What Goes Up Doesn't Always Come Down
In almost 50 years of spaceflight history, a lot of objects have been launched into space. Many of those have fallen back to Earth, either to land or to burn up in the atmosphere. A few of them have been launched beyond Earth's gravity, to travel to other worlds or to explore space. But, many of the objects that have been sent into space are still in orbit, endlessly circling the Earth.
Image to right: There are millions of pieces of orbital debris circling the Earth. Credit: NASA
Orbital debris is the term for any manmade "junk" circling the Earth. On one extreme, debris can be as small as tiny flecks of paint that have come off spacecraft. On the other, large debris could be the upper stages of launch vehicles or satellites that are no longer working. The most common source of orbital debris larger than 1 centimeter [cm] (0.39 inches) is the explosion of objects orbiting the Earth. These are often rocket upper stages containing fuel or high pressure fluids. There are about 11,000 known objects of a size greater than 10 centimeters (3.9 inches). Scientists believe that there are more than 100,000 pieces of orbital debris between 1 cm and 10 cm in diameter, and tens of millions smaller than 1 cm. The U.S. Space Surveillance Network tracks pieces of debris larger than 10 cm. Ground-based radar can detect objects as small as 3 millimeters [mm] (0.12 inches) in diameter. That information is used to estimate the number of small pieces of debris. Though the system cannot detect every piece of debris, it can provide a sampling from which the total amount can be extrapolated. Estimates of the amount of orbital debris smaller than 1 mm (0.04 inches) are based on damage caused to orbiting spacecraft by debris impacts. When the Space Shuttle returns from a mission, scientists count the number of impacts it suffered. By comparing that to the volume of space the Space Shuttle traveled through, they can estimate how many of the tiny objects are in orbit around the Earth.
Image to left: This shield protects spacecraft from debris. Credit: NASA
In low Earth orbit, most orbital debris is traveling at speeds of 7 to 8 kilometers per second [km/s] (4.3 to 5 miles per second). When orbital debris strikes a spacecraft, however, the impact speed is even greater than that, since both objects are moving. The average impact speed of a piece of orbital debris colliding with another object is 10 km/s (6.2 miles per second), or 36,000 km per hour [km/h] (22,370 miles per hour [mph]). Because of the great speeds involved, a piece of orbital debris does not need a lot of mass to have substantial force at impact. To generate the same force as a 1,500 kilogram [kg] (3,300 [lb]) car traveling at 97 km/h (60 mph), a piece of debris would only have to be 4 kg (8.8 lb) in mass.
Thanks to the information provided by the U.S. Space Surveillance Network, crewed spacecraft are able to dodge larger pieces of orbital debris. When an object is expected to come within a few miles of the Space Shuttle, and there is greater than a 1 in 10,000 chance of collision, the Space Shuttle changes its path to avoid the object. During normal flight operations of the Space Shuttle, this happens about once every year or two. The International Space Station (ISS) can also maneuver away from debris in its path. In addition, the ISS is also the most heavily shielded spacecraft ever, able to withstand impact with smaller pieces of debris. Since the smallest particles of debris cannot be tracked, occasional collisions with them are inevitable. The Space Shuttle frequently returns to Earth with tiny impact craters from being hit by orbital debris. Astronauts have reported seeing very small cracks formed in the Space Shuttles' front windows when they strike objects. While impact by small pieces of orbital debris is routine, the odds of two pieces of debris larger than 10 cm in diameter colliding is very low. In all of spaceflight history, there is only one recorded incident of two such objects from different missions accidentally colliding.
Image to right: International Space Station is the most heavily shielded spacecraft ever and is able to withstand impact from smaller pieces of debris. Credit: NASA
But, as more and more satellites and spacecraft are launched, will Earth orbit eventually turn into a dangerous, crowded junkyard? Space agencies around the world are working to make sure that does not happen. Since 1988, the United States has had an official policy to minimize the creation of new orbital debris. NASA has an Orbital Debris Program Office at Johnson Space Center that performs research related to orbital debris. Many U.S. aerospace firms also voluntarily follow guidelines to reduce the creation of debris. The Russian, Japanese, French and European space agencies also have debris mitigation policies. There is also a great deal of international cooperation in this area. Both the United Nations and the International Space Debris Coordination Committee have been studying orbital debris and making recommendations for its curtailment for more than 10 years.
Several things are being done to reduce the problem of orbital debris. Launch vehicle upper stages and some satellites are being placed in lower orbits so that they will re-enter the atmosphere and disintegrate soon. (Debris in orbits below 600 km [373 miles] usually falls back to Earth within a few years, while objects at altitudes of more than 1,000 km [621 miles] can remain in orbit for more than a century.) Many operators of satellites in geostationary orbit boost old spacecraft into higher orbits at the end of their missions to leave room in geostationary orbit for new satellites. Engineers are also researching new ways to combat the problem of orbital debris. For example, tethers are being researched as a way to cause satellites to drop closer to the Earth until they burn up.
For years, humans have understood the importance of protecting our environment on Earth. But, as we explore beyond our home planet, we are learning the importance of protecting the environment off the Earth as well.
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Adapted from NASAexplores