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Satellite Decommissioning In Low Earth Orbit: How To Do It Properly

April 19, 2017

To send a satellite in space you need a rocket with enough power to push your spacecraft at a speed in excess of 25,000 km per hour at an altitude where the atmosphere is too rarefied to significantly slow it down. In this condition, a spacecraft can stay in orbit for several decades, well beyond its operational life.

 

How do you get rid of an orbiting spacecraft?

 

The first approach is to just wait. Earth’s atmosphere doesn’t end abruptly at a well-defined altitude: it gradually fades, until it becomes so rarefied to be negligible. Solar activity expands and contracts the atmosphere, so the atmospheric density in the upper layers of the atmosphere changes over time. The result is that objects orbiting around Earth are still subject to a certain amount of atmospheric drag, whose intensity depends on the altitude and geometric characteristic of the body. This drag slowly erodes the satellite’s orbit until it is low enough to be captured by the atmosphere.

 

When a satellite re-enters the atmosphere, it smashes with the upper layers of the atmosphere at a speed high enough to break it and burn it up.

 

For satellites flying below 600 km, natural decay can take up to two decades. Between 600 km and 2000 km, the decay can take up to a couple centuries, and for satellites flying at about 35,000km in the so-called geostationary orbit (GEO), the natural decay can take thousands of years.

 

For spacecraft flying in low Earth orbit (2,000 km or lower), we can significantly reduce this time by performing a perigee-lowering maneuver. In simple terms, by applying a certain amount of thrust in the direction opposite to the velocity vector you can change the orbit from circular to elliptic, lowering one side of the orbit down to an altitude characterized by a higher atmospheric drag. This maneuver reduces the orbital decay to a time of 25 years or lower, as mandate by space debris regulations.

 

 Credits: iStock photo

 

The main reason why regulations set a 25 year limit instead of a direct re-entry is because the propulsion systems onboard most spacecraft are designed to execute only low thrust maneuvers to keep a spacecraft within its orbital path during the course of the mission. A decommissioning maneuver with this kind of propulsion takes a lot of time and consumes a lot of propellant, therefore reducing the life of a spacecraft by several months.

 

This solution is far from ideal, both because of the time frame and because the eventual re-entry will be uncontrolled. If a spacecraft is large enough, several fairly large pieces can survive re-entry and reach the surface of the Earth. Each year, 200 to 400 objects big enough to be tracked re-enter uncontrolled, falling in random places producing visible reentry fireballs. About two times per year, large debris like fuel tanks are discovered on the ground. The fact that nobody has been hurt by falling debris so far does not guarantee that it won’t happen in the future.

 

The ideal maneuver to dispose off of a LEO spacecraft promptly and safely is one that causes a direct and controlled re-entry. A direct decommissioning maneuver puts a spacecraft into an immediate re-entry path that that intersect the atmosphere within 30 minutes. A direct maneuver enables operators to control with precision where the satellite will hit the atmosphere, ensuring that no debris will fall on populated areas.

 

To execute a direct re-entry maneuver, you need a propulsive system with enough thrust to turn the satellite’s trajectory into a parabola, something that a spacecraft’s internal propulsion system is unable to do efficiently. Moreover, a significant percentage of spacecraft fail before the end of their mission, and once the main system is lost, it is not possible to get rid of it.

 

This is why we got the idea to create a dedicated device that can execute this kind of maneuvers with the least amount of mass necessary, and why we made this device independent from the main spacecraft.

 

D-Sat is about to test this innovative concept in a real space mission. This is history in the making: visit our KickStarter page, make a donation, and you’ll be part of it.

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