Just 40% of the total satellites that orbit the Earth are active, whilst the remaining 60% amount to useless metal shooting around the planet at speeds of 28,000kph, ten times faster than the speed of a bullet. However, inactive satellites are just a small fraction of the problem. Russia recently destroyed one of its own satellites to create over 1,500 pieces of trackable debris, in a similar fashion to China, India and America – the latter of which has done so twice. The net effect of this intentional obliteration has left millions more pieces of debris scattered across Earth’s low Earth orbit (LEO).
The Kessler syndrome
In 2009, the collision of Iridium communications satellite with the derelict Russian satellite Kosmos 2251 obliterated both spacecrafts into thousands of bits of debris. Experts warn that this may be the start of the Kessler syndrome, in which the overcrowding of Earth’s LEO will lead to a runway cascade of collisions that has the potential to render certain orbits unstable for human activities.
Atmospheric Entry: Heating Mechanisms
A meteor moving through the vacuum of space typically travels at speeds reaching tens of thousands of miles per hour. The von Karman line, which lies 100km above Earth’s mean sea level, defines the boundary between outer space and the atmospheric gases Earth; as a meteor crosses this line, it begins atmospheric entry.
During atmospheric entry, there are two main mechanisms that cause it to heat up: friction and compressive heating. Atmospheric drag (friction) a result of the hot gas flowing past the surface of the meteor. The latter phenomena (compressive heating) refers to the increase in temperature when the air in front of the meteor compresses rapidly. This can be understood from the ideal gas law, which describes the inverse proportionality between pressure and temperature. This causes the meteor to heat up so much that it glows and burns up until there is nothing left.
Radiative heating refers to the heating from the energetic shock layer that forms in front and to the sides of the meteor, early in atmospheric entry. Convective and radiative heating increase proportionally with velocity cubed and the eighth power of velocity, respectively. Due to its high velocity dependence, combined with its high wavelength dependence, radiative forces dominate during early atmospheric entry as the meteor travels at its fastest speeds, before frictional forces have had time to take effect.
Clearspace One Mission
Uncontrolled atmospheric entry of space debris does not occur fast enough naturally to clear up Earth’s orbit with the urgency required. To put timescales into perspective, if the Roman Empire had successfully launched a satellite in a LEO, it would only now be close to falling back to Earth. The European Space Agency has thus provided funding for the ClearSpace-One project, which aims to exploit aforementioned heating effects by forcing atmospheric entry, and therefore burning up, of space debris in order to clear up Earth’s LEO.
Remote Collision Avoidance
Simulations are run to estimate collision probability from satellite trajectories are currently used to decide whether a collision avoidance manoeuvre is necessary. However, spacecraft collision avoidance is particularly challenging at low altitudes, predominantly due to the difficulty of modelling atmospheric drag, the dominant force that impacts these satellites’ trajectories. Complex effects that arise due to coupling between the Sun and Earth’s environment lead to uncertainties in the atmospheric density (upon which atmospheric drag depends) create significant uncertainties in the satellite trajectories.
Starlink, a grid of 42,000 satellites that SpaceX are sending out into space, will circumvent human intervention. Implementation of artificial intelligence will allow the satellites to be fully automated, ‘ducking’ any incoming objects that are due to collide with them. Whilst this improves the efficiency of traditional collision avoidance methods, this is just a temporary mitigation strategy for this urgent global issue.