Satellite Deorbiting Methods

De-orbiting a satellite involves intentionally lowering its altitude to the point where atmospheric drag causes it to re-enter the Earth's atmosphere and burn up. There are several ways this can be done:

1. Use of Onboard Propulsion: Satellites are often equipped with thrusters or propulsion systems. By adjusting the orientation of the satellite and firing the thrusters, it is possible to lower the satellite's orbit gradually until it eventually enters the Earth's atmosphere.

2. Use of Atmospheric Drag: In a more passive approach, a satellite can be lowered into a less stable, lower orbit where the drag from the Earth's atmosphere gradually slows the satellite down. Over time, the satellite will drop lower and lower until it re-enters the atmosphere.

3. Electrodynamic Tethers: An electrodynamic tether is a long conducting wire extended from the satellite. When the tether cuts through the Earth's magnetic field, it can generate a current along the wire. This current, in turn, creates a magnetic field that interacts with the Earth's field, resulting in a force that can be used to reduce the satellite's speed and thereby lower its orbit.

4. Drag Sails: A drag sail increases the surface area of the satellite exposed to the very thin upper atmosphere, increasing the atmospheric drag and speeding up the deorbit process. This can be especially useful for small satellites or CubeSats.

5. Use of a Deorbit Module: A deorbit module is a separate device equipped with its own propulsion system. The module can be attached to a satellite and then used to guide the satellite back into the Earth's atmosphere.

6. Active Debris Removal (ADR): In this method, another spacecraft (or a "debris tug") is sent up to rendezvous with the satellite. The debris tug then either nudges the satellite into a lower orbit or attaches to it and uses its own propulsion system to lower the orbit.

It's important to note that de-orbiting needs to be done in a controlled manner to minimize the risk of debris from the satellite causing damage to other satellites or to the International Space Station. Some space agencies and companies are working on technologies and guidelines to ensure that future satellites can be de-orbited safely and efficiently, to help mitigate the growing problem of space debris.

Active Debris Removal (ADR) refers to techniques designed to remove non-functional satellites and other space debris from orbit. Given the increasing amount of space debris, ADR has become an important focus for space agencies and private companies worldwide. Here are some ADR methods currently in use or in development:

1. Robotic Arm or Manipulator: This involves the use of a robotic arm to grapple with defunct satellites or debris. After capturing the object, the ADR spacecraft can then either safely deorbit the debris or move it to a 'graveyard' orbit. An example of this method in action is the European Space Agency's (ESA) e.Deorbit mission, which plans to capture a defunct satellite using a robotic arm.

2. Harpoon Systems: A satellite equipped with a harpoon system can launch a harpoon to capture a piece of space debris. The harpoon is tethered to the ADR spacecraft and can be used to either tug the debris into a safe deorbit trajectory or to a graveyard orbit. The RemoveDEBRIS mission has tested this technique.

3. Nets: Another method tested by the RemoveDEBRIS mission is the use of nets. In this method, a net is ejected from the ADR spacecraft and expands to capture the target debris. Once captured, the net and the debris can be safely deorbited.

4. Tethers: These are long, durable cables that can be extended from an ADR spacecraft to a piece of space debris. Once attached, the tether can be used to change the debris's trajectory and guide it into the Earth's atmosphere.

5. Laser Systems: This is still a theoretical method and involves the use of ground-based or space-based lasers to alter the trajectory of space debris. The idea is to vaporize a small part of the debris with the laser, causing a reaction that changes the debris's orbit and leads to its eventual reentry into the Earth's atmosphere.

6. Ion Beam Shepherd (IBS): This is a contactless method in which an ADR spacecraft uses an ion beam (a stream of charged particles) directed at the debris. The impact from the ion beam can be used to nudge the debris into a new trajectory. This method has the advantage of being able to work over relatively long distances and not requiring physical contact with potentially spinning or tumbling debris.

It's important to note that while ADR is crucial for long-term sustainable use of space, it comes with technical, legal, and policy challenges. These include the technical difficulty of capturing and deorbiting debris, legal questions about who is responsible for debris and who has the right to remove it, and policy issues around the need for international cooperation and regulation.