Tuesday's collision marked the first time two intact satellites smashed together hundreds of miles overhead. It probably won't be the last we hear of high-altitude mismatches, but it will most likely remain a very rare accident. Though NASA predicts that we need to take care not to crowd our orbit, there is a lot of space out there.

The collision took place smack in the middle of low Earth orbit (LEO), usually defined as the 1200-mile tall swath of space that surrounds our planet. Nearly half of the roughly 900 operational satellites in orbit right now pass through low Earth orbit. Low Earth orbit is a big big place. The entirety of LEO has a volume of about 3.15782373 × 1011 cubic miles, however, the shell provides ample room for everyone to roam freely—about 350 million cubic miles per satellite. That's about four times the volume of the Atlantic Ocean.

On the other hand, the neighborhood where the defunct Russian satellite collided with the Iridium communications satellite is crowded—with man-made spacecraft, and a steady amount of tens of thousands of larger space debris continues to float. Most of the satellites are concentrated in orbits that take them near or over the north and south poles—which means the danger zone looks more like a donut than a tennis ball. But for the roughly 900 operational satellites, that's more than enough room; they are but a few fish in a giant sea.

It's usually a lonely world for satellites, since it's not hard to keep track of them and make sure they don't hit each other. But operational satellites aren't the only inhabitants of that swath. There are also the remnants of old, broken-down satellites: the rocket bodies, the engines and disabled satellites that haven't fallen apart yet. These difficult-to-track pieces are the main concern for NASA. The NASA Orbital Debris Program keeps an eye on these fragments, and of the more than 18,000 pieces of debris being tracked, about 10,000 are in the low Earth orbit. These are the wild cards: they don't tell us where they are, we can't control them without an outside force, and they're racing blindly through space at 20 times the speed of sound on Earth.

Wednesday's collision unleashed about 600 pieces of debris in LEO, which could in turn collide with other satellites or break apart other pieces of space junk. And that's just a preview of the storm that's brewing in near space: In 2006, NASA scientists warned that a cascade is coming. They estimated that the amount of space junk in LEO would remain constant for another 50 years or so, and then would increase as the debris from collisions will then cause other collisions.

While two large satellites crashing—as we saw today—will remain an exceedingly rare event, large satellites malfunctioning because of collisions with tiny, high-speed debris is likely to be more common. —Stephen Ornes