Sunday 23 May 2021

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.

Thursday 20 May 2021

The front group of Starlink V1.0 - L26

On May 15, yet another batch of SpaceX Starlink satellites was launched, and they are currently making evening and early night passes over Europe. Since launch, the resulting 'train' of satellites from the launch has split into two distinct groups passing a few minutes after each other.

The video above is from yesterday night (19 May 2021 ~23:17 UT) and shows the leading group, which is still quite tight. The trailing group (also captured but not shown in this video clip) is more dispersed. The video was shot with my DSLR: Canon EOS 80D + Samyang 1.4/85 mm lens.

A night earlier, I captured both the leading and trailing group with the WATEC 902H2 Supreme + FD 1.8/50 mm lens (see video below). They were naked eye during that pass.

Wednesday 19 May 2021

SBIRS GEO 5 Centaur fuel blowout imaged from Australia

click to enlarge

On 18 May 2021 at 17:37 UT, the United Launch Alliance (ULA) successfully launched SBIRS GEO 5 for the US Space Force from Cape Canaveral, using an Atlas V rocket. SBIRS GEO 5 is an Early Warning satellite that detects missile launches (SBIRS = Space-Based Infra-Red System). It was placed in a geosynchronous orbit. Two other small rideshare payloads were also launched on this launch.

Looking at the mission profile, I realized that the fuel blowout of the Centaur upper stage from the launch would be visible from Australia and Indonesia. So I alerted the Seesat-list and also sent a private alert to Paul Camilleri, an observer in Australia who in the past has made spectacular imagery of such Centaur fuel blowout events.

Paul grabbed his camera and went out. And returned with spectacular imagery, which I show here with his kind permission. According to Paul, the blow-out cloud reached magnitude +3.

Paul made his imagery with a Nikon D3200 with an F2.0/85 mm lens. They are 5-second exposures (fixed tripod) at ISO 6400.

In the first image shown, taken 18:55 UT just before start of the blowout sequence, you can see both the Centaur upper stage and the SBIRS GEO 5 payload, which had separated from the Centaur some 40 minutes earlier. In the second image shown, taken 5 minutes later, you can see a V-shaped fuel cloud and a circular ring of blown-out fuel. In the other images, you see further venting, creating a bright V-shaped cloud that slowly dissipated over the next tens of minutes. Paul imaged it untill 19:40 UT.


Click images to enlarge


Paul was not the only one imaging the fuel blowout. Australian astronomer Robert McNaught also captured the event on his all sky camera (image used with permission):



The fuel blowout happened at about 12000 km altitude. The Centaur upper stage was over the eastern Indian Ocean, just northwest of the West Australian coast at that moment (see map below).

click map to enlarge

Fuel blow-outs are done to get rid of left-over rocket fuels in the rocket stage. Venting them into space reduces the risk that vapours from the left-over fuel might ignite (e.g. because of static electricity buildup in the rocket stage) and cause a debris-generating explosion.


Animated image sequence by Grahame Kelaher from Australia:


Animated image sequence by Tel Lekatsas, also from Australia:

A movie from the all-sky camera of the Edward Pigot Seismic Observatory, courtesy of Michael Andre Phillips in Australia is here (look at the right of the image in the gap in the trees)

Thursday 6 May 2021

Reentry predictions for the Chinese CZ-5B rocket stage 2021-035B [UPDATED]


CSpOC/18th Space Control Squadron report that the CZ-5B rocket booster 2021-035B met a fiery end at 2:14 UT last night (May 9) over the Arabian peninsula and Indian Ocean. China reports reentry at 2:24 UT in the Indian Ocean near the Maldives

click map to enlarge

UPDATES 9 May 2021, 9:00 UT and 14:30 UT:

In the map above, I have plotted the approximate final trajectory (based on a SatEvo-evolved orbit propagated to reentry) and both the reentry locations reported by China (2:24 UT, 2.65 N, 72.64 E, in the Indian Ocean near the Maldives) and by CSpOC/18th Space Control Squadron (2:14 UT, 22.2 N 50 E, over the Arabian Peninsula).

Note how within minutes of the reentry, the rocket passed over the city of Riyahd in Saoudi-Arabia!

In looking at the plot, one should realize that a reentry is not an instantanious moment, but a process stretching over many minutes, where the object starts to break up and fragments burn up in the upper atmosphere. During this process, the object continues to move over a swat of trajectory that can be hundreds to a few thousands of kilometers long. It is very likely that this proces started over the Middle East or even over the Mediterranean already [edit: imagery from Jordan suggest that the object was still intact when passing over that location, but see a potential video from Oman below]. If fragments survived, they are scattered somewhere along the yellow line in the map, within the current uncertainties of the reentry location.

The video footage in the tweet below appears to show the (start of the) reentry, imaged from Oman, but is unverified for now:

I have tried, by fiddling with the area-to-mass ratio for the rocket in GMAT, to create a reentry trajectory that would match a splashdown in the Maldives around 2:24 UT, as reported by China. The closest I can get to the location reported by China is this result:

click map to enlarge

Of course, this is a simplified model (the proverbal "spherical cow in a vacuum") that does not take into account both mass loss from ablation and fragmentation over the reentry trajectory.

It however suggest that, if the time and location reported by China is right, the rocket stage would have been at an altitude of ~100 km around the position and time reported by CSpOC (the US military tracking network). It could be that the CSpOC position, with the quoted very small uncertainty in time of +- 1 minute, is based on a SBIRS satellite detection of the fireball (we have long suspected that TIP's with such 1-minute uncertainty quotes are based on satellite detections of the reentry fireball). 

About 100 km is a fair altitude for the ablative phase to start, similar to the altitude at which meteoric fireballs start. If the video from Oman above is the real deal, it is indeed suggested that ablation (and break-up) was starting over southern Arabia.

As a further update: interesting information about infrasound detections of the reentry from Djibouti, with source over the Arabian peninsula:

[end of update]

(below is the  pre-reentry  version of this frequently updated post):

(this part below last updated 8 May 23:00 UT)

On 29 May  April, China used a CZ-5B rocket to launch the core module of it's new Space Station Tianhe-1 ("Harmony of Heavens-1"). The module was initially placed in a 41.5 degree inclined, 382 x 171 km orbit and subsequently raised to a 385 x 352 km orbit.

The massive CZ-5B core stage from the CZ-5B rocket used for this launch was left in a 375 x 170 km orbit. Like it's predecessor that flew in May 2020, it was not deorbited after payload release. This likely means that it will come down in an uncontrolled reentry somewhere on May 8 or 9.

Compared to 'normal' rocket upper stages (which are typically 3 to 10 meters long and say 1000-2500 kg in mass), the CZ-5B core stage is huge. It is 31 meters long, 5 meters in diameter, and very heavy: sources differ on the dry mass, but it is somewhere between 17 and 22 tons.


The informal Space industry standard for objects that are heavier than 10 tons, and launched into Low Earth Orbit, is to have a deliberate deorbit over an empty stretch of Ocean. This did not happen with the CZ-5B core stage from the 5 May 2020 launch a year ago, and does not seem to happen now following the Tianhe-1 launch either.

An uncontrolled reentry of an object this large and heavy means that sizable fragments can survive reentry and reach the surface of the earth, with the risk that this happens over inhabited parts of the world. This is not something that you want, from safety concerns. Several analysts have gone on the record calling the lack of a controlled reentry for the CZ-5B core stage 'irresponsible' for that reason.

Indeed, there are risks. For example, sizable fragments from the core stage of the previous CZ-5B launch, that also came down in an uncontrolled reentry, rained down on well populated parts of Ivory Coast in Africa in May 2020.

With the first lauch and uncontrolled reentry of a CZ-5B core stage a year ago, the question was whether a deliberate deorbit was planned but failed, or whether the CZ-5B core stage simply does not have a deorbit capability. As history now seems to repeat with this second CZ-5B launch, it starts to look like the CZ-5B indeed has no deorbit capability. This is highly surprising, and, indeed, irresponsible imho.

At the same time, while there is a risk (see what happened a year ago in Ivory Coast), the risk should not be overstressed. The risk that a random passenger aircraft ends up falling on your house is still orders of a magnitude larger, and we all have come to accept that risk.

We should also realize that much of the 17 to 22 tons mass of the stage will burn up before it reaches earth surface. Moreover, as a large part of the Earth consists of Ocean, it is likely that it will come down harmlessly over some Ocean.

Yet, it cannot be excluded that it will come down over a populated area, so some worry is justified. With an orbital inclination of 41.5 degrees, locations between 41.5 N and 41.5 S are in the danger zone for this reentry. The danger is slightly elevated at the extremes of this (41.5 N and 41.5 S). The latitude range where the CZ-5B booster can come down includes the whole of the United States, Australia, Africa, the southern parts of Asia and much of southern America. Europe is mostly safe, except for Spain, Italy and the Balkans:


When the reentry happens, the rocket stage will break up in the atmosphere and any surviving parts that have not burned up completely will rain down along the ground path along a very long stretch of earth: the area where fragments fall down can be hundreds of kilometers long.



The diagram in the very top of this post gives reentry predictions. I will update it every time I run a new prediction. Below are the same predictions in table form. They are based on modelling of the orbital evolution in the General Mission Analysis Tool (GMAT).

Modelling is done for a mass of 17 tons (note that the true dry mass of the rocket stage is a bit uncertain: quoted values range between 17 and 22 tons, depending on the source) and with a drag surface at 60% of the maximum surface, as I have found that this generally fits well with tumbling rocket stages (which as a result of the tumbling have a variable drag surface). 

Each new prediction is based on a new orbital update from CSpOC. The MSISE90 model atmosphere is used, with past, current and estimated future Space Weather.

Note that these predicted reentry times are nominal values only. PLEASE NOTE THE VERY LARGE UNCERTAINTY IN THESE TIMES! 

The uncertainty margins shown are calculated as 20% of the time between the orbital epoch on which the prediction is based, and the predicted reentry time.

Note that the nominal position shown is only nominal: it is for the center of the uncertainty interval, but as long as the uncertainties in the reentry time measure in the hours, it is basically meaningless.


DATE      *NOMINAL* TIME        (issued)     (*nominal* position)

8 May     19:23 UT +- 22 hr     (May 4.22)

8 May     21:23 UT +- 21 hr     (May 4.46)

8 May     20:40 UT +- 21 hr     (May 4.53)

8 May     19:41 UT +- 20 hr     (May 4.59)

8 May     20:46 UT +- 18 hr     (May 5.08)

8 May     21:36 UT +- 18 hr     (May 5.20)

8 May     21:18 UT +- 16 hr     (May 5.51)

9 May     00:10 UT +- 16 hr     (May 5.58) 

9 May     04:40 UT +- 13 hr     (May 6.50)

9 May     03:52 UT +- 10 hr     (May 7.18)

9 May     04:01 UT +-  9 hr     (May 7.36)

9 May     03:36 UT +-  5 hr     (May 8.17)

9 May     03:10 UT +- 3.5 hr    (May 8.40)   (07 N 108 W)

9 May     03:11 UT +- 1.8 hr    (May 8.77)   (11 N 103 W)

9 May     02:54 UT +- 1.3 hr    (May 8.86)   (31 S 155 W) 

Uncertainties in the predicted reentry times will remain very large untill very shortly before the actual reentry.

click map to enlarge

Within the 2 hour wide uncertainty window of the current CSpOC TIP (9 May 2:04 UT +- 1 hr), which no doubt is more reliable than my amateurish efforts, the rocket stage can come down anywhere on the lines drawn in the map above (the lines show the rocket's trajectory over the uncertainty window). Within the risk window, the trajectory runs over central America, southern Europe, Arabia and the southern tips of Australia. China itself now appears to be out of the risk zone, ironically enough. The nominal point (but: with such a wide uncertainty that it is still basically meaningless!!!) is over Spain

The yellow dots in the map are those cities with populations of a million or more between 41.5 N and 41.5 S.

Other sources of predictions:

Most notably: Space-Track (the CSpOC portal: requires an account), which should be regarded as the most authoritive one.

Also: EU SST on Twitter

...and otherwise: Joseph Remis on Twitter, and Aerospace Corporation 

(note how all of these give somewhat differing forecasts: which shows you how difficult these reentry forecasts are!)

Several years ago I wrote a FAQ about reentries, and reentry predictions, that might answer any questions you have.

I will update this post with new data when I have the opportunity.