Satellites That Have Low Perigees
There are a number of satellites that are not in the "default official" CSpOC catalog, and some have low perigees (and there are others with various orbits). This page is about those low perigee satellites that will eventually reenter - but for whom there will be no alert about this reentry. People might be concerned by what they observe - for events that can be predicted. This page is here to help document this phenomenon and help predict future reentries - this will only list those objects that will not be shown on the official Aerospace Corporation reentry prediction.
These satellites have high eccentricities and also have two approximate inclinations - 25 and 65 degrees. These indicate that they were launched from Cape Canaveral (and are upper stages left in transfer orbits) or if they are the higher inclinations they are likely payloads or debris in the final, operational orbit. These objects are tracked so they must be larger, almost certainly large enough to survive to impact the ground (or sea) below.
First, it is unusual to find satellites which have perigees that will rise, low altitude satellites (perigee below 500 km) that most people are familiar with normally will show a steady decrease in perigee. Most low altitude satellites also have very low eccentricities, the increase in perigee is only seen in highly eccentric satellites where apogee is far above perigee. This orbit is sometimes called a "semi-synchronous" or Molniya orbit. I am told by people that know what they are talking about that lunar and other perturbations can increase the perigee of these eccentric orbits while apogee steadily decreases. At some point, apogee is going to be pulled so low that perturbations will not affect perigee much - and the satellite will likely steadily decay after that. But what apogee might that be? Perhaps over 10 thousand kilometers?
What do we look at to determine if a satellite is at risk of reentering in the next few years? Here is a typical high eccentricity satellite, 99208:
Satellite 36869 (was 99208)
99208 has had some fascinating behavior and it is a rare "observer" satellite that I have a lot of data for. As I did with 90059 (Acknowledging Some Overlooked Satellites) I looked at the historical TLEs for this object (with a great deal of assistance from my colleague Brian Chambers). First it was important to verify that they were on the same object, it is possible that some observations were mis-tagged and the TLEs might have been on multiple objects. But if we plot the TLEs and the Right Ascension of the Ascending Node (RAAN) it should show multiple lines if we have more than one object - here we see that there is one line that resets to 360 when the RAAN gets to zero degrees. This plot strongly indicates that we are dealing with the same object.
Fortunately I have been able to convince observers to track this object more frequently and so we have more points now.
Next let's look at the change in apogee - it should decrease over time.
And it does. At first it declined steeply but has since slowed it's rate of change. That is not what we would normally expect, we would expect a steady decrease in apogee followed by a rapid decrease.
This plot shows perigee over time and is really interesting.
These TLEs indicate that the satellite survived having a perigee of below 150 km for months; during that time the apogee dropped rapidly. Now that it's perigee is much higher, the apogee has dropped much more slowly. It is very very surprising to see a satellite which could survive that low of a perigee for that long. We should conclude that this object was tracked shortly after it went into orbit, it is hard to imagine it surviving with a perigee even lower than this. Since the perigee is still well below 400 km it continues to lose energy and the apogee will continue to drop. At some point the apogee is low enough that perturbations cannot pull the perigee up and both apogee and perigee will decrease until it reenters.
This object is being tracked fairly regularly now (thanks Kevin Fetter!). And as of Dec 2018 we know that this object is 36869. Now that this object is in the Satellite Catalog it will be covered in decay predictions but I leave this information to show how the analysis is done.
Satellite 32707
The story of how the astronomer community, reacting to a satellite reentry, found which satellite had impacted the ground is a fascinating one and is a big motivator to look at objects which will reenter in the future. The story, mainly written by Ted Molczan, is here:
http://www.satobs.org/reentry/2008-010B/2008-010B.html
and it is well worth reading. Figure 3 is very interesting. Apparently his analysis was done with orbital information from ISON.
In the case of satellite 32707 (in the analysis that Ted and company did in the link above), it had an orbit with a perigee of approximately 275 km and an apogee of approximately 38,000 km when it began it's final drop into the atmosphere. Interestingly, it apparently did not renter at perigee. If we could observe a reentry and track it back to the object that apparently reentered, couldn't we predict reentry on at least a few satellites?
With 99208, it had a far lower perigee which evidently then increased - but the apogee was only 20,000 km or so. From this it would seem that 99208 could have not survived as long as it has.
A study done by Ted Molczan strongly suggests that once the perigee of a satellite like this goes down to 150 km, apogee also begins dropping swiftly and reentry is a short time later.
Satellite 90059
The article that I wrote about the behavior of 90059 is: Acknowledging Some Overlooked Satellites it also has a low perigee and it's perigee is not steadily decreasing.
The "official" definition of reentered is when a satellite gets down to 85 km, but with small satellites such as CubeSats they will not be tracked if they reach 160 km. These satellites on this page are large, dense objects.
Satellites That Will Reenter One Day
There are currently several other satellites that I am watching. These satellites have very low perigees - approximately at or below 300 km. For reference, the ISS orbits at 400 km or higher. They do have higher apogees and so they are not in the drag of the Earth's atmosphere for their entire orbit, but still they encounter significant drag near perigee. Now perigee, for various reasons, can increase as well as decrease for these satellites but perigee will always be down in the appreciable atmosphere. This is in approximate order of reentry.
Some Historical Notes (These Satellites Are No Longer Tracked)
Satellite 28385
First - Thanks Leo Barhorst for tracking this object!!
This object was very interesting, it had an orbit of 217x6793 km. Even with that low apogee the perigee increased in 2018 and, this satellite, with an inclination of 57.4 degrees, could fall on many parts of the Earth. By looking at the next two plots we can see how the apogee and perigee dropped over 2.5 years; I have a more updated plot for apogee and perigee but they just show the same trends.
Both of these 2017 plots showed a near linear trend (based on the points that we have) with the satellite reaching 100 km perigee in late 2020 (that point is not shown here but we can extrapolate).
The TLE was updated in Oct 2018 and the perigee was increasing. In March of 2018 the orbit was 211 x 7040 km and perigee kept dropping until Sept 2018, suddenly in Oct perigee went up to 217 km - where it had been in January 2018. This is the second time that this object's perigee has risen suddenly.
I didn't put the RAAN plot here since it just tells us that the TLEs are from the same object.
This object previously had a sudden increase in perigee, a very talented colleague, Björn Gimle, plotted some TLEs from 2013 and got a very interesting result. A plot of perigee shows:
So the perigee was below 200 km but rapidly increased, heading towards 250 km.
At that same time the apogee steadily decreased as we have learned to expect. I thought that with an apogee of well below 10,000 km - the perigee should no longer increase, but it has.
Apogee continues to steadily decrease and this will send our friend into the atmosphere at some point in the near future.
Björn is a noted amateur astronomer who lives in Sweden. He used a different software package to generate his plots and he saved them as GIFs, if that causes any problem with anyone's browser please let me know.
We can see that the apogee was well on it's way down to 7000 km or so at that time. By looking at the behavior of perigee for these eccentric orbits we can see that perigees can do some surprising things.
Satellite 32379
This object was also interesting, it had a low perigee but an apogee of almost 15,000 km - so the perigee appeared to be increasing. We got several later points and they are shown below.
The apogee shows a steady decrease as is typical for this kind of orbit.
Satellite 90108
This satellite was tracked for several years and shows that perigee can increase and decrease. The plot of perigee shows:
And we can see that it does increase and decrease - let's compare that to the apogee:
and we see that the apogee has steadily decreased. As the energy stored in the orbit is lost when the object goes through perigee, eventually the object will reach a lower apogee that cannot be affected by lunar, etc forces and so the perigee will stop going up. When the perigee increases, drag decreases and the apogee does not drop as much.
Satellites 90114, 90113
90114 had a perigee that was increasing but it turned around and begun to decrease. The apogee was still very high.
90113 has shown a semi-sinusoidal curve for perigee as well:
but it's apogee is much lower than 90114 - perigee is increasing but likely cannot increase much.
Comparison With Similar Satellites
With great assistance from my colleague, Brian Chambers, I looked at four large objects with similar high eccentricity orbits and got the following results.
You can see that two objects show the "rising perigee" behavior. 25485 may be beginning to have a rising perigee as well but it is perilously low.
The apogees seem to be well behaved, slowly decreasing as we would expect.
Future - NOSS Satellites
One class of satellites that will cause us concern in a few years is the NOSS satellites. These are the Naval Ocean Surveillance System, they started out at an altitude of about 1000 km but the early ones (satellite number 8818 for instance) are down to an orbit of 387 x 1524 km, with a low perigee like that they will reenter in the mid-2020s or maybe earlier. These TLEs were publicly released until about 1983 (as I recall) and some are being released again.
These analyses could not have been done without Brian Chambers - Brian is much better at writing C code than I am and is very good with data analysis. We get Two Line Element (TLE) sets from an international optical observer network. I combine that with what the international radio observer network has documented. This page could not have been done without the invaluable assistance of Greg Roberts, a very friendly astronomer in South Africa.
Some of the deep space satellites have also had breakups. For more information on that, see the other page in this set: