of debris that circles our planet in low-Earth orbit. Space planners have long warned that the growing belt of cosmic junk would eventually lead to collisions, and on Tuesday it happened, when an American satellite and a defunct Russian satellite totaled each other 500 miles above Siberia. This has sparked new worries that space is simply becoming too dangerous a place to travel. Things aren't nearly that severe yet — but they're getting worse all the time.

The human species has a prodigious power to litter, but the popular belief has been that the sheer enormousness of space prevents humans from doing too much damage. That's true enough — or at least it would be if we traveled throughout the entirety of the cosmos. But the fact is, the vast majority of our space exploration amounts to little more than wading offshore. The shuttle orbits at about 220 miles; the International Space Station stays at 270 miles. Our highest-flying satellites hang in space at about 22,000 miles — which sounds like a lot until you realize that still gets you only one-tenth of the way to the moon.

Like popular commuter routes, orbital corridors have been growing increasingly crowded since the 1950s. Every time we put even a small satellite into orbit, after all, much more than just the satellite comes along for the ride. There are spent booster stages, discarded adapter rings, bolts and panels and bits of insulation and even chips of paint being shed in the process.

The two ships that just collided are sources of a whole lot of potential new junk. The American craft — one of 65 communications satellites in an orbital flock known as Iridium — weighed 1,235 lb. The Russian craft, a now defunct satellite launched in 1993, weighed a ton.

"I think we will almost certainly see hundreds if not thousands of pieces of tracked debris," says Mark Matney, an orbital-debris scientist at NASA's Johnson Space Center. "It all depends on how efficient the impact was. Was it a glancing blow or a full body hit?"

A consortium of government organizations including NASA, NORAD and even the FCC keeps track of all the planet's high-flying rubbish, and so far, its running count is flat-out scary. There are currently at least 17,000 objects measuring 4 in. or greater circling the Earth — and in some ways, that's the good news. The government estimates that there are 200,000 objects in the 1-in.-to-3-in. range and tens of millions smaller than an inch.

And if you think those tiny pieces of junk can't do much harm, think again. According to a back-of-the-envelope rule the Apollo astronauts used, given the speeds involved in traveling in low-Earth orbit, a one-tenth-in. bit of chaff would collide with an oncoming spacecraft with as much force as a bowling ball traveling 60 m.p.h.

So why is space travel survivable at all? Because all speed is relative. A satellite orbiting Earth may be moving at 17,500 m.p.h., but so is every other object in the same orbital corridor. Relative to one another, they're standing still. If one happened to speed up to 17,505 m.p.h., the most it could do is nudge another ship at 5 m.p.h. Attaining orbit is like entering an expressway: the tricky part is merging; once you're there, all you have to do is maintain your speed, and you'll be fine.

The trouble comes when speed — or worse, the angle — of orbit changes. Things in higher orbits move more slowly than things in lower ones. A dead satellite or one that has lost gyroscopic control could go tumbling down to lower and lower orbits, colliding with objects moving at different speeds along the way. Similarly, the International Space Station and its three astronauts do, in theory, lie in the path of the debris created by Tuesday's collision, and while international space officials believe the danger to the crew is low, they do not rule it out.

"At this time and in the near future, there's no threat," was all Alexander Vorobyev, the head of Russia's civilian space agency, was willing to concede.

True killer collisions occur not when spacecraft traveling in the same band or orbital plane bump each other but when there's a full-blown crash between two ships in different planes — say, between one ship in an orbit that carries it over the U.S. and Central Asia, and another in an orbit that carries it over Western Europe and Eastern Asia. That's what happened on Tuesday.

"Think about two cars colliding in an intersection," says Nicholas Johnson, head of NASA's Orbital Debris Program. "The individual speeds [of the satellites] were about 17,000 m.p.h., but the collision speed was 26,000."

It's hard to bear thinking about what would have happened if there had been astronauts aboard either ship — but space officials can't afford not to think about it. There is currently no international treaty governing space debris, though the U.S., Russia, Japan, France and the European Space Agency have rules they follow to keep the junk to a minimum. Additionally, an international committee overseen by most of the world's space agencies consults on the issue. Still, it's a problem that isn't going away.

"It was only a matter of time before something like this happened," says Matney. Worse, it is almost certainly just a matter of time before it happens again.