The cosmos, as the name suggests, is mostly empty. However, after the first launch of the satellite in 1957, humanity began to inhabit Earth’s orbit with all sorts of spaceships. On the downside, space is also becoming increasingly cluttered with non-existent or shattered rocket scenes and satellites. Moving at a speed of nearly 30,000 km / h, even the smallest object can drill a hole in your spacecraft. Therefore, space debris poses a real threat to both manned and unmanned spacecraft, and space agencies are therefore stepping up their efforts to track, avoid and dispose of it.
The Earth’s orbit becomes crowded
According to the Office of the NASA Orbital Debris Program (ODPO), the Earth’s orbit currently hosts approximately 500,000 marble debris and a huge 100,000,000 objects with 1 mm or less. As shown in the picture, there are two main fields for debris. While most of the debris is located in low-Earth orbit (LEO) at altitudes <2000 km, in geosynchronous orbit (GEO) there is also a ring of space debris at an altitude of ~ 36,000 km.
Space debris includes abandoned spacecraft such as the Vanguard I satellite, which has been in orbit for more than 60 years and thus holds the record for the oldest man-made object in space. Other culprits are the upper stages of the rockets, which have broken or exploded, which is why nowadays they are usually “passivated” by venting their unburned fuel.
In 2007, China won much criticism for blowing up their Fengyun-1C meteorological satellite as part of a missile test. Together with the accidental collision of the US communications satellite Iridium-33 and the non-existent Russian satellite Kosmos 2251 in 2009, these events are responsible for much of the large debris currently in orbit.
To protect against micrometeorites and orbital debris (MMOD), spacecraft use so-called Whipple shields, consisting of several thin layers that are spaced apart. Upon impact, the outermost layer breaks the projectile, thus spreading its kinetic energy over a large area as it passes. To avoid collisions with known larger objects, spacecraft sometimes have to perform evasive maneuvers.
For the ISS, such a maneuver is ordered if the chance of impact is greater than 1 / 10,000, which occurs on average once a year. In 2012, a record number of four such moves had to be made, which are always expensive due to the large amount of fuel that needs to be consumed. NASA space shuttles have often been drilled by the MMOD, but fortunately all catastrophic collisions so far have been limited to unmanned spacecraft. One example is that of the French satellite Cerise, which was hit in 1996 by part of the Ariane rocket accelerator. And we have already mentioned the Iridum-Cosmos catastrophe.
Track all junk
It is vital to catalog and track all debris floating in orbit to prevent future crashes and prevent further contributions to the space debris problem. The most comprehensive catalog of space is garbage held by the American Space Observation Network (SSN). They currently track more than 22,000 man-made objects orbiting the Earth that are 10 centimeters or larger.
Depending on their altitude, objects of sufficient size can be detected by ground-based radars and optical telescopes. Optical telescopes measure sunlight reflected from debris, while the distance can be accurately determined by laser distance. The method is based on measuring the reversal time of a short laser pulse fired from the ground and reflected from the object. The technique has long been used to track satellites that are equipped with a reflector. Because the diffuse reflection from space debris is much weaker, measurement is significantly more difficult. Until now, the technique could only be used at dusk, when the laser station on Earth is in darkness, but the debris is still illuminated by the sun. With improved imaging techniques, Austrian scientists have recently been able to use space debris located during the day, doubling the viewing window.
NASA’s Long Exposure Device (LDEF) has taught us a lot about space debris. In essence, this was a target that was left in space for about six years before being recovered from the Columbia space shuttle in 1990. The LDEF hosted 57 individual scientific experiments designed to study the long-term effects of the space environment on various electronics materials. and biological samples. Due to its large surface area and long exposure, a lot of statistical information has been obtained from the study of the Swiss cheese model that formed on its surface, as shown in the photo.
Cleaning the orbit
Since 2002, all major space agencies have followed some general guidelines for reducing the growth of space debris. GEO spacecraft must move to the cemetery orbit at higher altitudes after completing their mission. Objects passing through the LEO region must be deorbed or at least placed in a reduced-life orbit.
Due to the atmospheric resistance, all orbital debris will eventually fell back to Earth. At altitudes of 800 km, however, this can take decades, while more than 1,000 km of orbital debris will usually continue to orbit the Earth for a century or more. We add space junk faster than it rains. Therefore, in the long run, we must not only stop the continuing pollution of space, but also actively get rid of some of the space debris that is already in orbit. Otherwise, the density of the debris may become large enough to create a cascading effect in which the fragments created in a collision cause new collisions. This scenario is known as Kessler’s syndrome and is well explained by Donald Kessler himself in this video.
Plans to actively remove space debris include the ESA-funded mission of ClearSpace-1 from the Swiss startup ClearSpace, which is scheduled to launch in 2025. ClearSpace-1 will use robotic weapons to capture part of the Vespa (Vega Secondary Payload Adapter) upper stage to the left in orbit from a previous ESA mission. Both spacecraft will then be neutralized to burn up in the atmosphere. Ultimately, the goal is to have a “tow truck” in space that can capture and remove multiple objects on a single mission.
The same lasers that are used to track space debris can also be used to remove it. There are several concepts for using powerful ground-based or space-based laser systems to remove debris 1 – 10 cm in size from LEO space. The laser evaporates the material from the object, which forms a jet that slows down the target so that it re-enters the atmosphere faster.
Now that we have polluted the Earth to a devastating degree, it would be good not to see the same thing happen in space. It would be unfortunate if scientific progress and communication technologies allowed by space missions were stopped by Kessler’s syndrome. With future new mega-constellations of satellites such as the Starlink project, this risk is quite immediate.