The average life span of a LEO satellite is approximately 5 years, but the average life span for a GEO satellite is approximately 8 years. Why is this?
$\begingroup$
$\endgroup$
3
-
3$\begingroup$ Do you have a source for the life span values? $\endgroup$– James JenkinsCommented Aug 13, 2013 at 14:56
-
3$\begingroup$ I agree, the question contains a mistaken assumption. This should be fixed, but otherwise I find it sufficiently unique and relevant. $\endgroup$– AlanSECommented Aug 13, 2013 at 17:44
-
$\begingroup$ The limiting factors are covered in the answer to this space.stackexchange.com/questions/1204/… $\endgroup$– Dave GremlinCommented Sep 6, 2017 at 20:30
Add a comment
|
5 Answers
$\begingroup$
$\endgroup$
3
According to Wikipedia on Low Earth orbit:
A low Earth orbit (LEO) is generally defined as an orbit below an altitude of approximately 2,000 kilometers (1,200 mi). Given the rapid orbital decay of objects below approximately 200 kilometers (120 mi), the commonly accepted definition for LEO is between 160 kilometers (99 mi) (with a period of about 88 minutes) and 2,000 kilometers (1,200 mi) (with a period of about 127 minutes) above the Earth's surface.
At this altitude, there are atmospheric molecules present, leading to increased drag that translates to orbital decay (more so during solar maxima, due to the expansion of gases in the atmosphere). Hence, these LEO orbited satellites need to be constantly reboosted to overcome this drag, otherwise their orbital velocity decreases, and they spiral into the lower atmosphere.
Meanwhile, the geostationary orbit is a circular orbit 35,786 kilometres (22,236 mi) above the Earth's equator, and the atmospheric drag is comparatively smaller at these heights.
So, in a nutshell, the difference in atmospheric drag is the main reason for longer orbital lifetime of GEO orbited satellites, than LEO orbited satellites.
-
6$\begingroup$ I'm pretty sure that atmospheric drag at 35,786km out is non-existing and the lifetime constrained mainly by how long components last. $\endgroup$ Commented Aug 14, 2013 at 10:23
-
1$\begingroup$ @MichaelBorgwardt, the lifetime is constrained mainly by the fuel supply. In LEO, most of your fuel expenditure goes towards countering drag; in GEO, there's no drag, and all your fuel can go towards staying in your orbital slot. $\endgroup$– MarkCommented Oct 21, 2015 at 23:03
-
2$\begingroup$ In GEO there's still the tidal forces drag, but it's very minor. The GEO satellites will die out of malfunction caused by the harsh space conditions long before they run out of RCS propellant. $\endgroup$– SF.Commented Oct 23, 2015 at 18:16
$\begingroup$
$\endgroup$
I note that the question asks about lifespan, not limited to orbital lifetime.
Ironically, luni-solar orbital perturbations at GEO have till now had a very strong life limiting effect on GEO satellite lifetimes through the typical mission requirement to control the evolution of orbital inclination, though advances in propulsion technology are easing the limitations of propellant lifetimes.
However, despite this, the basic premise of the question still holds, design lifetimes of commercial geostationary satellites have crept up from 7 to 15 years since the 1970s whilst those of, for example LEO remote sensing missions have evolved from 3 to 10, though I confess to being a little more shaky on the latter evolution.
I think the reasons relate to the high level emergent behaviour of the customers when faced with their own economic cycle of which I can only suggest some starting ideas:
- it costs more to get to GEO so the businesses of these satellites could be more sensitive to economies
- the functions of most GEO satellites for communications is electrical power intensive, thus mass and launch cost enhancing, thus reinforcing the benefit of longer life to maximise the return on capital
- some features of LEO, such as ~15 eclipses per day, may have been technically demanding in terms of electrical and thermal stress.
Any other suggestions?
$\begingroup$
$\endgroup$
2
GEO satellites tend to be large satellites, because they have to be larger to support their missions. When you have to make them larger, then they are built with additional redundancy. In addition, GEO satellites do not have as many eclipses, they have them at most once a day for 90 minutes during eclipse season.
For LEO satellites, their death tends to be due to poor batteries. The batteries are very stressed due to the constant use that they require. GEO satellites tend to end their lives when the fuel runs out. Fuel requirements are usually easier than batteries.
LEO satellites tend to require a constellation to work effective. The redundancy in many ways is that there are usually other satellites in orbit that can "pick up the slack" if one of them fails. GEO satellites tend to require being in a specific location, and cannot support that kind of redundancy.
Bottom line is, LEO satellites are built smaller, cheaper, and with lower lifetimes, and have more battery stress than GEO satellites.
answered Sep 6, 2017 at 16:47
-
$\begingroup$ I think you are quite right about the greater stress on LEO batteries because of the typical 15 cycles a day and short recharge time compared to GEO. However I think there is more to it. Major LEO missions, whether scientific or commercial usually size their batteries to be 20% DoD, or even less in the "new" Li-Ion era (compare typical 80% for GEO) meaning that they carry a large weight penalty to guarantee battery life despite battery fade. $\endgroup$– PuffinCommented Sep 7, 2017 at 11:21
-
$\begingroup$ Battery life tends to be the optimal "Slow Death" of most LEO satellites, but the truth it, they serve different purposes. LEO satellites tend to be smaller, and thus more easily replaced, than GEO satellites. $\endgroup$– PearsonArtPhoto ♦Commented Sep 7, 2017 at 12:16
$\begingroup$
$\endgroup$
There is a design lifetime, and an environment lifetime.
The design lifetime is lower in LEO partially due to cost - the cost of launch to GEO is much more expensive so satellites have to be designed to make a better use of that cost for a greater period of time to justify the expense. LEO satellites are cheaper and so they are typically not designed for as long a lifetime - they will also encounter more drag, but if that were a concern then propulsion and attitude/orbit control would be more mainstream. It's still cheaper to have a propelled satellite in LEO than a GEO satellite. LEO costs ~\$5000/kg to place in orbit, whereas GEO is still ~\$30,000/kg.
The environment lifetime at GEO is very long, as others have stated, mostly due to the more sparse atmosphere. In LEO at 250km, there may be as many as $10^{-10}g/cm^3$ particles, whereas in GEO at 35,786km this is more like $10^{-20}g/cm^3$. Spacecraft in GEO experience other effects from the solar wind (surface charging and other electromagnetic effects).
$\begingroup$
$\endgroup$
Another contributing factor is that the LEO satellite, at altitudes of 1-6 Mm, is getting pounded by the inner Van Allen belt.
