From what altitude does space debris drop vertically?

While gearing up for the CZ-5B reentry in the first week of May, an interesting exchange developed on Twitter between @SpaceTrackOrg, @DutchSpace and me, regarding the way space debris falls down in the last few tens of kilometers before hitting ground surface. 

It was triggered by the comment by @SpaceTrackOrg that the coordinates in their TIP messages typically refer to the object at 10 km altitude, not ground level:

As I pointed out in the Twitter thread, increasing drag acting on the fragments during reentry will not only make them start to ablate (and fragment), but will also slow them down, to a point where they finally have lost all initial forward momentum. From that point onwards they drop straight down.

During that tweet exchange, I decided to prove my point to initial disbelievers with a General Missions Analysis Tool (GMAT) model. I constructed an orbit for a hypothetical satellite about to reenter. I next ran this object through a GMAT model, modelling descent through the MSISE90 model atmosphere: initially for a 10 kg mass and 1 m² drag surface, but later I ran the model for 5 kg and 50 kg masses too, capturing a range of area-to-mass ratio's. The initial speed was orbital (7.4 km/s) and the starting orbital altitude was 80 km, just below the tipping point between orbital and suborbital altitude (in this way, rapid reentry in the model was assured).

The movement in latitude and longitude from the model output was next converted to movement in meters at the earth surface (I did this in QGIS), i.e. horizontal displacement, yielding this diagram that maps the horizontal component of movement of each fragment against atmospheric altitude:

click diagram to enlarge

As can be seen, all three objects indeed reach a point where horizontal movement becomes essentially zero - they drop down vertically from a certain point. 

These points where the horizontal movement becomes zero are located at about 45 km altitude for a 5 kg object  (with a 1 m² drag surface), about 35 km for a 10 kg object (with 1 m² drag surface), and about 25 km for a 50 kg object  (with 1 m² drag surface).

So our GMAT model demonstrates what I argued: from a certain point, well above 10 km atmospheric altitude, fragments from a reentry loose their forward momentum and basically start to drop down vertically, essentially a free fall.

But the reality is, of course, a bit different and more complex than this model suggests. Apart from atmospheric drag and gravity, there is another force that starts to act on these fragments once in the (upper) atmosphere, one that GMAT does not account for. The force in question is high altitude winds, which above 50 km altitude can be very strong.

So the reality is, that these high altitude winds at a certain point start to become the main force of horizontal displacement - fragments are litterally being blown away by these winds. As a result, the actual fall from the mentioned altitudes is not straigth down: falling fragments can be blown away laterally from the initial trajectory, or foward along the trajectory, and even be blown backwards along the initial trajectory, depending on the direction of the high altitude winds! The displacement, especially for fragments that are relatively large for their mass (space debris fragments usually are, as they usually are not solid), can be many kilometers.

This effect is well known to meteor astronomers, as it is a complicating factor in calculating where any meteorite fragments from a fireball might have landed. Like space debris, meteorites likewise are slowed down once descending through the atmosphere, and from ~25 to ~15 km altitude (their initial speed is faster than that of space debris and they are more dense, hence they penetrate deeper before losing their cosmic speed) they start the same kind of free fall, moving primarily under the effects of high altitude winds.

As an aside: I would love to see someone add the capability to import and effect high altitude wind profiles into GMAT, so this kind of displacement could be modelled in GMAT!

Note that, in interpreting the diagram above, one should realise that it maps horizontal displacement relative to altitude in the atmosphere. The modelled fragments do not end up in the same geographic location. 

For a given drag surface, low mass objects will come down earlier along the trajectory than heavier objects. This can be seen in the diagram below, which also shows you that the debris footprint of a reentry can easily be hundreds of kilometers long, something to keep in mind when looking at reentry coordinates in TIP messages:

 

click diagram to enlarge

It takes quite a while for these objects to come down through the lower layers of the atmosphere too, especially if they are large but lightweight:

click diagram to enlarge

The actual fall durations are heavily influenced by the area-to-mass-ratio. Relatively solid fragments (low area-to-mass) will come down faster, sheet-like or hollow objects (high area-to-mass) will come down slower. Surviving fragments will trickle down over tens of minutes. This is one reason why the time windows given for hazard areas during a controlled rocket stage reentry are usually an hour or so in duration.

From meteoric fireball studies, we know that as a rule of thumb, ablation (mass loss, i.e. burning up) of fragments stops once their speed is below ~3 km/s. Note that for low melting point materials like aluminium, the speed might actually be somewhat lower (meteorites are rock or iron with melting points at ~1100-1500 C, while aluminium has a melting point at ~660 C).

For the three modelled fragments (all modelled for a drag surface of 1 m²), the 5 kg fragment reaches this point at 77 km altitude; the 10 kg fragment at 73 km altitude; the 50 kg fragment at 61 km altitude. Note that the results will be different when modelling with the same masses but a different drag surface (for a smaller drag surface, the altitudes for a given mass will get lower, as they don't slow down as rapidly). Also note my earlier remark about materials with low melting point temperatures. But in general: anything that survives to below ~50 km in the atmosphere, will probably reach ground surface.

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

No, the failed Venus lander from Kosmos 482 is not about to come down yet

Venera landing craft (photo: NASA)

Late February 2019, a number of news outlets (e.g. here and here) copied a story that originally appeared on Space.com, titled: "Failed 1970s Venus Probe Could Crash to Earth This Year".

It concerned an unusual object launched 47 years ago, called the Kosmos 482 Descent Craft (1972-023E, CSpOC nr 6073). Word was that it was about to reenter into the atmosphere, maybe even this year.  But will it?  Short answer: almost certainly not.

The source of the prediction is attributed to Thomas Dorman in the Space.com article, but how the prediction was done is not clear from the news coverage. On the request of David Dickinson, who was preparing an article on the topic for Universe Today, I made my own assessment of the issue. I looked at the orbital decay of 1972-023E since 1973 and did some GMAT modelling to gain insight into how the orbital decay will develop in the future.

As I will show in this post, my modelling suggests the Kosmos 482 Descent Craft is not to come down yet for several years.

Kosmos 482, a failed Venera mission

During the 1960-ies and '70-ies, the Soviet Union launched a number of Venera space probes destined for the planet Venus. Some of these probes did reach Venus and even briefly took pictures before succumbing to the very hostile atmospheric environment on this planet. But not all of the probes reached Venus. Several attempts went awry.

Kosmos 482, a probe similar to and launched only a few days after the Venera 8 probe, was launched from Baikonur on 31 March 1972. Reaching a highly elliptic parking orbit around Earth, its apogee kick motor failed to put it into an heliocentric orbit. The space probe broke up into at least four pieces that remained in Low Earth Orbit. Two of these, parts of the rocket engine, reentered within weeks of the failure. Another piece, presumably the main space probe bus, reentered in 1981.

A fourth piece, 1972-023E, is left on orbit, and it is interesting, as it most likely concerns the Descent Craft, the lander module in its protective cover that was to land on Venus, similar to the Venera lander module imaged in the photograph in the top of this post. That makes this a highly interesting object, as it will likely survive reentry into the atmosphere (it was designed to survive reentry into Venus' atmosphere after all).

Orbital decay 1973-2019

Initially stuck in a highly elliptic ~9600 x 220 km, 52.25 degree inclined orbit 47 years ago, its orbit has since decayed considerably. Currently (March 2019) it is in a ~2400 x 202 km, 52.05 degree inclined orbit:

click to enlarge

The diagram below shows how the apogee and perigee changed between January 1973 and March 2019. The orbit has become markedly less eccentric. Orbital decay strongly acted on the apogee altitude. The apogee altitude (blue line in the diagram) has come down steadily and by a large amount, from ~9600 km to 2397 km.This lowering of the apogee is to continue over the coming years. By contrast, the perigee altitude (red line) has changed only minimally, from 210 to 202 km over the past 46 years.

click diagram to enlarge

The apogee altitude will continue to come down. Once it is below ~1000 km, in combination with the low perigee at ~200 km. decay will go progressively fast.

Modelling future orbital decay

To gain insight into the validity of the claim that object 1972-023E might reenter this year, I modelled the future decay of the orbit using General Mission Analysis Tool (GMAT) software. Modelling was done for a 495 kg semi-spherical lander module 1 meter in size, using the MSISE90 model atmosphere.

The result suggests that the Kosmos 482 Descent Craft still has at least 5 to 7 years left on orbit. My model has it nominally reenter late 2025. Taking into account the uncertainties, a reentry between late 2024 and late 2026 seems most likely. That is still several years away.

click diagram to enlarge

click diagram to enlarge

The model result fits well with the trend in the actual tracking data, which gives confidence in the results (the thick lines in the diagrams above are actual tracking data, the thinner lines the GMAT modelled future orbital decay. The latter extend the previous trend in the tracking data well, there are no clear pattern breaks).

It should be well noted that modelling the decay of highly elliptic orbits with high apogee and low perigee is notoriously difficult. Yet, both the past and current orbital parameters and my modelling forecast do lead me to think a reentry is not imminent.

I am not the only one casting some doubt on a reentry of 1972-023E this year. Both NBCnews and Newsweek quote earlier results by Pavel Shubin that predict reentry around 2025-2026, quite similar to my results. They also quote well-known and respected space analyst Jonathan McDowell who is similarly opting for a reentry several years into the future, rather than the coming year.

Conclusions 

From my look at the current orbital decay rate and my modelling of future orbital decay, supported by assessments from other sources, it appears that the newsreports suggesting that the reentry of the Kosmos 482 descent craft is imminent and might even occur this year, are in error.

As to the why of the discrepancy: in the Space.com article, Dorman is quoted claiming "Our guess is maybe as much as 40 to 50 percent of the upper spacecraft bus may still be there". It is not clear at all what this "guess" is based on. My own modelling shows that the mass and size of the landing module only (i.e. without other parts still attached), fits the current orbital decay rather well. It is not clear how Thomas reached his conclusion, but modelling with a wrong mass and/or size might explain the discrepancy between my result and that claimed in the Space.com article.

I am hesitant with regard to accepting the high resolution imaging attempts by Ralph Vandebergh featuring in the Space.com article as evidence for object 1972-023E being more than the lander module only, as the weak and rather irregular protrusions visible might be image artefacts from atmospheric unrest and camera shake rather than real structure. Even when telescopically imaged at minimal range in perigee, we are talking about apparent object sizes at the arcsecond level and single pixel level here, conditions under which it is very challenging to image detail. Under such challenging conditions, spurious image artefacts are easily introduced.


Acknowledgement: I thank David Dickinson for encouraging me to probe this issue.


UPDATE May 2020:

On 7 May 2020 I imaged a pass of the Kosmos 482 Descent Craft using the WATEC 902H and a Samyang 1.4/85 mm lens. Here is the video:



This is a stack of 564 frames from the video:

Click to enlarge