By HE Dr Mohammed Al Ahbabi, Director General of the UAE Space Agency
There are more than half a million pieces of human-made material in orbit around our planet. These materials, referred to as space or orbital debris, range in size from that of a school bus to a thumbtack. Generally space debris is made up of a mix of no longer functioning spacecraft, components of booster rockets and the remains of upper stages of launch vehicles discarded after use in addition to equipment that is lost during extravehicular activities (EVAs), such as space walks.
To keep astronauts and spacecraft safe, scientists use radar and telescopes to keep track of pieces of debris in orbit larger than 10 cm and keep a record of these objects and their orbits or trajectories. More than21 thousand such objects have been cataloged and tracked so far. This information is used to estimate the number of smaller pieces of debris which cannot be directly detected or monitored and scientists have estimated the population of particles between 1 and 10 cm in diameter to be approximately 500,000, while the number of particles smaller than 1 cm could exceed 100 million.
The higher the altitude, the longer the orbital debris will typically remain in Earth orbit. Debris left in orbit below 600 km normally fall back to Earth within several years. At altitudes of 800 km, the time for orbital decay is often measured in decades. Above 1,000 km, orbital debris will normally continue circling the Earth for a century or more.
In low Earth orbit (below 2,000 km), orbital debris circle the Earth at speeds of 7 km/s. However, the average impact speed of orbital debris with another space object may exceed 10 km/s. Consequently, collisions with even a small piece of debris may cause catastrophic damage. Fortunately the probability of two large objects, greater than 10 cm in diameter, accidentally colliding is very low, but can happen as was the case when a Russian satellite accidently collided with a US satellite in 2009.
So where is the threat? While movies like Gravity are currently considered an exaggeration of the threat posed by space debris, the risk is real and growing. Launch vehicles have to have their paths carefully calculated to avoid colliding with, among other things, the thousands of derelict satellites orbiting earth. Satellites have to be equipped with debris shields such as the Whipple shield, a type of hypervelocity impact shield used to protect manned and unmanned spacecraft from collisions with micrometeoroids and orbital debris within a range of velocities (3-18 kilometers per second). The real threat, however, stems from the ever increasing number of objects being launched into space and the probability that this crowding will lead to collisional cascading, more specifically the Kessler syndrome (also called the Kessler effect).
The Kessler syndrome was proposed by the NASA scientist Donald J. Kessler in 1978, it involves a scenario in which the density of objects in low Earth orbit is high enough that collisions between objects could cause a cascade, with each collision generating space debris that increases the likelihood of further collisions. It is a bit of a stretch, but theoretically the distribution and density of debris in orbit could reach a point where space activities and the use of satellites in specific orbital ranges would become unfeasible for generations.
That being said, a lot is being done to ensure that the Kessler syndrome remains nothing more than a theory. Through prevention, mitigation, and remediation, nations and space programs across the planet are working to slow and eventually reverse the current trend of ever increasing quantities of space debris (more than a 100 new satellites are launched into space every year).
Prevention entails ensuring that all future launch vehicles and satellites are equipped with passive or active devices that allow them to be disposed of at the end of their life cycles. This typically means safely directing them into Earth’s atmosphere where they burn up. While the technology is still being developed, ideas include devices such as tethers, balloons, and solar sails.
Mitigation involves the tracking of all current space debris to try and limit the number of future collisions as well as venting pressure vessels and fuel tanks and discharging batteries to prevent explosions in space. Tracking is already an integral part of many space programs these days, and venting and discharging of batteries has begun as it is relatively easy to implement.
Finally, remediation, by far the most complex solution to the ever growing amount of space debris. Remediation generally refers to the removal and elimination of the garbage that is already in space. The technology and methods by which debris can be removed from orbit are currently in development, but organizations such as the European Space Agency (ESA) have already launched programs to tackle the challenge. The ESA’s Clean Space initiative is studying an active debris removal mission called e.Deorbit, which would target an ESA-owned derelict satellite in low orbit, capture it, then safely burn it up in a controlled atmospheric reentry. e.Deorbit will be the world’s first active debris removal mission.
e.Deorbit aside, for the time being we will have to rely on the United Nation’s active set of non-binding guidelines on space debris mitigation. These guidelines are based upon common principals such as preventing on-orbit break-ups, limiting the objects released during normal operations, and removing spacecraft and orbital stages that have reached the end of their mission operations from the useful densely populated orbit regions.
The UAE Space Agency is currently considering legislation on orbital debris mitigation, mimicking the previously mentioned non-binding guidelines promulgated by the U.N. Outside of the UAE, policies on space debris vary amongst the many organizations and governments involved in the space industry. NASA, Russia, China, Japan, France, and the European Space Agency have all issued orbital debris mitigation guidelines. However, private industry is often left to regulate itself. Many firms do voluntarily adhere to measures designed to limit the growth of orbital debris.
In summary, we still have the ability to clean up the debris that is floating around space. Through the implementation of new prevention, mitigation, and remediation technology we will be able to ensure that our grandchildren will continue to benefit from the vast array of services and information provided to us by low earth orbiting satellites.