For more than 50 years, humans have been breaking the tether of Earth's gravity and launching objects into space, but remnants of these stellar explorations are left circling the planet, posing a risk to future missions.

At the moment, NASA has tracked more than 500,000 pieces of orbital debris, or space junk, trapped by gravity and traveling at speeds up to 17,500 miles per hour.

"NASA uses the term 'orbital debris' to describe human-made debris in Earth orbit and uses the term 'space debris' for orbital debris and micrometeoroids, which come from asteroids and comets," Chief of NASAs Orbital Debris Program J.C. Liou said.

Orbital debris can consist of anything such as flecks of paint or bits of metal from spacecraft to larger debris like entire defunct satellites. Each day at least one piece of debris plummets back to Earth, according to NASA.

Astronauts aboard the International Space Station took this image of southern Scandinavia just before midnight under a full moon. (Photo/NASA)

"In general, orbital debris dominates the near-Earth environment below about 4,000 km altitude," Liou added.

Most debris burns up during atmospheric reentries. However, some satellite components have survived the reentry process and landed on the ground, Liou said.

Some examples include a propellant tank of a Delta 2 launch vehicle, which landed near Georgetown, Texas, in January 1997.

"Fortunately, no injuries or property damage due to debris reentries have been reported," he said.

In space, however, some of the smallest pieces of debris pose the most significant risks to both crew and spacecraft, Liou said.

"According to a recent NASA independent study, orbital debris in the millimeter-sized regime represents the highest penetration risk to most uncrewed spacecraft," Liou said.

The International Space Station is much better protected against orbital debris than uncrewed spacecraft, he added, stating that the space station's modules are equipped with debris impact protection shields. These modules are protected against orbital debris about 1 cm and smaller.

"However, when astronauts are conducting a spacewalk outside the ISS, orbital debris as small as 0.3 mm can penetrate certain portions of the suits the astronauts are wearing," Liou added.

Orbital space debris depicted in low Earth orbit. (Image/NASA)

In order to mitigate the risk that orbital debris poses to both spacecraft and crew, NASA conducts observations and has guidelines in place to maneuver around it.

"For the space situational awareness and orbital debris community, 'tracking' means using sensors to detect objects, conduct routine follow-up observations and then use the data to develop and update objects orbits on a regular basis," Liou said.

The orbits of these tracked objects can then be used for assessments and potential collision avoidance maneuver operations.

The U.S. Joint Space Operations Center (JSpOC), uses its global sensor network to track objects about 10 cm and larger in low Earth orbit, the region below 2,000 km altitude.

For objects ranging 1 meter and larger in geosynchronous orbit, or the region around a 36,000 km altitude, NASA uses different ground-based radars, telescopes and space-based data to make observations.

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While 500,000 pieces of orbital debris is the estimated quantity of objects that are 1 cm or larger, other smaller pieces may be too tiny to be tracked by JSpOC. Even though these pieces are smaller, they are still large enough to pose a threat to human spacecraft and robotic missions, according to Liou.

"For orbital debris 1 mm and larger, the population is on the order of 100,000,000," Liou said, citing debris larger than 0.3 mm is still a major safety concern.

In April, Liou gave a lecture during the European Conference on Space Debris to address some of the latest advancements and goals for the future in detecting and mitigating the risk posed by orbital debris.

The agency intends on advancing its research by conducting statistical surveys in both low Earth orbit and in geosynchronous orbit. However, there is currently no on-site data for particles below 0.3 mm at altitudes beyond 600 km. At the moment, NASA operates nearly 20 missions between 600 km and 1,000 km altitudes.

According to Liou, one of the main goals is to reach full autonomous operations for routine geosynchronous orbit debris surveys by 2019.

This will expand the measurement coverage and improve modeling capabilities in order to provide better risk assessments for future missions.

Liou said cooperation with the U.S. and international communities can improve mitigation efforts from global orbital debris.

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500000 pieces of space junk whirl around Earth: How this fast-moving debris poses risks to spacecraft, crew - AccuWeather.com