Space Debris
Introduction
Space debris, in other words known as orbital debris, term used by American National Aeronautics and Space Administration (NASA), represents all of the man-made satellites in geocentric earth orbit that no longer serve a useful purpose. Their sizes can vary from few micrometers to meters depending on the debris’ origin. The majority of the larger objects is regularly observed, their origin is well known, and their orbits are tracked and computed. Such orbits can be found in a public catalogue released at www.space-track.org.
Catalogued objects are mostly objects with radius greater than 10 cm, have a specific orbit and require a particular method for their observation. For example, if the debris size is around 10 cm and it is at an altitude 1000 km above the ground it can be observed by a radar. In case its orbit is at an altitude 37 000 km and higher it will not be detectable neither by radar nor by any other optical instrument. In altitudes high as this it is possible to observe objects with diameters greater than 1 meter, the population of smaller objects is more difficult to detect and hence is insufficiently mapped.
Space debris' catalogue includes satellites that are no longer functional, upper stages of launch vehicles, debris created during explosions or mechanical parts released after impacts or due to thermal stress. U.S. Space Surveillance Network is currently monitoring more than 22 300 pieces of catalogued debris which are regularly monitored and tracked. However according to the European Space Agency’s (ESA) statistical models there are more than 34 000 objects greater than 10 cm. Detailed statistics about the evolution of space debris population, total mass and surface area can be seen in the figures 1a, on the right.
Figure 1b shows examples of large compact bodies located in close vicinity to Earth. On the left is a functional US navigational satellite GPS, in the middle is the oldest, no longer functional satellite Vanguard 1 and finally on the right is Agena D, a launch vehicle used during the launch of Gemini mission in the 60s.
The population of small objects consists of space debris smaller than 10 centimeters which is difficult to track and catalogue. These can be, for example, tiny fragments of paint, peeled off thermal insulation, so called MLI (multi-layer insulation) or particles created during the burning process in solid rocket motors (SRM) which are mostly made from aluminum oxide, referred to as mission related debris. When a crash with meteoroid or other space debris occurs in orbit even more pieces of space debris are created which remain trapped in the orbit. Such pieces of material thrown out as a result of an impact are known as ejecta.
Figure 1c shows different types of small space debris. On the left is shown the aforementioned residue created during the burning process in solid rocket motors. When aluminum is added into the fuel, after the burning aluminum oxide is created as a leftover product in form of few centimeter sized particles or dust. In the middle is an impact crater on the Challenger spacecraft. Detailed analysis revealed that the spacecraft was hit by a 0.2 mm fragment of paint. On the right are tiny dipoles in a form of needle compared to a human finger which were a part of the Westford Needles project, mentioned in the following article.
On top of all that, there are debris particles with even more exotic origin, cooling substances (NaK droplets) from nuclear reactors in soviet satellites RORSAT, which were released after the separation of the core from the reactor. This separation occurred in orbit 16 times meanwhile 16 clouds of NaK droplets were discharged above the Earth.
Rising Threat
The amount of space debris in Earth’s orbit is steadily increasing, as suggested by Figure 1a. ESA’s statistical models predict that in year 2019 there were more than 900 000 objects with size ranging from 10 to 1 centimeter and about 128 million objects smaller than 1 centimeter and greater than 1 mm in Earth’s orbit. The total mass of all space objects in Earth’s orbit is estimated to be more than 8 400 tones.
Probably the biggest threat is not the varying size of these objects but the speed at which they are moving in the orbit. It can be as high as 10 km/s which is almost ten times the speed of a bullet shot from a rifle. Severe damage can be done to a functional satellite if an impact with fast-speed space debris occurs. Especially vulnerable are pressurized compartments or solar panels since even a small hole in the coating can lead to total disfunction. Space missions therefore must be designed to avoid such impacts.
Figure 1d shows an impact crater which 0,2 mm fragment of paint left on the surface of Space Shuttle Challenger’s front window. (zdroj: Real Engineering:The truth about space debris, 2019)
The international space station, for example, needs to execute one collision avoidance maneuver per year. One of the recent events occurred on September 22nd, 2020, when the crew of three astronauts had to be evacuated from ISS into Russian spaceship Sojuz. Luckily an upper stage of Japanese rocket which endangered the ISS passed within the safe distance of 1.39 km and the crew returned unharmed back to the station. However this was the third collision maneuver the ISS had to perform in year 2020, as said by NASA administrator Jim Bridenstine: „The @Space_Station has maneuverer 3 times in 2020 to avoid debris. In the last 2 weeks, there have been 3 high concern potential conjunctions. Debris is getting worse!”
Moreover, Russian station MIR in April 1997 suffered fatal consequences after a Progress cargo vessel collided with the station on docking, piercing the module that contained the American living quarters and forcing it to be permanently sealed off.
Figure 1e shows the damaged solar arrays attached to the module which caused the Mir station to power down and left it drifting in space until repairs were made. (zdroj: NASA, Micrometeoroid/Debris Photo Survey of Mir)
Surveying Space Debris
National space agencies are devoted to study space debris in order to safely plan the space missions, mainly to protect the missions with live crew on board. Hence it is essential to map the population of space debris as thoroughly as possible. Tracking and overall observations can be done by optical telescopes, such as AGO 70-cm telescope at AGO Modra observatory operated by FMPI CU, shown in figure 1f. The telescope was installed in autumn 2016 and tested in an observation campaign by the year 2018.
Figure 1f
Besides the 70-cm telescope, different types of space debris can be detected and observed by the main 60-cm telescope, which is hosted in the dome shown in Figure 1g.
Figure 1g
Bright objects in Low-Earth Orbits (LEO) can be detected even by the all sky camera systems, such as AMOS-Cam and the all sky spectral camera AMOS-Spec, originally used to observe meteors. Figure 1h shows AMOS-Cam in front and the dome hosting the AGO70 telescope at the back.
Figure 1h
