The space around Earth is littered with upper stages of launch vehicles, as well as derelict satellites. There are over 500,000 pieces of space debris, moving at speeds in excess of 28,000 kilometers per hour.
At the end of its operational period, the energy required to increase the altitude of a satellite is less than the energy required to deorbit the satellite. Because of this, defunct satellites are moved into what is known as the junk orbit or the graveyard orbit, to reduce the possibility of collisions and creating more space debris.
A Chinese anti satellite missile test in 2007 was responsible for creating the largest amount of space debris in a single event. Over 3,000 pieces of debris were tracked, with each piece being gold ball sized or larger. The debris created by the test passed six kilometers away from the international space station in 2011. In 2009, a commercial satellite built in the US collided with a Russian military satellite, creating over 2,000 pieces of trackable debris pieces.
With each collision, the chances of further collision increases. A chain reaction of hypervelocity collisions, as seen in the movie Gravity, is known as the Kessler effect. It is critical for future spaceflight operations that efforts are taken to reduce orbital debris. The deorbit approaches include active and passive propulsion systems on board the spacecraft, external removal by other satellites, and ground based solutions as well.
Nets and Harpoons
This might sound like a fishing expedition, and is an active debris removal approach. Tiny CubeSats deploy a net through a harpoon, to capture the debris. The satellite subsequently deorbits along with the catch. The ESA has plans to demonstrate such a mission with a satellite called e.Deorbit, which is scheduled for a 2023 launch. The “space tug” has technologies that are similar to an autonomous car, as it will be difficult to manually control the satellite, when it manoeuvres to capture the debris, from ground based stations. The guidance and navigation systems will take inputs from a combination of optical and multispectral cameras, as well as lidar. The agency is looking into using a net or harpoon to capture the target.
The Surrey Space Centre and Airbus are part of a consortium that plans to demonstrate deorbital with the RemoveDEBRIS mission later this year. The satellite will be launched to the ISS on board a SpaceX rocket. There will be two CubeSats which will act as targets for the demonstration. A number of satellite capture and deorbital strategies will be tested by the mission. The tests include using a harpoon to fire at a target, using a net to capture a target, and using a dragsail to accelerate the deorbital. Cameras on board the satellite will be used to check the effectiveness of the strategies.
There are two types of sails, active sails and passive sails. Solar sails use the energy from the sun to steer the satellite towards the ground. A dragsail, is a passive propulsion system that uses the particles in the upper reaches of the atmosphere, to make the satellite fall back to the Earth faster. Both sails can be incorporated into the satellites themselves. The technologies for deploying the sails are susceptible to jamming, and there is an altitude limit to the dragsail.
A dragsail uses the resistance of the thin air in the upper atmosphere to speed up the rate at which a satellite deorbits. Using a dragsail to deorbit a satellite is still a relatively slow process. It could take anywhere between a few months to 25 years or more to deorbit. That is still faster than the decades for which the junk satellite would have otherwise remained in orbit. Satellites have already demonstrated the viability of dragsails to deorbit a satellite. The sails are typically folded up and packed into the satellite. There are designs that fit into a CubeSat.
One of the ground based plans by NASA to hasten the reentry of derelict satellites, is to reduce their velocity with ground based lasers. The laser broom would slow down the debris by 1 mm per second, and would require firing over multiple passes. Recently, researchers from the Chinese Air Force Engineering University conducted a study, and found that it was viable to use high energy lasers from a space station to destroy orbital debris, and break it down into smaller, less harmful pieces.
The Indian Army plans to build ground based laser blasters that can take down satellites in space as well, but this is for building up the defence capabilities, and not for the specific purpose of cleaning up space junk. The 2018 Technology Perspective and Capability Roadmap lists a requirement for such a laser weapon system.
An active propulsion system can be incorporated into the satellite, specifically for actively deorbiting the satellite. This approach is more viable with the smaller nanosatellites, as well as advances in miniaturisation and fuel technologies. Using propulsion systems for deorbiting is faster than using passive approaches such as a dragsail. However, the cost is higher, and the satellite may not be able to accommodate the fuel required. A company known as D-Orbit has developed a propulsive decommissioning device known as D3, with variants available for various orbits. The device is modular and small enough to fit on a CubeSat.
Similar to using a net or harpoon, a chaser satellite targets a junk satellite, captures it using the tether, and then deorbits itself and the target. The Kounotori 6 satellite launched by JAXA last year was an effort to test a deorbital strategy that allows a satellite to capture debris with an electrodynamic tether. Unfortunately, there were problems with the deployment of the tether. Tethers unlimited is also developing solutions for deorbital, including a Terminator Tape for low altitudes and a Terminator Tether for higher altitudes. The Terminator Tape uses the energy collected by moving through the Earth’s magnetic field to induce a passive drag force on the satellite, which reduces the altitude of the satellite.
ISRO has launched a number of satellites that tested deorbiting strategies
ISRO has launched a number of satellites on its workhorse rocket, the PSLV, that were attempts to demonstrate various deorbital strategies.
Launched by the PSLV-C40 mission, along with 30 other satellites, the satellite is an attempt to check test the viability of a drag sail to deorbit a satellite. The sail works by interacting with the particles in the upper atmosphere. As a passive deorbiting measure, drag sails have the potential to be used on nanosatellites and microsatellites, which have limited space available for fuel and active propulsion systems. The satellite was built by students of the Chungnam National University in Korea.
The PSLV-C38 mission had thirty one passengers on board, out of which three were technology demonstration satellites for deorbiting technologies. The Inflatesail project was handled by the Surrey Space Centre, which has announced that the mission successfully demonstrated the viability of a dragsail to deorbit a nanosatellite. The sail was folded up and is designed to fit in a standard 3 unit of the CubeSat standard, a platform for nanosatellites.
The URSA MAIOR satellite launched on the PSLV-C38 mission, measured the electron density in space. At the end of the mission, it has a dragsail payload as well, to safely deorbit the satellite. The dragsail is developed by Space Mind and is called ARTICA, short for Aerodynamic Reentry Technology In CubeSat Application. The sail occupies a standard 1 unit on a CubeSat, and is designed to be a cost effective, modular deorbiting solution.
Also launched on the PSLV-C38 mission, the D-Sat was built by a company that specialises in creating technologies for deorbiting satellites, and is called D-Orbit. Their main product is the D3 decommissioning device, which uses active propulsion to deorbit satellites in various orbits. The onboard propulsive device on the D-Sat fired successfully, but a direct and controlled decommissioning was not achieved.
Launched on the PSLV-c35, the NLS-19 is also known as the CanX-7 and was built by the University of Toronto Institute for Aerospace Studies in Canada. The satellite successfully demonstrated four drag sails to deorbit a nanosatellite, with each sail measuring about one square metre. The modular drag sail actually fits on the outside of a CubeSat, so there is ample space on the interiors for the primary payloads.