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According to Systems and methods for a self-deploying vehicle drag device (US 8616496 B2) patent's background of the invention:

The United States Federal Communications Commission (FCC) requires CubeSats and other picosatellites to be designed to re-enter the atmosphere within 25 years of the end of their useful lifetimes. Without an assistance, it is estimated that a CubeSat may take over 150 years to de-orbit from an 800 km altitude.

The purpose of deorbiting devices is to cause the satellite to deorbit within the 25-year post-mission lifetime limit, as also recommended by the Inter-Agency Space Debris Coordination Committee (IADC) and ISO 24113 Debris mitigation requirements and compliance:

(e) The orbital lifetime of objects passing through LEO (lower than 2,000 km) shall be shorter than 25 years after the end of operation.

Why does it take 150 years for a CubeSat satellite in 800 km Low Earth Orbit (LEO) to de-orbit naturally, without the use of deorbiting devices?

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If you're asking why the time limit is set to 25 years then it's more for legal reasons than anything astrodynamic related. The choice of 25 years was at this point the owners of the satellite are not culpable should it re-enter, crash into another satellite etc. So the rational was give people as long as possible, without it being so long that they are not responsible for their actions.

Unfortunately I can't give a linked reference for this, since it was an answer to a question at the ESA Space Debris conference in 2013.

If you're asking why a satellite might take 25 years to de-orbit, it's because of the very high velocity of orbiting object (~7 km/s) and hence the objects having a high kinetic energy. For low Earth orbits, de-orbiting only passively occurs due to drag. To reduce your very high kinetic energy you need to collide and lose that kinetic energy with the rarified gas atmosphere. It take such a long time because the atmosphere is extremely thin. If you are interested in knowing how long an object will take to de-orbit due to drag, the key variables are altitude and ballistic coefficient.

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  • $\begingroup$ That's an interesting input - the reason for the choice of 25 years. It surprises me because the idea of culpability sounds as if it would be specific to a particular national legal system, rather than having been adopted by the IADC and ISO standard as Romean points out. As far as I know, there is no connection with 25 years in the Liability Convention of the Outer Space Treaty. Can you recall more of the context that you heard it? $\endgroup$ – Puffin Oct 21 '15 at 20:44
  • $\begingroup$ @Puffin way to resurrect! The question was asked at the 6th EU conference on space debris (back in 2013); the person who answered was one of the authors of the standard. He was quite candid that the reasons had nothing to do with science. $\endgroup$ – ThePlanMan Oct 21 '15 at 20:50
  • $\begingroup$ Curious. Now I think about it I do recall about ten years ago there was some disagreement amongst the national representatives on the IADC as to whether 25 years or 50 was appropriate. I suspect the 25 years wasn't unanimous. It seems as if this hasn't gone away as I checked the recent update to the guidelines, 2014, and it still couches the 25 years in very loose terms. It now makes more sense that perhaps the "legal" origin of 25 years as you have suggested simply came from an existing statute of limitations in one of the IADC member countries. $\endgroup$ – Puffin Oct 23 '15 at 12:54
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There's more history on the NASA Orbital Debris Program web site.

In 1995 NASA was the first space agency in the world to issue a comprehensive set of orbital debris mitigation guidelines. Two years later, the U.S. Government developed a set of Orbital Debris Mitigation Standard Practices based on the NASA guidelines. Other countries and organizations, including Japan, France, Russia, and the European Space Agency (ESA), have followed suit with their own orbital debris mitigation guidelines. In 2002, after a multi-year effort, the Inter-Agency Space Debris Coordination Committee (IADC), comprised of the space agencies of 10 countries as well as ESA, adopted a consensus set of guidelines designed to mitigate the growth of the orbital debris population. In February 2007, the Scientific and Technical Subcommittee (STSC) of the United Nations' Committee on the Peaceful Uses of Outer Space (COPUOS) completed a multi-year work plan with the adoption of a consensus set of space debris mitigation guidelines very similar to the IADC guidelines. The guidelines were accepted by the COPUOS in June 2007 and endorsed by the United Nations in January 2008.

The 1997 Standard Practices document was already talking about a 25 year expectation.

More formally, the IADC says:

A study on the effect of post- mission orbital lifetime limitation on collision rate and debris population growth has been performed by the IADC. This IADC and some other studies and a number of existing national guidelines have found 25 years to be a reasonable and appropriate lifetime limit."

Unfortunately, I haven't been able to find those original studies. There are later IADC studies available that address the question, but I haven't found any that are early enough to have motivated the 1995 formulation.

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