Thursday 31 October 2013

About half-way: another short update on GOCE, 10 days after its engine cut off

Ten days ago the ion engine of GOCE, the European Space Agency's 1-tonne Gravity field and steady-state Ocean Circulation Explorer scientific satellite (2009-013A), cut off after the satellite ran out of fuel.

Originally orbiting at an average orbital altitude of 227.5 km, it is since coming down (see earlier posts here and here and the first two diagrams below). Its perigee is now below 205 km and it is currently coming down at a rate of 2.7 km/day. That rate is increasing (see third diagram below): it is already a factor 3 larger than it was on October 21st just after the ion engine of GOCE cut off.

click diagrams to enlarge

As I wrote earlier, for various reasons it is still too early to provide reliable re-entry estimates. I nevertheless estimate that GOCE is now about half-way, in terms of days not altitude, from inception of its fall to re-entry.

After the latest orbital updates, the nominal re-entry date appears to slowly creep to an earlier date. Currently I have re-entry forecast for a several-day window around November 10.

That date can still (dramatically) shift if anything happens to GOCE, for example if it loses its current drag-reducing attitude (flight orientation - see also a previous post) or if we see a drastic change in solar activity.

The Sun has been very active the past few days, spewing several CME's. This solar activity has an influence on the density of the upper atmosphere, and this in turn has an influence on the magnitude of the atmospheric drag on GOCE, influencing how fast its orbit evolves.

I do expect the prognosed re-entry date to slowly creep to an earlier date over the coming few days. It therefore does seem that GOCE has less than two weeks, and possibly closer to a week, of lifetime left.

Wednesday 30 October 2013

Tracking Prowler

Last week I imaged Prowler again, the classified spacecraft we amateur trackers indicate with the informal Cospar code 2009-097E. The object (no longer operational) is in a librating geosynchronous disposal orbit located over the eastern Pacific. It is hence not visible from the Netherlands. I therefore use a 'remote' telescope in the US to observe it.

Prowler is (or was) one of the most secret and enigmatic objects in existence. It was probably an experimental satellite for close covert inspection of Soviet geostationary satellites. I have posted on the background story of this object in more detail before. Clandestinely launched from Space Shuttle STS-38 in 1990, its launch and existence have never been acknowledged by the US government.

The images below were made on October 25 and 27, using the 61-cm F/10 Cassegrain of Sierra Stars Observatory in Markleebergville, California.

click image to enlarge

Both images have an exposure time of 30 seconds. The brightness difference between the two images is mostly due to a difference in phase angle.

Tuesday 29 October 2013

A Space Buff Watches 'Gravity'


Last Wednesday the GF and I went to the cinema to see Gravity, the latest Space-themed blockbuster movie starring George Clooney and Sandra Bullock. Emersing ourselves in the hyperrealistic 3D graphics, we were so captivated by it all that we went for a second screening last Saturday, and highly enjoyed that too.

I should ad that while I am acknowledged to be a space buff, my GF is not (I think she initially primarily went for Clooney). So my GF’s insistence that she wanted to see it a second time, says something about the impact this movie made on her. And on me, because I happily went along with the idea of going for a second screening!

In this blog post I will give a brief impression of the movie as I experienced it, and next provide some comments on the scientific reality and accuracy of it.

Don’t expect me to go all negative, bashing the movie for incorrect science. While I do have several things to comment on from the Space science point of view later in this review, such comments are basically nothing more than nitpicking in order to satisfy the inner Geek in me. It is fiction after all, not reality, and moviemakers, like all artists, have artistic license to alter reality if that suits their creative process. As one of my friends said: “if you want accuracy, you should watch a documentary, not a movie”.

PART I: the movie 

With regard to the movie as a whole: it is, in one word, FANTASTIC!

I can thoroughly recommend you to go and see it. You will be at the edge of your seat from the beginning to the end, and the 3D graphics are stunning.

This is also the first time that I really thought the 3D was worthwhile, adding to the movie experience. Movies in 3D until now did not quite captivate me. With most movies, I actually don’t see the extra value of it: in my opinion it is more a gimmick than that it really ads to the experience. But Gravity is a clear exception to this rule. With this very ‘spatial’ movie, the 3D really does ad to the experience. Maybe because it is so extremely well done and the storyline is so extremely suited to it. It sucks you really into the movie, making it life-like. For example, when the International Space Station started to disintegrate and fragments flew off all over the place, I was instinctively ducking and making avoidance manoeuvres in my seat. Wow!

One of the very cool things about this movie is actually how much (compared to many other disaster and space-themed movies) it gets right, certainly in the details of the space hardware. ISS, Soyuz; they look very realistic. While I am not sure they are correct down to the individual rivet so to speak, they truely do look like the real deal. The moviemakers have clearly documented themselves extremely well on this point. The graphics are moreover realistic to the point where you really can’t tell where real-life filming ends and CGI graphics take over. Rendered in amazing and highly realistic detail, you truely get the idea that you are watching real NASA or Roscosmos footage. It is all hyper realistic.

For an astronomy buff like me, it was also very fine to note that the starry backgrounds of space featured recognizable star patterns: I noted Aquila and the Arrow, Hydra, and Auriga with the Pleiades and Hyades for example. The only odd frowning moment was when in a certain scene the moon was rising from behind the limb of the Earth, apparently south of the head of Hydra (which is too much south of the ecliptic to be possible). It was the only potential gaffe I could discern in the astronomical rendering of the starry backgrounds.

The story of the movie is simple and the storyline can be summed up in one sentence: astronaut stranded in space under constant threat of disaster, tries to get home. That’s all there is to it. The story is not very complex: it runs on the action and superb Oscar-worthy 3D graphics, not on a great storyline.

And frankly it turns out that this is enough to make this a very captivating movie. I had a slight “Meh” reaction only twice, and that was when Bullock brings up her dead child in the conversation with Clooney, and later when she addresses his spirit on it again from the Soyuz.It was probably meant to ad to the general movie theme of “letting go” (except from letting go of life preservation instincts, in Bullocks case) but to me it was a bit cheesy. For the rest, the movie didn’t annoy for a moment.

There appears to be an attempt to put some symbolism into the movie every now and then. There is for example a sort of  'rebirth' scene where Ryan removes here spacesuit once inside the ISS and curls up in foetus position, with cables mimicking an umbilical cord.

Scenes that particularly impressed me, for either their action or their beauty:

- The opening scenes, with the Shuttle and docked Hubble slowly appearing into view. It sets the tone for the movie;

- The violent destruction of the ISS after a volley of space debris hits it (and the fire onboard causes an explosion?). This is one of the truely breath-taking action scenes in the movie, taking full advantage of the 3D effects;

- The Soyuz undocking from the Space Station, with the nice detail of the noise from the burning Space Station instantly ceasing. This is simply a beautiful scene, also in the way it introduces sudden tranquillity after mayhem (and then back to mayhem again only a short bit later);

- The re-entry scenes of the Tiangong space station, fragmenting into a stream of parallel moving ablating debris in the wake of Bullock’s Shenzou landing module. This was beautifully done.

Other noteworthy details:

- I noted there was an ATV (the European space cargo ship) docked to the ISS. Nice detail!

- There are some visual jokes every now and then: such as the table tennis bats floating about in the Chinese Space Station, and chess pieces (Russians are renowned chess players) floating in the Russian section of the ISS. There also is a small Marvin the Martian figurine floating in the Space Shuttle: and a Russian icon with Saint Christophoros in the ISS, as well as a Buddha in the Chinese Space Station. And of course there are the jokes by Kowalski about the Vodka stash of the Russians, although I bet it would be packed in small sacks rather than bottles as in the movie (but then, that was Bullock hallucinating through Oxygen deprivation, so needn’t be realistic anyway).


So what struck me in terms of scientific impossibilities or other oddities in the movie, but also things that neatly match reality?

I should note here first that I haven’t been particularly paying attention to other science reviews of the movie (except for Phil Plait’s review in Slate). I have no doubt others have commented on some of the issues I raise below too and I have no pretension to be original in my comments.

In a few cases below, I will do the actual math, so that might be different from more generalized reviews. In those cases the math is based on tables and equations from “Space Mission Analysis and Design” (Third Edition) by Wertz and Larson (eds.), Springer, New York, 1999.

Let’s start with a few things I noted:

- Above I already noted the potential gaffe of a moon located in Hydra;

 - The Shuttle: it bears the name “Explorer” and is identified as “STS-157”, i.e. 22 flights after the true last Shuttle flight, STS-135 in July 2011. The plot therefore necessitates a re-invigorated Shuttle program and a new Shuttle orbiter (hopefully without the inherent flaws of the older Shuttles) to have been built. There is no existing Space Shuttle “Explorer”. This all would seem to necessitate many years. In the past Shuttle schedule, launching 22 Shuttle flights took about 5 years, and there is the time needed to build the new Shuttle as well. So logic would place the events in the movie well into the future, i.e. multiple years from now.

- The Soyuz: the Soyuz that Bullock uses to get from the ISS to the Chinese Space Station Tiangong, is identified by her as Soyuz TMA-14M when she tries to contact Houston. TMA-14M is an existing Soyuz, on schedule to be launched carrying part of the ISS expedition 41 crew to the ISS in September 2014. It will return to Earth in March 2015. This would hence place the events in the movie between September 2014 and March 2015, i.e. in a not too far away future, contradicting the presence of a Shuttle (which is an anachronism anyway: the Shuttle program is history).

- The ATV: there is an ATV (the European Space Agency’s Automated Transfer Vehicle robotic space cargoship) docked to the ISS. The fifth and last ATV, ATV-5 George LemaĆ®tre, is scheduled to launch to the ISS in June 2014. As the ATV’s usually stay docked to the ISS for about half a year, this would place the events in the movie (assuming it is not the current ATV-4, which incidentally undocked yesterday) somewhere during the second half of 2014. This tallies with the presence of Soyuz TMA-14M. It doesn’t solve the riddle of the Shuttle.

- The Chinese Space Station Tiangong: Tiangong exists. It was launched in 2011 but is currently much smaller than it is portrayed to be in the movie (where it looks somewhat like the past Soviet Space Station MIR). So as with the Shuttle, the movie makers take some artistic liberty here.

- The Space suit: when Bullock does her EVA from the Soyuz in a Russian space suit to get rid of the parachute, she must be donning an Orlan suit. The suits used onboard a Soyuz normally would be Sokol suits, but these are not fit for an EVA. So how did she get hold of that Orlan suit? She certainly did not take it with her from the ISS when she fled from the fire, dashing into the Soyuz.

- The plot premise: the plot premise itself (an ASAT test creating a Kessler Syndrome in Low Earth Orbit) is clearly modelled after the ASAT test on the Fengyun 1C satellite done by the Chinese in January 2007, even if the latter did not lead to a Kessler Syndrome situation. Following all the critique on China after their 2007 test and all the problems with space debris it created, would Russia really risk to do an ASAT in low earth orbit at altitudes that would create debris at the orbital altitude of the ISS? I seriously doubt that they would be this reckless. They are not stupid, and have kosmonauts onboard the ISS themselves that they certainly would not want to endanger.

Frankly, given that movies often represent US sentiments of the time, I was surprised it were the Russians and not the North Koreans that were cast as the villains, certainly now North Korea has a proven launch capability and a reckless disregard of what the International world thinks of their actions. To pick the Russians instead appears to be a weird, somewhat anachronistic choice by the script writers. Maybe it originates in resentment about the fact that the US human spaceflight program is currently completely dependant on the Russians?

[note added: as Brian Weeden rightly remarks in a comment to this blogpost, a Kessler Syndrome takes a long time to develop. It is incommensurable with the timespan of events depicted in the movie]

- The lost communications: In the movie, communications with Houston are lost. This is because the communication satellites that do the relay from the spacecraft to the groundstation, are knocked out by the swarm of space debris, according to the movie plot.

This does not tally for various reasons. The debris is in Low Earth Orbit (the altitudes of Hubble, ISS, Tiangong). Communication relay satellites used by the Shuttle and ISS are however TDRS satellites, and these are in Geocentric Geostationary orbit, i.e. much higher than a debris stream in Low Earth Orbit. A Kessler Syndrome scenario in Low Earth Orbit would not lead to satellites being taken out at Geostationary altitudes. Communications would hence not be lost.

In addition, during parts of the orbit direct communications with groundstations in the US, Europe and Russia would be possible via FM during direct overflight (which gives about 10 minutes where communications are possible). Soyuz spacecraft frequently communicate directly with ground stations in Russia at 121.75 MHz FM and occasionally the ISS does as well via 143.625 MHz. It is also weird that Bullock would be able to receive and communicate with Aningaaq in Greenland, but not with groundstations in the US or Russia.

- Speaking about the radio contact with Aningaaq (1): what radio operator would not know what “Mayday” means?

- The radio contact with Aningaaq (2): Interestingly Soyuz voice (121.75 MHz) is FM, not AM, while Bullock says she receives Aningaaq in AM. The Space Shuttle voice modulation was AM (at 259.700 MHz). So perhaps someone advising the script writers confused the Soyuz and Shuttle radio modulation modes.

The airband emergency frequency (121.5 MHz) is close to the 121.75 MHz Soyuz frequency, so Bullock might have been using that for her Mayday call.

- The orbits (1): this is another point where reality notably had to be sacrificed to suit the plot. Apart from the Shuttle, three spacecraft play a role in the story: The Hubble Space Telescope; the International Space Station ISS; and the Chinese Space Station Tiangong. In the movie, these appear to be portrayed as being in quite similar orbits and constantly in relatively close proximity, with visibility to the astronauts. Tiangong for example is close to the ISS for the better part of the movie, with a multiple times referred to distance of “100 miles”. This is all far from reality.

To start with, the three objects are at quite different orbital altitudes: an average orbital altitude of 350 km for Tiangong; 420 km for the ISS; and 555 km for Hubble. As a result they will move at different speeds and hence not remain in close proximity for long. Hubble is moving at an orbital velocity of 7.58 km/s; ISS at 7.66 km/s; and Tiangong at 7.70 km/s.

In addition to this, their orbits are dissimilar in orbital inclination as well. Tiangong has an orbital inclination of 42.77 degrees: ISS of 51.65 degrees; and Hubble of 28.47 degrees. Their RAAN (Right Ascension of the Ascending Node) values are quite different most of the time too, although there are moments that the RAAN values of ISS and Tiangong almost coincide. Hence, they are never truely in close proximity. Most notably, they are moving in quite different orbital planes, apart from the different orbital altitudes.

Because of the inclination and altitude difference, Tiangong would never be in eyesight range of ISS for a prolonged time period and their mutual distance would rapidly change (and mostly be very large). The same is true for the ISS as seen from the Hubble orbit.

In order to get from Hubble to the ISS, Clooney (tugging Bullock on a tether) in his modest Manned Maneuvering Unit (MMU) would have to have lowered his orbital altitude by 135 km and changed the inclination of his MMU’s orbit by 23.18 degrees. He would likely also need to change the RAAN of his orbit, perhaps considerably. This cannot be done by a simple MMU. In fact, it even couldn’t be done by a Space Shuttle, which is why after the Columbia disaster, Shuttles were barred from flying to anything else than the ISS (because from another orbit they wouldn’t be able to get to the ISS for a safe retreat, if the tiles of their Shuttle would turn out to be damaged after launch). So Clooney is flying a Hell of an MMU there!

Let’s do some of the math (and I hope I am not making any mistakes here). In order to lower the perigee of his orbit from the altitude of the Hubble to the altitude of the ISS, a speed change delta V of about 38.3 m/s would be necessary. And this would need to be done at a carefully chosen moment, not a random moment, so that reaching perigee matches the ISS passing the same point in space. The maximum delta V capability on existing MMU’s such as employed on some Space Shuttle EVA’s was about 24.4 m/s: i.e. the MMU in the movie needs to have at least 1.6 times as much thrust as existing MMU’s. Assuming it is an improved version (the conversation between Houston and Kowalski in the start of the movie seems to imply that), that is perhaps possible.

Still, this concerns the difference in orbital altitude alone and does not solve the much more serious problem of the difference in orbital inclination, and any differences in RAAN. To change this would need quite some extra thrust. For the inclination change alone, a delta V of 3.077 km/s would be necessary, certainly impossible for an MMU. Changing RAAN is even more complex, and takes a lot of time.

Bullock flying the Soyuz to Tiangong with a single retrorocket fire is similarly problematic. She would have to change her orbital inclination by 8.88 degrees and lower her orbital altitude by 70 km, and likely change the RAAN of her orbit significantly as well. This is a complex manoeuvre. A single retrorocket fire would not do the trick. I doubt a Soyuz could even do this all with the main engine working.

- Failure to rescue Clooney: It is also somewhat silly that Clooney (after he let her slip, which was already unnecessary, see later) indicates to Bullock (who says she will fetch Clooney with the Soyuz) that he already has “too much of a head start”. Clooney’s MMU and the ISS and Soyuz are moving coplanar at that moment, at similar orbital altitudes, and in close proximity even if Clooney is slowly drifting away (as he should, his drag coefficient is different). So Clooney should be way more easy to reach for the Soyuz, than the Chinese Space Station Tiangong is. Compared to reaching Tiangong the needed velocity changes to reach Clooney are very small and necessary orbital plane changes are nihil. Not only did Clooney let go of Bullock in a way that was unnecessary: his insistence that she cannot use the Soyuz to fetch him is stupid, for she could! (that the Soyuz was out of fuel, Clooney did not know yet at that time).

- The orbits (2). Just as Bullock is reaching Tiangong and about to leave her Soyuz to try to rocket herself to Tiangong using a fire extinguisher as propulsion (!), we can see her passing over Scandinavia, north of Denmark (Jutland and Bornholm are recognizable, and the northern tip of Jutland is pointing to the spacecraft, so the Soyuz must be passing North of it). Slightly later the Polish coast is briefly visibly too. This is implying a pass at latitudes above 55 degrees north. Tiangong, with an orbital inclination of 42.8 degrees, in reality never comes higher in latitude than 42.8 degrees north. So a pass north of Denmark at 55+ North latitudes would be completely impossible. Bullock should have left the Soyuz for Tiangong over Southern France, not Scandinavia.

- The orbits (3): the swarm of space debris appears about every 90 minutes. As a ballpark figure that is right for the orbital altitude in question: at the altitude of the Hubble the orbital period for a circular orbit is about 95.5 minutes, at the orbital altitude of the ISS it is 92.5 minutes.

Yet the fact that the swarm of Space debris is still so concentrated that it appears only briefly every 90 minutes, does not tally with the implied Kessler Syndrome that is the premise of the movie plot. The Kessler Syndrome is only possible when debris starts to spread far and wide.

- Debris speeds are twice mentioned. Houston mentions “20 000 miles per hour” which is about 8.9 km/s if Statute miles are meant, or 10.29 km/s if Nautical miles were meant. The first value is somewhat correct, certainly if we regard the “20 000 miles per hour” as a ballpark figure only. Any debris orbiting in more or less circular (and prograde) orbits at the altitudes of the ISS and Hubble will move at about 7.6 km/s.

Later, Clooney (if I have heard it correctly) talks about “50 000 miles per hour”. The latter value translates to 22.35 km/s (Statute miles) or 25.7 km/s (Nautical miles) and are speeds impossible for debris in orbit around earth. At this speed they would fly off into interplanetary space.

[note added: as Brian Weeden correctly remarks in a comment to this blogpost, relative speeds between two objects can be higher than the mentioned orbital speed of 7.6 km/s: up to 15.2 km/s (7.6 + 7.6 km/s) if the objects move in opposite directions. So, was Houston talking about orbital speed, or relative speed? The speed Clooney seems to mention is clearly too high however]

- The orbits (4): As Bullock is holding on to Tiangong, the debris swarm approaches again. Tiangong is already starting to re-enter (more about that later) and starting to sport what looks like the beginning of a plasma tail (the “streamers”, looking like a sort of contrails – I have no idea what the movie-makers really meant to visualize here). In this part of the movie, one hence can infer the direction of movement of Tiangong in relation to that of the debris swarm. It appears that the debris swarm is coming from the exact opposite direction. That would only be possible if the debris is moving in a retrograde (east-west) orbit. This is highly unlikely as almost all satellite and space debris circling the earth is in a prograde (west-east) orbit.

 - Speed differences: on a related note, with speed differences up to several km/s, I doubt Clooney would be able to visually identify debris pieces as being part of a particular satellite type. Debris should have zipped by much faster than portrayed in the movie, unless it is in a very similar orbit as Clooney and Bullock are.

- The re-entry of Tiangong. Bullock almost reaches the Chinese Space Station Tiangong too late. It is about to re-enter into the atmosphere. It is not at all clear why, however. Space debris impacts would not make it re-enter early, certainly as the station appears to be still quite intact. From its current orbit with no orbit boosts, I calculate it would take Tiangong at least five months to come down to re-entry altitudes, not a mere few hours. Tiangong in the movie is much larger (and hence experiencing more drag) than the real Tiangong currently is, but still: it would take weeks to months to come down, not hours.

On another note, I doubt that the Shenzou reentry vehicle would have survived the kind of violent tumbling that the Shenzou displays at the start of the re-entry, already sporting prominent plasma phenomena. In this stage of the re-entry descent, it would have probably not been nice on the physiology of Bullock either.

After Bullock has landed in the lake and emerged from the sinking landing module on the lake surface, re-entering space debris appears in the sky. I assume this is meant to be the final demise of Tiangong. In reality, given the amount of time it takes the landing capsule to re-enter (during which it decellerates) and parachute down, any debris of Tiangong would already have decayed many minutes before.

 - Being peppered by space debris: I found it odd that in both ISS and Tiangong, as well as the Soyuz and Shenzou, the Space Stations and spacecraft were still pressurized despite having been peppered by space debris. One would expect damage, punctures, and hence loss of pressure, certainly given what happened to the much smaller Shuttle earlier. If I was Bullock, I would have kept a pressure suit on at all times. In fact, situations like this require that and I strongly doubt a trained astronaut would strip off her suit under these circumstances.

- I found it unrealistic that Kowalski (Clooney) would not know where his team member Ryan (Bullock) would live, nor that he would not know whether she was married or not. It is not that they would not have met, extensively trained together etcetera before the start of this mission. Astronauts, certainly those on the same mission team, would generally know each other’s personal life quite well.

- Why did Clooney let go? Phil Plait also remarked on this. Once the parachute ropes captured Bullock and Clooney tethered to her, there would have been no clear pull on the pair anymore. Clooney didn’t need to let go and die! And again: Bullock could in theory have used the Soyuz to fetch him.

- The fire in the ISS: With a fire of that magnitude, I would expect oxygen to get depleted very quickly. Bullock would have asphyxiated.

Concluding remarks

So far my science takedown. Now, does this all mean that the movie is rubbish? No, not at all: as I already mentioned, one of the very cool things about this movie is actually how much (compared to many other disaster and space-themed movies) it gets right, certainly in the details of the Space hardware.

This is a movie, and movies sometimes take shortcuts with reality. It is fiction, after all. In other space-themed movies, the science is usually much worse than it is in this movie. Violating the rules of orbital mechanics is a common theme in SciFi movies, where movies like Star Wars are much worse offenders than Gravity is.

In the end, the thruth is that the makers of Gravity have managed to produce a movie that is grabbing your attention from start to end, with visualizations that, if not always scientifically adequate, nevertheless appear superbly realistic to the eye in the way they are graphically visualized. Only a Sheldon Cooper would not appreciate this and focus on the scientific inadequacies only.

So my advise: just go and see and enjoy it. You won’t regret it!

(Note added later: ESA astronaut Samantha Cristoforetti has a Google+ post with more scientific and engineering inaccuracies in the movie here (pt. 1) and here (pt. 2), worth a read)

Saturday 26 October 2013

A short update on GOCE, five days after engine cut-off

It is now five days ago that the ion engine of GOCE, the European Space Agency's 1-tonne Gravity field and steady-state Ocean Circulation Explorer scientific satellite (2009-013A), cut off after the satellite ran out of fuel. Originally orbiting at an average orbital altitude of 227.5 km, it is since coming down (see earlier post here). The diagrams below provide an update on the evolution of the orbit's apogee, perigee and Mean Motion (see also earlier post here for explanations) .

 click diagrams to enlarge

As I write this, the average orbital altitude has dropped over 8 km already since the ion engine shut down on October 21st. Yesterday afternoon (25 October) the average orbital altitude had already dropped below 220 km altitude, with the perigee now below 217 km (see first diagram above).

GOCE is currently losing altitude at a rate near 2 km per day. That rate will notably increase over the coming days, as GOCE will drop faster and faster.

It is too early yet to provide meaningful estimates of the re-entry date. My current prognosis (for which I used Alan Pickup's SatAna and SatEvo software) suggests a re-entry in the second week of November, but note that this date will probably shift to an earlier date over the coming week.

Currently, the satellite is still maintaining an attitude (orientation) that is optimizing for low drag. It does so using magnetic torques. At some point close to re-entry, that system will probably fail and GOCE will then lose attitude control. As it does so, drag will increase, which will seriously influence the re-entry date, shifting it to an earlier time. Meanwhile, the sun has been quite active the last few days, which speeds up the decay as well as the density of the Earth's outer atmosphere changes under the influence of solar activity. My very preliminary prognosis was made with an average forecast F10.7 cm solar flux of Fx=135 for the coming 20 days, but unexpected solar outbursts might alter the true picture.

I expect that several sources will start to provide re-entry estimates in the days before re-entry. The chief authoritive sources will be ESA itself, and the TIP-messages by USSTRATCOM on their  Space-Track portal (needs an account to access).  I expect that several independent analysts will provide re-entry estimates as well. The Aerospace Corporation provides re-entry estimates based on their own re-entry models, but is usually lagging behind. Their predictions sometimes strongly differ (up to several hours) from the final re-entry times and locations determined by USSTRATCOM (which I consider to be a more reliable and authoritive source).

(note: in the first diagram above, the values for perigee and apogee show some short-term fluctuation during the first 2-3 days after engine cut-off. These fluctuations are the result of errors in the orbit determinations, which easily occur (and are inevitable) when the data-arc used is still short and fitting error margins are hence wide. As the observational arc grows, the orbital determinations become more stable, which is indeed what we see over the last two days.
The apogee and perigee altitudes in the first diagram have been calculated from the values for Mean Motion and eccentricity using a fixed Earth radius of 6378 km, ignoring Earth's oblateness. Orbital elements are from USSTRATCOM (needs an account to access) with a secondary source (open access) here)

Note 31 Oct 2013: a new update here.

Tuesday 22 October 2013

[Updated diagrams] GOCE is falling!

[diagrams updated 23 Oct 2013, 9:15 UT] 

GOCE, The European Space Agency's 1-tonne slick Gravity field and steady-state Ocean Circulation Explorer scientific satellite (2009-013A), is now truely coming down.

During the night of October 17-18, fuel reserves became so low that the pressure in GOCE's ion engine fuel system dropped below a critical 2.5 bar.  Next, between October 21.12 and 21.54, the ion engine stopped functioning, and as a result GOCE is now clearly losing altitude.

click diagrams [updated 23 Oct 9:15 UT]  to enlarge 

The first orbital determinations after the engine cut-off on October 21 are still inaccurate and as a result they are fluctuating, as the observational arc is still very short. But in the diagrams above, it can be clearly seen that the Mean Motion (the number of orbital revolutions per day that the satellite makes, i.e. how many times it circles the earth each day) jumps to much higher values. More orbital revolutions per day means that the orbit is getting smaller. The orbit getting smaller means the satellite is coming down.

This can be seen in the second diagram too. The apogee (the highest point in GOCE's slightly elliptical orbit) is steadily coming down since yesterday. The perigee (the lowest point in GOCE's slightly elliptical orbit) is dropping too.

GOCE's ion engine, when still working, provided a force countering the drag that the satellite experienced from the outer layers of the atmosphere in its low ~225 km orbit. As a result the drag parameter Bstar fluctuated around zero. When the ion engine cut out, the satellite suddenly experienced the full force of atmospheric drag. This can be seen in the lowermost diagram, which shows that the drag parameter Bstar made a strong jump to high positive values. The drag slows down the satellite, and as a result it drops in orbital altitude.

Over the coming days GOCE will rapidly lose altitude. So shortly after ion engine cut-off it is still too early to provide an accurate prediction about when it will truely re-enter and largely burn up: but as a ballpark figure this will happen somewhere between 2 to 3 weeks from now, somewhere during the first two weeks of November. In the days before re-entry, I will update re-entry forecasts on this blog.

Most of GOCE's one-tonne mass will burn up on re-entry, but some 250 kg (in many small fragments) might survive re-entry. At this point, it is still impossible to predict where (and when) these fragments may come down as that is dependant on many contributing factors, some of which are difficult to predict (e.g. the effect of fluctuating solar activity on the density gradient of the atmosphere). It will only be possible to predict this with some confidence in the final hours directly before re-entry.

Although the satellite is now without propulsion, its scientific sensors are still working. GOCE will continue to gather important scientific data on the Earth's gravity field until very shortly before its final demise.

The satellite controllers at ESOC have told me they have put the satellite in Fine Pointing Mode: a series of magnetic torques which react to the Earth's magnetic field keep the satellite stable in attitude (orientation), preventing it from tumbling, even though it has lost propulsion.

Since 2009, the GOCE satellite has gathered highly detailed data on the Earth's gravitational field and ocean surface heights.

Note: the apogee and perigee altitudes in the 2nd diagram were calculated with a fixed Earth radius of 6378 km, ignoring Earth oblateness

My last view of GOCE, an image taken on 29 September 2013 during a twilight pass over Leiden (click image to enlarge)

Saturday 12 October 2013

Past and future of the KH-11 Keyhole/Evolved Enhanced CRYSTAL constellation (part 4)

In a number of previous posts from the last month (this one being the most pertinent one), I probed the changes to the KH-11 Keyhole/CRYSTAL optical reconnaissance satellite constellation over the past 8 years, aiming to predict what will happen now USA 245  has been added to the constellation on 28 August 2013 (launch NROL-65).

The previous analysis was focussed on the orbital planes of the satellites. In this fourth post in this series, I will take a look at other orbital parameters, such as apogee and perigee heights, eccentricity and mean motion.

West plane KeyHole/CRYSTAL satellites:
 USA 129: launched in 1996,
now in secondary West plane, 
probably soon to be de-orbited?
(imaged 28 Sep 2013)

 USA 186: launched in 2005,
soon to switch from primary West plane to 
secondary West plane?
(imaged  5 October 2013)

USA 245: launched 28 August 2013
into the primary West plane
(imaged 5 October 2013)

Let me first briefly summarize the previous analysis. In these I showed that the KH-11 constellation consists of two primary orbital planes separated by 48-50 degrees in RAAN. In addition, each primary orbital plane has an accompanying secondary orbital plane, 10 degrees more west for the West plane and 20 degrees more East for the East plane.

Satellites are initially launched into one of the primary planes, in their primary mission: after a couple of years, and after a replacement has been launched into the same orbital plane, they shift to the accompanying secondary plane, going from primary mission into secondary extended mission.

For example, USA 129 did this in 2006 after the launch of USA 186; and USA 161 did this in 2011 after the launch of USA 224. I pointed out that I expect USA 186 to do the same early 2014 following the recent launch of USA 245 into the West plane. I also expect USA 129 to be de-orbitted.

The graphic summaries given in that previous post, were these two images (see previous post for discussions):

Shifting from primary to secondary orbital planes is however not the only thing that happens. When we look at various orbital parameters, we can see other, accompanying patterns, notably in the apogee and perigee heights:

(click diagrams to enlarge)

(note: all the orbital parameters used in the diagrams above have been determined by Mike McCants from amateur observations, including mine).

New plane, lower apogee altitudes, and more circular orbit

For example: in the previous post on this topic it was discussed how USA 161 (2001-044A) in the East plane manoeuvred from the primary East plane to the secondary East plane late 2011 by changing its RAAN by 20 degrees (i.e., by rotating its line of apsides). This followed the launch of USA 224 (2011-002A) into the primary East plane, as a replacement for USA 161.

In the diagrams above, we can see that other orbital changes took effect as a result of the same series of manoeuvres. In addition to its orbital plane, USA 161 (blue dots in the diagrams) also changed its orbital eccentricity and its apogee and perigee heights. The apogee height was significantly lowered (which initially confused analysts at the time), from about 960 km to eventually about 390 km altitude. The perigee height was raised somewhat, from 310 km to 390 km altitude. The result is a much more circular orbit.

The inclination of the orbit was also changed, by about one degree. The reason for this can be seen in the lowermost diagram: with the changes in apogee and perigee altitudes, the orbital inclination had to be changed to make the resulting orbit sun-synchronous again.

In all, although much of this was accomplished within 6 months after the massive manoeuvre of late August 2011, it took USA 161 about a year to settle in its new orbit.

A repeat of an earlier case

Earlier, in 2006-2007, changes in the orbit of USA 129 (1996-072A) in the West plane can be seen to follow a somewhat similar pattern.

After the launch of USA 186 (2005-042A) into the primary West plane in 2005, USA 129, by that time already 10 years old and hence quite of age, moved to the secondary West plane by changing its RAAN by 10 degrees. Accompanying this move, is a change in perigee and apogee altitudes. The perigee is gently raised from about 280 km to eventually 310 km altitude. The apogee is lowered from about 1020-1030 km to eventually about 770 km altitude. The orbit becomes much more circular as a result.

With USA 129, this process took much longer than with USA 161 and the changes are less drastic. Yet the ideas behind them are clearly similar to what USA 161 did five years later: change orbital plane from primary to secondary plane, lower apogee significantly, raise perigee gently, and circularize the orbit (although not to the degree like USA 161 later did).

The more gentle approach taken by USA 129 in 2006-2007 compared to USA 161 in 2011-2012 might implicate either of these two scenarios:

(a) USA 129 had less fuel reserves left in 2006 than USA 161 had in 2011;
... or (and I prefer this explanation):
(b) it was anticipated in 2006 that the lifetime of  USA 129 needed to be prolonged untill well after the initial lifetime estimates, putting restrictions on fuel use for manoeuvres.

Remember: this is around the time the KH-11/CRYSTAL follow-up program, the FIA Optical program, entered delays and was next cancelled. So option (b) could well be the case.

What to expect?

Based on these past patterns, I expect USA 186 to do the following things by means of  a series of manoeuvres starting the first months of 2014:

1) change RAAN by 10 degrees (i.e. rotating its line of apsides), moving itself from the primary West plane into the secondary West plane (see previous post here);

2) drastically lower apogee (currently at about 1020 km) to about 390 km altitude;

3) gently raise perigee (currently at 260 km) to about 390 km altitude.;

4) circularize its orbit as a result of (2) and (3);

5) change inclination by about one degree to re-attain sun-synchronicity after the altered apogee and perigee altitudes.

These changes should take a few months and be completed towards the end of 2014. They will likely be initiated by a large manoeuvre early 2014 (in February or March likely).

As mentioned earlier I expect USA 129 to be de-orbited this winter or spring.

Why the apogee and perigee changes?

One question pertaining is: why these changes in perigee and notably apogee? Is a circular ~390 x 390 km orbit easier to maintain? Is there instead some operational reason behind this change in altitudes, in terms of desired track-repeat intervals or equipment performance (e.g. demands of image resolution)? If  so, why are similar changes not made to the orbits of the primary plane objects but only to the secondary plane, extended mission objects? I have no answers, and at best I can speculate from a few ideas I have. That is not for this blog, however.

This post benefitted from discussions with Ted Molczan and Cees Bassa. Interpretations and any errors theirin are mine.