Humanity has been shooting things into space for a few decades now, and we've gotten pretty good at it.

What we haven't gotten so good at is bringing things back down.

Scientists have been sounding the alarm about the buildup of space junk for years, a point that was reinforced at the recent European Conference on Space Debris.

The message was clear: we need to stop talking about doing something and actually do it before space gets too crowded.

We've launched some 7,000 spacecraft as a species since 1957 when Sputnik kicked off the space race. however, there are considerably more than 7,000 objects to worry about up there.

Earth-based radar stations track more than 18,000 objects in orbit, and only 7 percent of them are operational satellites.

The rest are derelict or just pieces of debris from old launches.

The European Space Agency (ESA) started speaking up about the threat posed by space debris after one of its satellites was hit. The Sentinel-1A radar imaging satellite was struck on one of its solar panels last August.

Luckily, the satellite survived the impact, but it could have been much worse if the object struck elsewhere on the spacecraft.

The impactor wasn't even very big. In order to remain in orbit, this junk has to be traveling at a high rate of speed. That means the impacts can be catastrophic even for a small object.

According to Holger Krag, Head of the ESA Space Debris Office, there are around 5,000 objects larger than 1 meter in orbit, 20,000 larger than 10cm, and 750,000 larger than 1cm.

Any of those could seriously damage or destroy a spacecraft.

There are also around 150 million objects up there between 1mm and 1cm in size, including the one that blasted a 40cm hole in Sentinel-1A's solar panel. This isn't only a danger to expensive satellites, but also to the crew of the International Space Station.

Four times in its history the crew has retreated to the docked Soyuz lifeboat due to a potential impact.

Damage to Sentinel-1A

from collision with a 1mm object.

The problem with space debris is only getting worse, and it comes at a time when private space firms are preparing for their own manned space missions.

Krag showed data at the conference that indicates we may be headed for the "Kessler syndrome," an exponential increase in space junk. It's like a chain reaction where one object produces more debris, which then goes on to hit other objects and produce still more debris.

The ESA is already committed to increased tracking of orbital debris and the development of automatic collision avoidance systems.

The design of spacecraft may also be changed to avoid adding to the problem. Getting the debris that's already there out of orbit is a much more difficult task.

A recent ESA mission called e.deorbit was cancelled when it failed to get enough support in the EU.

Japan is working on a technology that could drag objects out of orbit using an electrodynamic tether, but that's still highly experimental.