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Everything seemed normal in space. Astronauts were hard at work on the International Space Station. Suddenly, alarms sounded. Mission Control ordered them to abandon the station. They rushed to the escape pod, where they waited for further orders. What was the emergency?

A small piece of space junk was speeding toward the space station at 28,000 kilometers (17,500 miles) per hour. Mission Control feared it would slam into the station. Ten minutes later, Mission Control sent an all-clear order. Phew! The station was safe.

This close call happened on March 12, 2009. It wasn't the first time space junk threatened astronauts' lives. And it won't be the last. I know. I've been interested in trash since I was 12 years old. I don't mean the trash that's lying around your bedroom that makes your parents mad. I mean the trash that's littering the space around our planet.

We put a lot of trash in space. Each year, we add more. It's sort of like space trash is out of sight, so it is out of mind. Yet space junk has caused a lot of problems.

The first artificial satellite was launched in October 1957. Since then we have launched thousands of satellites. I used one of them to hunt for black holes. Many are still in orbit. Only one in five still work. The rest are junk. Satellites come in many shapes and sizes. Some are as large as a school bus. Despite their size, most aren't that dangerous. They are large enough to track. We can map where they are.

Burned-out satellites are just one kind of space junk. Rockets streak through space to place satellites there. Usually one section of a rocket ends up staying in space, too.

These once valuable rockets now are part of a growing band of space waste. Many still contain rocket fuel that can explode. If one blows up, it can splinter into hundreds of small pieces. Each one of those small pieces adds to the amount of space junk.

Some space junk already has been smashed into smaller pieces. The United States and the former Soviet Union used to test weapons by destroying old satellites. These target satellites shattered into hundreds of small pieces.

China did the same thing a few years ago. One of its missiles crashed into a large weather satellite. The missile and satellite shattered into many pieces, becoming the single worst case of space littering—ever. So far, scientists have counted more than 2,800 pieces.

Space junk even can make more space junk. Just last year, a broken Russian satellite smashed into a working U.S. satellite. What happened? Both satellites exploded into a lot more space junk.

The smash-up littered space with more than 1,600 large pieces and countless smaller ones. Each of these pieces threatens the astronauts who bravely work in space. Each piece could also destroy working satellites.

With each new space mission, it seems like more and more small pieces of space junk pile up. Some of this trash winds up there accidentally. Others are put there on purpose.

Missing tools, lost screws, dropped gloves, and chipped paint all can become space junk. So can regular trash. Russian cosmonauts on a space station threw away trash by tossing it into space. As a result, about 300,000 pieces of space junk larger than one centimeter (about half an inch) now litter space.

All this trash can cause problems. Objects orbiting Earth the same distance as the International Space Station whip through space at 7.7 kilometers (almost five miles) per second. At that speed, an object the size of a nickel packs the same wallop as a car going 80 kilometers (50 miles) per hour.

This can cause lots of damage. Space trash has cracked windows. It has chipped heat shields. It has ripped holes in solar panels. Nothing orbiting Earth is safe from this debris. Space junk also threatens Earth. On average, one piece of junk falls back to Earth each day. Friction with particles in Earth's atmosphere causes most of this stuff to burn up in the air. Big pieces slam into the ground or splash into the oceans. Luckily, only one person has been hit by falling space junk. She was not hurt.

To protect astronauts and valuable satellites, scientists track space junk. By using radar and telescopes, they can track pieces of trash larger than a grapefruit. This allows scientists to keep tabs on 19,000 pieces of junk. This is only the largest space waste, though. Millions of pieces are much smaller. These small scraps can do big damage. Yet there is no way to know where they are.

What can be done about space trash? A good first step is to make less of it. Scientists are designing tools that are harder to lose. For example, it can be easy for an astronaut to drop a camera lens cap. It's harder to lose the cap if it is tied to a camera.

A rocket can use its leftover fuel. That keeps the rocket from blowing up and making even more trash. Special shields can protect astronauts and spacecraft, too. Astronauts wear spacesuits with a layer of bullet-proof material. This protects them from small pieces of trash, which could slice through their suits.

Making less trash and protecting astronauts are only the first steps. We know that the junk already in space is very dangerous. Currently, about 13,000 close encounters take place between working spacecraft and space junk each week. Worse yet, the junk already in space is making more junk. So something needs to be done.

Scientists are looking for ways to clean up space waste. One idea is to shoot lasers at litter. The lasers would move the litter away from working satellites. The only problem is that Earth's gravity would pull the litter back. So the litter would become a problem again.

New spacecraft also can act like garbage trucks. Nets on these high-flying garbage trucks could catch litter and dump it into Earth's atmosphere. The litter would burn up before hitting the ground. Some of the burning trash might light up the sky.

One thing is clear: Scientists must work together to find solutions. No one country is responsible for cleaning up space. Many countries tossed junk into space. Now we have to clean it up so space is safe!

Article by Jonathan McDowell, Astrophysicist, Harvard-Smithsonian Center for Astrophysics. Top-of-page image by European Space Agency. "Space Junk" appears in the May 2010 issue.