Launching cubesats from the X-37B OTV 5: lifetime modelling with GMAT

image: USAF

Last week, CSpOC issued catalogue entries for three cubesats released as part of the X-37B mission OTV 5.

It concerns USA 295 (2017-052C), USA 296 (2017-052D) and USA 297 (2017-052E). No orbital data are given, but the catalogue entry did explicitly indicate that all three are no longer on orbit.

That cubesats were released as part of this X-37B mission had been clear from a US Air Force statement made after completion of the OTV 5 mission in October last year. The wording of that statement is however ambiguous: while most analysts take it to mean the cubesats were released by OTV 5, it is also possible that they were released as ride shares by the upper stage of the Falcon 9 rocket that launched OTV 5 in 2017.

In this blog post, I will do an academic exercise aimed at guessing when, at the latest, these cubesats could have been released by OTV 5, assuming release from the latter.

OTV 5, the 5th X-37B mission, was launched from Cape Canaveral on 7 September 2017. It landed at the Kennedy Space Center Shuttle Landing Facility on 27 October 2019, after 780 days in space. Unlike previous missions that were all launched in 38-43 degree inclined orbits, this one was launched into a 54.5 degree inclined orbit. Combined with the fall launch date, this meant it took our tracking network a while to locate it on-orbit: the first positive observations were made in April 2018, half a year after launch.

From April 2018, when we started to track it, to October 2019, when it landed, OTV 5 orbitted at various orbital altitudes between 300 and 390 km altitude (see diagram below):

click diagram to enlarge

The CSpOC catalogue entry lists all three cubesats that were released as part of this mission as "no longer on orbit". Assuming they ended their orbital life by natural decay (rather than, for example, being retrieved by OTV 5 again at a later stage, which is in theory certainly possible!), the fact that they were no longer on orbit by 11 February 2020 might yield some constraints on when they could have been released.

To get some idea of the orbital lifetime of a cubesat released from OTV 5, and spurred on to do so by Jonathan McDowell, I ran several GMAT models in which I modelled a 5 kg 3U cubesat released at three altitudes: 400 km, 360 km and 325 km.

We do not know the actual orbital altitude of OTV 5 at that  moment. Nor do we know when the cubesats were released. Hence the three altitude variants. The start point of the modelling was an assumed release into the OTV 5 orbit on October 7, 2017, one month after launch of OTV 5.

For each cubesat, the models were run in two variants: one with the cubesat in minimal drag orientation (0.01 m² cross section), and one with the cubesat in maximal drag orientation (0.03 m² cross section). I used the MSISE90 atmosphere in the model, with historic Space Weather data for October 2017 to February 2020 and estimated solar and geomagnetic activity parameters from the 'early cycle' variant of the GMAT Schattenfile for dates past early 2020.

For the three assumed orbital altitudes and an assumed release one month after OTV 5 launch, the GMAT data produce the orbital decay plots below. In these plots, the red data are for minimal drag orientation, the blue data for maximal drag orientation. If the cubesats in question were similar to NRO's Colony II cubesats, then the red minimum drag orientation curves probably represent the orbital evolution best. If they were more like Colony I cubesats, then the blue maximal drag curves are more representative.

Taking the minimal drag variants, and under the assumption that the cubesats were 3U cubesats and not retrieved on-orbit by OTV 5 at a later stage, the suggestion is a release below 350 km. Released at higher altitudes, they would still be on-orbit.

Assuming reentry before 11 February 2020 after natural orbital decay, a minimal drag orientation and release no lower and no higher than 325 km, the latest possible moment of release would be late August 2018, give or take a month to account for the uncertainties.

It appears we can rule this out however, because we know that OTV 5 was orbiting at 380 km altitude, not 325 km altitude, at that time. So the best guess (although one under many assumptions) is a release some time before August 2018, i.e. within 1 year after the launch of OTV 5.

It is still possible that the cubesats were released at a later date, but next retrieved while still on-orbit by OTV 5. If the cubesats were smaller than a 3U cubesat, a later release than August 2018 is possible as well.

Finally, given the ambiguity in US Air Force Statements on the matter, it is also possible that the cubesats were released from the Falcon 9 upper stage on the day of launch.

For more about the X-37B, and especially the active myth-making that seems to be at play around this secretive space-plane, see my earlier post here.

OTV 5 rising in April 2018. Click image to enlarge

X-37B fact and fiction

X-37B. Photo: USAF

If there is one classified space object that speaks to the public's imagination, then it is the US Air Force's  X-37B robottic space plane, also known as Orbital Test Vehicle (OTV).  These 9 meter long uncrewed spacecraft have wings, with a wingspan of 4.5 meter, and look like a mini Space Shuttle. They are launched on a rocket like a normal satellite, but return to earth by landing like an airplane (or indeed like the Space Shuttles did). They have a payload bay of 2.1 by 1.2 meter in which they carry experiments and from which they could perhaps also release and retrieve small satellites. They are launched in very low orbits, between 250 and 450 km orbital altitude (i.e. generally below the orbit of the ISS).

The US Air Force has two X-37B's and is currently flying it's 5th OTV mission with one of them, with 685 days on orbit on the day of writing.

The winged design and the coloquial 'space plane' lead many people to think the X-37B flies and banks like an airplane or a Star Wars X-wing fighter while in space - its infamous purported "manoeuverability", a notion recently fuelled again by remarks of former SecAF Heather Wilson (see below).

This is mostly a misunderstanding and part of the mythos that surrounds the X-37B: in space, the wings of the X-37B are useless and it behaves and orbits the earth like any other satellite. The X-37B does not change its orbital plane at a whim - or at least not generally. That is quite clear from amateur monitoring of the five OTV missions so far.

In this post I will show that the only significant manoeuvers the OTV's make are frequent alterations of their orbital altitude: they do not significantly change orbital plane during a mission. Periodically changing orbital altitude is something other satellites do too, so the X-37B is not special in this either, except that during recent OTV missions the X-37B's have done this more often than ordinary satellites typically do. And let me add, so you understand me well: you don't need (or indeed use) wings for that. These orbital altitude changes are done with an engine burn, just like 'normal' satellites do.

The X-37B OTV 5 filmed by the author on 26 June 2019

The wings of the X-37B are not for manoeuvering in space, but primarily for use in the lower atmosphere upon its return to earth, when it lands like an aircraft (as the Space Shuttle did). Yet every now and then, the myth of the supposed wing-supported "manoeuverability" pops up again, and connected to it is a whole ecosystem of suspicions and theories about the potential "function" of the X-37B - most notoriously the (almost certainly incorrect) notion that it is some kind of "Space Bomber" ready to be flown to any target on earth within 90 minutes to drop a destructive weapon. The Space Treaty, to which the USA is a signatory, prohibits to deploy weapons from space, and it is really unlikely that the X-37B is such a 'space bomber'.

The X37-B instead likely is a testbed for new space hardware, testing new technologies under real space conditions and then returning them to earth for inspection. We know for example that during the OTV 4 mission, a XR-5a Hall-effect thruster was tested. The frequent changes in orbital altitude are part of this: testing space hardware under various drag regimes.

So what about that "manoeuverability" then? New fuel was fanned on the idea of extraordinarily "manoeuverability" recently by intriguing statements made by former SecAF Heather Wilson. She claimed that the X-37B:

"Can do an orbit that looks like an egg and, when it's close to the Earth, it's close enough to the atmosphere to turn where it is. [...] Which means our adversaries don't know -- and that happens on the far side of the Earth from our adversaries -- where it's going to come up next. And we know that that drives them nuts."

Two things are apparently being claimed here:

(1)  The X-37B can manoeuvre by briefly dipping into the upper atmosphere;

(2)  This makes the X-37B difficult to track.

The wording of the statement is wonderfully opaque, but Wilson seems to suggest that the X-37B can seriously change its orbital inclination by briefly dipping into the upper atmosphere and using its wings to manoeuvre.

I have two problems with this. One is that bringing the X-37B down into the upper atmosphere by an engine burn (there is no other way), have it change orbital plane by using the wings, and then do a burn to get back to orbital altitude again, probably costs as much fuel as a more regular on-orbit engine burn to change orbital plane. So where is the gain in using this dip-and-wing-manoeuvre?

The other problem I have, is that I do not see the claimed behaviour in our tracking data. Contrary to the impression that Wilson is trying to give us, i.e. that the X-37B's are difficult to track due to the tricks they perform, the X-37B OTV missions have been regularly tracked by our amateur network. And we do not see significant changes in the orbital plane during a given OTV mission.

The X-37B OTV 5 imaged by the author in April 2018 (click to enlarge)

Looking at the tracking data we have for these X-37B missions, they show only very minor changes in orbital inclination during a given mission. There is no evidence for sudden, significant changes in the orbital plane, as is illustrated by these diagrams that for each OTV mission plots the orbital inclination against time (the data are from observations by the satobs amateur network):

The only exception appears to be mission OTV 4, which does show a temporary change in orbital inclination and then back again in the last quarter of 2016. The orbital plane change is of little significance however (only 0.6 degrees) and could have been done by a normal engine burn. So if the X-37B indeed can use a drop into the upper atmosphere to make use of it wings to significantly change orbital plane, they so far do not seem to have clearly demonstrated this capability.

(the changes in orbital inclination at the end of the OTV 2 and OTV 3 missions, probably are in preparation for landing).

What the X-37B missions in contrast do have demonstrated, especially during the last two missions, are repeated changes in orbital altitude and orbital eccentricity (in Wilsons words: it "can do an orbit like an egg"). This is illustrated by these plots of the apogee and perigee altitudes against time for the five OTV missions so far:

As I already mentioned this is something other satellites do too, so the X-37B is not particularly special in this either, except that during recent OTV missions the X-37B's do this more often than ordinary satellites typically do. The changes in orbital altitude probably are related to testing equipment under different drag, gravity and irradiation regimes.

So the X-37B missions so far set themselves apart from regular satellite missions by their low orbital altitudes and frequent changes in orbital altitude (in which the wings play no role at all). They can do so because their missions are relatively short compared to a typical satellite mission. Unlike a regular satellite, at one point they will land and be refuelled, and then relaunched after a while.

But as intriguing as the suggestions are, the orbital history of the five X-37B OTV missions so far do not evidence the alledged manoeuverability in orbital plane.

Nor of course, are the X-37B that difficult to track as is claimed. Our amateur network regularly observed and observes the OTV missions. We might lose the OTV for a (usually brief) moment when it has made a manoeuvre to a higher or lower orbit, but a plane scan is enough to relocate it (and as the diagrams above show, they do not manoeuvre daily or even weekly).

So Wilson's remarks appear to be just part of the myth-making around the X-37B.

The X-37B OTV 5 is manoeuvering to a higher orbit

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The image above shows the classified robottic X-37B space-plane OTV 5 of the US Air Force, a kind of unmanned mini Space Shuttle, in the sky above my home on August 20. It had manoeuvered in the previous days (probably on August 17 or 18), from an approximately 316 km orbital altitude to 325 km orbital altitude, an orbit raise of ~9-10 km. The video below shows it the next night, passing through Delphinus:





Just two days later, on August 22, OTV 5 was a no-show, indicating another, and major manoeuvre. Three days later, Leo Barhorst found it again, and subsequent observations showed it to had moved into a 387 x 395 km orbit. A total orbital raise of some 75 km in series of manoeuvers spanning a few days.

As can be seen in the diagram below, which is based on amateur tracking data, the orbit of OTV 5 had been rather steady from when Cees Bassa first located it in late April 2018 up to mid August, at an orbital altitude of ~316 km. The orbital raises mid and late August to ~325 km and next to ~391 km could point to a new test regime for the experimental equipment onboard.

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The X-37B (image: US Air Force). Click to enlarge

Capturing a pass of the X-37B OTV-5, and imaging an ISS transit over the Sun

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Yesterday evening was very clear, and the moon low in the south no real hindrance. I observed a very fine pass of the X-37B secret space plane OTV-5. It was an easy naked eye object. The photograph above (10-second exposure with an EF 2.0/35 mm lens) shows it ascending in the southwest, through Bootes (Arcturus is just above the open window).

The next morning (26 June) at 10:17:21 local time (8:17:21 UT), the International Space Station ISS was predicted to make a transit over the solar disc as seen from my house in Leiden.

I set up the Celestron C6 telescope in the courtyard, put a Baader Solar Foil filter in front of it, and hooked up the Canon EOS 60D to the prime focus. Instead of photographing at rapid burst, the technique I used for imaging with previous transits, I this time put the camera in HD movie mode. While this yields a lower resolution image than photography, the upside is that it yields more images showing the ISS silhouetted in front of the sun. And the ISS is big enough that the reduced resolution is not a real problem, the solar panels of the ISS are still well visible.

The image below is a composite of 21 frames from the resulting movie:

click to enlarge

Here is the movie itself, showing you how rapid such an ISS transit over the sun is (the total duration was only 0.8 seconds - it is over in a blink of the eye). The ISS had an apparent size of 45.8" during the transit, with the sun at 41 degrees elevation in the east:

The movie was made in the prime focus of a Celestron C6 (15-cm, F1500 mm Schmidt-Cassegrain, equiped with a Baader Foil solar filter) with a Canon EOS 60D DSLR in HD movie mode at 25 frames/second, with each frame having an exposure time of 1/4000th of  a second to avoid blurring the ISS. The track and time of the transit had been checked before the observation by loading the latest orbital elements for the ISS into Guide.

The biggest challenge with this kind of imagery is always to focus properly, certainly when the sun is spotless as it was this day. I always find focussing on the sun cumbersome. The focus this time turned out to be reasonably good though.

Pinpointing the OTV 5 orbital manoeuvre on 19 April 2018

click map to enlarge

As related in a previous post, the X-37B robottic space plane OTV 5 made an orbital manoeuvre on the 19th, lowering its orbital altitude from ~355 km to ~315 km.

It has been observed in its new orbit enough by now (pass predictions for yesterday evening were spot on), to allow an analysis to reconstruct the time and location of the manoeuvre. This can be done by looking for a moment where the positions in the old orbit and the new orbit were close.

Using Mike's pre-manoeuvre OTV 5 orbit of epoch 18104.41928168 and my own post-manoeuvre orbit solution of epoch 18112.84880111, and feeding these into the COLA program written a long time ago by Rob Matson, the resulting time of coincidence is 19 April 2018 at 5:20 UT.

OTV 5 was near perigee and in its descending node at the time, over west Africa, as can be seen on the map above. Manoeuvres typically happen near the nodes and near either perigee or apogee, so that fits well with this reconstructed moment of manoeuvre.

Since the manoeuvre entailed both a lowering of the perigee and a lowering of the apogee, the time and location listed above is likely the second of two manoeuvre moments.

The first manoeuvre burn probably happened near 4:35 UT, near apogee and the ascending node of the original orbit, south of Hawaii. This burn lowered the perigee altitude of the orbit to 310 km. Next, a second burn lowering the apogee altitude to 323 km was conducted half an orbital revolution later at 5:20 UT, near perigee and the ascending node of the orbit over west Africa. The two points are depicted by red circles in the map above.

Past OTV missions frequently made such manoeuvres between different orbital altitudes. They probably are meant to be able to test experimental technology in the payload bay under various thermospheric density and irradiation regimes.

Meanwhile, we continue to track OTV 5 in its new orbit. My observations yesterday were hampered a bit by an untimely field of clouds, but I did get some astrometry. Here is some imagery from yesterday, showing OTV 5 ascending amidst a thin cloud cover (bright star in clouds at right is Capella):

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OTV 5 or Zuma? A brief explanation why this object is OTV 5 and not Zuma

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The image above shows the US Air Force's "secret" X-37B space plane OTV 5 ascending in the western sky (the two bright stars above the roof are Castor and Pollux), in the evening of 21 April 2018.

I was asked the question: "how do we know this is OTV 5? Why can't it be Zuma?". I will explain here why it is definitely OTV 5 and definitely not Zuma.

The key is in the orientation of the orbital plane. Both OTV 5 and Zuma were launched from Cape Canaveral into a northwest direction, towards azimuth 40-50 degrees (see map with launch hazard zones below). That direction establishes the orbital plane the objects were launched into.

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From our tracking of the OTV 5 candidate the past 10 days, we have the orbital plane this object is moving in. We can project that orbital plane back to the launch dates of both OTV 5 and Zuma.

For the launch date and launch time, it should pass over the launch site, with a correct orientation in terms of direction. That means, in this case: it should pass over Cape Canaveral, into a northeastern direction.

Now let us first do that for OTV 5, which was launched by SpaceX from Cape Canaveral pad 39A on 7 Sept 2017 at 14:00 UT. The 3D plot below shows the orbital plane of the object we track projected backwards, for the moment of OTV 5 orbit insertion (7 Sep 2017, ~14:09 UT):

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As can be clearly seen, the orbital plane we established for the object we have been tracking the past few days, for this date and time lines up with the launch site, and it is oriented into the correct direction (southwest to northeast). This strongly indicates that the object we track is from the OTV 5 launch.

If we do the same for the Zuma launch, we do not get a good match. Zuma was launched by SpaceX on 8 Jan 2018 at 01:00 UT from Cape Canaveral pad 40. The 3D plot below shows the orbital plane of the object we track projected backwards, for the moment of Zuma's orbit insertion (8 Jan 2018, ~01:09 UT):

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As we can see, the orbital plane we established for the object we have been tracking the past few days, for this date and time does not line up with the launch site, and it is moreover oriented into the wrong direction too (northwest to southeast instead of southwest to northeast: a 90-degree angle!). This strongly indicates that the object we track is not from the Zuma launch.

(As avid readers of this blog know, Zuma presumably failed to detach from the Falcon 9 upper stage due to a faulty adapter provided by the satellite's builder Northrop Grumman, and reentered with the upper stage a few hours after its launch).

So the object's orbital plane lines up with a launch from Cape Canaveral on 7 Sept 2017 and orbit insertion at 14:09 UT, the launch date of OTV 5. Ad to this the very low orbit which was also typical for past OTV missions, and it is very clear that the object we are currently tracking is the X-37B mission OTV 5.

Below is a video of OTV 5 which I shot yesterday evening, 21 April 2018:

Imaging the X-37B Space Plane OTV 5 post-manoeuvre

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The image above shows the secretive X-37B Space Plane OTV 5, a robottic mini space shuttle flown by the US Air Force, over my house in Leiden, cruising through Leo (the bright star above the chimney is Regulus). It was a bright, easy naked eye object with a brightness of magnitude +1.

In a previous post I detailed how (and why), following the launch in September 2017, we had a hard time tracking down the whereabouts of this fifth OTV mission. Untill Cees Bassa located it on April 11th, in a 54.4 degree inclined orbit. It is the first OTV mission bringing it to the latitudes of the Netherlands.

Clouded weather in the Dutch coastal region after Cees' recovery prevented me from seeing it untill yesterday. During the past week, OTV 5 moved from morning passes to evening passes. Weather improved too medio last week, but still OTV 5 initially escaped me. Because it manoeuvered!

On April 18th, a week after it was first located in orbit, OTV 5 made a manoeuvre. It was a no-show for several observers, including me, on the 19th, but two observers, Tristan Cools in Belgium and Marian Sabo in Slovakia, reported an "unidentified" object some 8 minutes earlier (which means it passed while I was setting up my camera on the 19th). Based on Tristan's photograph of that object, a post-manoeuvre orbit was guessed by Mike McCants as well as by me. Yesterday evening on the 20th, we were ready to look for it, and we did recover OTV 5, a few minutes in front of the estimated new orbit.

The new orbit is still preliminary, but it seems as if the orbit has been lowered from a ~355 km circular orbit to a 307 x  320 km orbit. In a few days, when we have more observations, we'll know more about the new orbit, and when the manoeuvre exactly happened.

The video below which I shot yesterday evening shows OTV 5 cruising through the Coma Berenice cluster:



This was my very first observation of an X-37B! Very cool to see this enigmatic object pass in my own sky. Given that previous OTV missions frequently manoeuvered, it will be an interesting object to follow.

All kinds of nefarious motives and purported specific targets have been ascribed to the X-37B program by the aluminium hat brigade, but the reality probably is that the X-37B is an experimental test-bed for new space technologies, testing these under real space conditions and at various thermospheric regimes, over a prolonged time period, before retrieving them.

I do find it interesting though that this new OTV mission is in a 54.4 degree inclined orbit, rather than the previous 38-43 degree inclined orbits (see comparison in my previous post). Over the past year we have now seen three experimental missions going (or planned to go) into 50-55 degree inclined orbits: USA 276; the failed Zuma; and OTV 5. All three are clearly experimental missions. For Zuma, I suspect it was meant as an experimental radar satellite, and maybe OTV 5 tests radar as well. Or maybe not.

At any rate, I welcome this new attention to ~50-55 degrees inclination, as objects in such orbits are well observable from my 52-degree latitude in the Netherlands.

X-37B OTV-5 mission located on orbit

OTV-5, The fifth mission of the US Air Force' X-37B  robottic mini-shuttle, was launched from Cape Canaveral on 7 September 2017 on a SpaceX Falcon 9 rocket. Until last week, OTV-5 had not been located by amateur satellite trackers, and that was somewhat curious, as we did locate and track the previous four missions.

But now OTV-5 has been finally found. In the early morning of April 11, 2018, Dutch satellite tracker Cees Bassa imaged a bright unidentified satellite in a ~54 degree inclined orbit. It was seen again by Cees two days later, on April 13. Ted Molczan managed to link it to a lone sighthing of an unidentified object done by Russell Eberst in Scotland back in early October 2017 that was already suspected to perhaps be OTV-5 at that time (several of us, including me,  had tried to recover the object Russell observed in the next few nights that October, but failed).

OTV-5 immediately was suspected as the identity for this object. It was in a very low, ~355 km circular orbit, which is lower than usual for satellites, but which fits with the characteristics of previous OTV missions.

The orbital plane the object is moving in passed over Cape Canaveral at the moment OTV-5 was launched (see below, which shows the location of the orbital plane for the moment of OTV-5 orbit insertion on 7 September 2017). So that fits nicely, and as a result we are quite confident that this is OTV-5.

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There is a difference with previous OTV missions: OTV-5 is in a 54.5 degree inclined orbit, which is a substantially higher orbital inclination than that of previous OTV missions which were flown at orbital inclinations between 38.0 degrees and 43.5 degrees, as can be seen in this diagram below where the current OTV-5 mission orbit is white, and previous OTV mission orbits are red:

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But this actually fits with information released on the OTV-5 mission by the US Air Force, which prior to the launch of OTV-5 stated that:

"The fifth OTV mission will also be launched into, and landed from, a higher inclination orbit than prior missions to further expand the X-37B’s orbital envelop." 

I am very happy that OTV-5 was launched, as it now turns out, into a 54 degree inclined orbit, as for the first time this will give me a chance to see an X-37B OTV mission from the Netherlands. OTV-5 will actually pass over my country (and even somewhat north of it), while previous OTV missions passed over southern Europe only. The previous four missions therefore were not visible from my country, due to their lower orbital inclination.

An obvious question is: why did it take so long to find OTV-5? I have some answers to this that might explain.

First, I think many amateurs subconsciously reckoned it would be in a 38-43 degree inclined orbit like its predecessors. Indeed, the initial search elements we used were for a 43-degree orbit.

Second, this was an autumn launch and the very low orbital altitude means it is not well visible in wintertime from the Northern hemisphere, where almost all currently active satellite trackers are located. Almost all wintertime passes are in Earth shadow.

Now spring has arrived, OTV-5 is emerging out of these shadows, into the light. Weather has not been cooperating for me in the coastal area of the Netherlands where I am located so far, but I hope to be able to joing tracking this object soon. It is an interesting object to track, as previous OTV missions frequently manoeuvered between different orbital altitudes. Plus, the shuttle-like character of this object makes it a special one to track as well.