When is it time to turn off a satellite?
by Charles D. Phillips
|One thing that motivates organizations to use a satellite as long as possible is that getting them into the right orbit is still not guaranteed, so once someone has an operational satellite they will try to use it as long as possible.|
Another reason to turn off spacecraft is to try to make them inert, so they will hopefully not shed pieces. For instance, the US Air Force has been flying the Defense Meteorological Satellite Program (DMSP) satellites since May 1962, and two of the three latest models of that satellite program have shown a tendency to break up and scatter pieces into their orbits. The National Oceanographic and Atmospheric Administration (NOAA) flies a similar series of polar spacecraft, the Polar Orbiting Environmental Satellites, (POES) and they have shown a tendency to break up as well.
So lets take a look at some satellites and what kind of factors go into a decision to turn off a satellite. Our examples are DMSP F-14 and NOAA 15 (for more information on these two spacecraft please, see the earlier story on these constellations.) These two satellites have exceeded their design life and some of their subsystems have failed. While they do provide some data and thus are still being used, they will be passivated in the near future (unless they fail first.)
The Air Force launched nine of the “5D-2” model of the DMSP and all except for the latest one have shed pieces in orbit. Even passivating them has not guaranteed that they will not shed pieces. F-11 was decommissioned in August 2000 but broke up in orbit in April 2004. F-12 was decommissioned in April 1997 but broke up in October 2016.
Sometimes the decision to passivate a satellite is not made in time. For example F-13 was launched in March 1995 and broke up in Feb 2015.1 The F-13 satellite was functioning until it began to show signs of a battery overcharge. The situation quickly got out of control and the satellite broke up before it could be passivated.
DMSP F-14 has been in space long enough to break up (it was launched in 1997) and a battery overcharge could quickly cause it to do that. It launched two years after F-13, and it has been in space as long as F-13 was when F-13 broke up.
The situation with DMSP F-13 is similar to what happened to NOAA 8. In December 1985 that satellite also showed signs of a battery overcharge, and shortly after that it shed pieces.2 When these situations occur the operator may not be able to gracefully turn off a satellite. So, when we look at these two specific constellations of spacecraft, both of them have shown that they can shed pieces with little notice. Most likely the chance of them shedding pieces will be lower if they can be gracefully passivated. The program offices for those two constellations certainly have plans to turn them off; they may wait for them to fail or they might proactively turn them off.
When a satellite operator passivates a satellite they do things like open valves to vent fuel and pressurized gases, and they disconnect the batteries from the charging assembly.3 This will not totally remove the chance of it breaking up, since DMSP F-11 was decommissioned in August 2000 and still broke up in April 2004. The NASA list of breakups speculates that it was due to a hydrazine ignition, but the exact cause of that might be may never be known. There is also the case of DMSP F-12.
But could we have just lowered their orbits until they re-entered? Unfortunately, we could not have deorbited those DMSP satellites in spite of the fact that they have a fairly powerful engine on board. To deorbit a spacecraft, we have to lower the perigee to less than 200 kilometers, but these satellites orbit at about 850 kilometers. From even 400 kilometers, a large spacecraft will renter the atmosphere within a year or so.
|Unfortunately, we could not have deorbited those DMSP satellites in spite of the fact that they have a fairly powerful engine on board.|
The earlier DMSP spacecraft have an engine and fuel that is used to separate from the upper stage, for initial attitude control, and to circularize the orbit. After that, it is a common practice to try to expel the remaining hydrazine fuel from spacecraft (as was done with DMSP F-14) so that there is no worry of hydrazine freezing in fuel lines and possibly causing unknown problems during the extended lifetimes of the satellites. The DMSP spacecraft, while operational, only have small nitrogen thrusters for attitude control. More recent DMSP spacecraft (starting with F-17) were launched with the Atlas V and it had the ability to put the satellite into its final orbit, so they did not have hydrazine propellant loaded at all.
One factor that makes it easier to consider turning off F-14 is that there is a fully functional satellite on the ground, just waiting to be launched. However, bureaucratic inertia appears to mean that the Air Force will not launch this needed spacecraft. It will likely end up in a museum instead of providing badly needed weather data.
The second satellite, NOAA 15, is a part of the NOAA POES system. It is time to think about turning it off, as it was launched in May 1998 and is similar to others that have shed pieces. Certainly NOAA must have plans to turn it off it when shows signs of failing.
As with the DMSP satellites, passivating NOAA 15 will not guarantee that it will not shed pieces. NOAA 16 was turned off in June 2014 but still broke up in November 2015.
A new NOAA spacecraft, the first Joint Polar Satellite System (JPSS), should be flown later this year. After a long and costly development effort it appears that NOAA is finally ready to replace the NOAA POES spacecraft. When the first JPSS satellite is operational it will be renamed NOAA 20. So a decision to passivate NOAA 15 should have less impact – of course it is smart to wait until the new satellite is on station and operational before letting the old one go.
The table below lists some satellites that have been documented as being passivated, and shows that even that process does not prevent a satellite from shedding pieces. It is difficult to get confirmation that earlier satellites were passivated; though we can assume that some were, still earlier ones (even those that later had pieces cataloged) are not listed. We can see that at least three passivated satellites have later shed pieces. We could list DMSP F-13 and NOAA 8, but they shed pieces prior to a graceful passivation. These three satellites probably did not experience battery overcharge that led to them shedding pieces, but possibly there is some other battery failure mode, or perhaps residual hydrazine reacted. There is some speculation that hydrazine freezes in space but there is no way to confirm that; frozen hydrazine would be far less reactive.
|DMSP F-11||21798||Nov 1991||Aug 2000||Apr 2004||Speculation about hydrazine ignition|
|DMSP F-12||23233||Aug 1994||Apr 1997||Oct 2016|
|NOAA 16||26536||Sep 2000||Jun 2014||Nov 2015|
|NOAA 17||27453||Jun 2002||Apr 2013|
The European Space Agency’s Environmental Satellite (ENVISAT), which was launched in March of 2002, illustrates the concerns behind this satellite decision. During its mission it had fuel so it could adjust its orbit and move to avoid collisions. ESA had also intended to use that fuel at the end of its life to lower its perigee; ENVISAT is currently in a circular orbit at about 770 kilometers. In 2010, the orbit was lowered about ten kilometers (from 780 kilometers), but a failure on board prevented ESA from lowering it further. According the mission’s website, it could never have deorbited. If it had used all of its fuel as soon as it got into its final orbit, it could have lowered its altitude, but not far enough. ESA estimated that it would have decayed within 25 years if the altitude was about 600 kilometers, and it never had enough fuel to get down to that altitude. There is some thought that hydrazine could freeze in the fuel lines and be a cause of later piece shedding; if so, the remaining hydrazine in ENVISAT is a concern.
|It is definitely time to passivate both DMSP F-14 and NOAA 15, but it would be comforting to have a replacement satellite operational before that is done.|
Predictions about how long a satellite will remain in orbit are always difficult to make. A major factor is the solar cycle that causes the Earth’s atmosphere to periodically expand and contract, and we do not completely understand what drives that cycle. Generally we expect that satellites with perigees over 800 kilometers will be in orbit for over 150 years. Even estimating a lifetime of 25 years for a satellite at 600 kilometers is, at best, a rough estimate.
There is a need to put satellites into those altitude bands but there is no economical way to also build them so that they could lower their altitudes at the end of their lifetimes. It is definitely time to passivate both DMSP F-14 and NOAA 15, but it would be comforting to have a replacement satellite operational before that is done. For NOAA, they should have a replacement in severalmonths; unfortunately DMSP does not have that kind of option for a replacement.
As an Orbital Analyst for the Air Force, it was always a privilege to work with our Canadian allies. People like Stan Brown: he was a warrant officer that worked with us in the Space Defense Center; the Canadians mainly dealt with deep space objects (probably because they had most of the Baker Nunn deep space tracking cameras). They were professional and provided some needed stability in a military unit that had the normal high turnover.