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.
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.
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.
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.
Space distribution of artificial bodies around the Earth is directly proportional to the distribution of satellites. Geocentric orbits of artificial bodies can be divided into categories according to the shapes of the orbits.
Low Earth Orbit (LEO) is an orbit where the period of one revolution of an object is shorter than 2,2 hours (mean altitude above the surface is under 2000 km). These types of orbits are the most beneficial economically and from the point of elimination of dysfunctional satellites since they will eventually burn in the Earth’s atmosphere. Eccentricities of these orbits are very small, under 0,1. Only about 1% of the bodies on LEO orbits has an eccentricity above this value.
On Medium Earth Orbits (MEO) are bodies with the period of revolution from 2,2 to 24 hours (mean altitude above the surface from 2000 km up to 35 700 km). Eccentricities of these orbits have different values. There are Semi-synchronous Earth orbits (SEO) which are circular and are occupied mostly by navigation satellites. Their period of revolution is around 12 hours. Satellites on these orbits are in agglomerations (American GPS, Russian GLONASS, future European GALILEO and Chinese BEIDOU), where a couple of dozens of satellites have similar orbits which are mutually spatially rotated. MEO types of orbits also consist of orbits with high eccentricity. These can be for example occupied by Russian telecommunication satellites Molnija, which have eccentricity of about 0,7, a period of revolution 12 hours and inclination towards the equatorial plane around 6 degrees. High eccentricities are typical for Geosynchronous Earth Orbits (GEO). Satellites on these orbits are through their entire revolution around the Earth located above the same geodetic point. They are circular orbits with the period of revolution one stellar day (23 hours, 56 minutes, and 4,1 seconds). Their inclination varies between 0° and 15°. Special types of these orbits are geostationary orbits with the inclination to the equator 0°.
High Earth Orbits (HEO) have one period of revolution 1 stellar day and their eccentricities are above 0,2. These orbits are except satellites occupied by a great number of rocket bodies. Transfer between LEO, MEO and HEO types of orbits is continuous.
On top of all of these types of orbits there exists special Super HEO type of orbit, which is occupied by satellites with perigee above the altitudes of GEO. Objects on these orbits are usually not included in accessible catalogues. They are mostly rocket bodies which were used to escape Earth’s gravity in Moon-landing missions with live crew on board or scientific missions designed to study the Sun, the planets, or other bodies in our solar system. Examples of types of orbits are in the Figure 3. The aforementioned division of orbits is set by the author of this article and designed for easier orientation.
Figure 3 – Shows the types of orbits by the author. On the left view from the north pole, on the right view from the equator. Spring equinox pointing towards the right. Displayed is the low orbit LEO of Iridum satellite, mean orbit MEO of satellite Glonass, mean orbit GTO of a rocket body Centaur and geo-synchronous orbit GEO of satellite Astra.
According to the Space Environment Statistics led by ESA, by the date 14th of October 2020 are majority of catalogued satellites, up to 55%, located at the LEO type of orbit. Catalogued bodies are satellites greater than 10 cm in size and include mostly objects which were created in disintegration, leftover debris from space missions, satellites, and rocket bodies. Low Earth Orbit is very well mapped thanks to radar observations.
On MEO type of orbits are approximately 19% of all catalogued objects. Out of these only 1,35% are located on SEO, where no debris’ fragmentation was observed (Johnson et al., 2008). Moreover 10% of overall catalogued population consists of LEO – MEO crossing orbits. In the past were the least known populations in GTO, GEO, HEO and Super HEO orbits. Nowadays they compose more than 16% of all catalogued satellites. Out of these, the majority is on the MEO – HEO crossing orbits. GTO type of orbit is over one quarter of the MEO population. These are orbits with eccentricities between 0,6 and 0,75, and inclinations 0° - 30° (Musci et al., 2005). Majority of bodies on GTO orbits are either rocket bodies, debris from fragmentation, or debris from space missions. The most frequently used orbits, whether for commercial, amateur, civil or defence purposes are LEO and GEO. Statistics of European Space Agency – Figure 3a & 3b - show the increase mostly in the number of commercial missions in LEO as well as in GEO orbits.
Figure 3a – The number of space missions of different type in different time periods, LEO orbit. Source: ESA
Figure 3b - The number of space missions of different type in different time periods, GEO orbit. Source: ESA
With the increasing traffic in these regions is also increasing the risk of collisions and the demand for better catalogues. However, the low magnitude of these objects does not allow observations of bodies smaller than 0,5 to 1 meter. Therefore, the real population is suspected to be much bigger, this supports the observational campaign of Schildknecht et al. (2006). GEO region is after LEO region second most used. Satellites in GEO orbits compose roughly 12% of all objects in public catalogues, these are mostly satellites and rocket bodies. The population of GEO region is until this date not very well known due to the low apparent magnitudes of objects residing in these orbits. Up until November 2020, only two fragmentation events were recorded here (Johnson et al., 2008), although many observational campaigns suggest that more fragmentation events occurred in these regions (Schildknecht et al., 2004 & 2005). According to statistical models of Oltroge et al., 2017 the collision probability in GEO orbit is four times as higher as previously predicted.
On HEO orbits are located mostly satellites, such as American scientific satellite Chandra X-ray Observatory, which period of revolution around the Earth is 2,6 days. Satellites with the highest altitudes are American satellite Interstellar Boundary EXplorer with altitude 268 679 km and Russian satellite SPEKTR-R with period of revolution 8,3 days and the altitude at the apogee 330 000 km. Objects on these types of orbits compose more than 4% of all of the catalogued satellites. In figures 4 and 5 are shown distributions of geocentric elements of catalogued bodies. In the animations in figures 6, 7 and 8 are shown spatial distributions of satellites, rocket bodies and the remaining debris around the Earth.
Figure 4 – Distribution of average altitudes of LEO, MEO and GEO populations, by March/April 2009.
Figure 5 – Distribution of the inclination as a function of the ascending node. Different populations can be distinguished in this representation, with data generated by March/April 2009.
Figure 6 – Animation showing the spatial distribution of functional and dysfunctional satellites around the Earth.
Figure 7 – Animation showing the spatial distribution of rocket bodies around the Earth.
Figure 8: Animation showing the distribution of space debris, except dysfunctional satellites and rocket bodies around the Earth.