China's Tiangong-1 space station: Perhaps not a disaster yet, but keep looking-up just in case... 9

[JohnRBaker]

China's Tiangong-1 space station will crash to Earth within weeks

Experts say it is impossible to plot where module will re-enter the atmosphere, but the chance is higher in parts of Europe, US, Australia and New Zealand

https://www.theguardian.com/science...pace-station-will-crash-to-earth-within-weeks

John R. Baker, P.E. (ret)
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Irvine, CA
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[JAE]

Chicken Little

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[TenPenny]

Somehow I like this paragragh:
"The US-funded Aerospace Corporation estimates Tiangong-1 will re-enter the atmosphere during the first week of April, give or take a week. The European Space Agency says the module will come down between 24 March and 19 April."

Yes, technically, there is a slight difference between 'the first week of April, give or take a week', and 'between 24 March and 19 April', but in this context, the difference is not particularly meaningful.

 

[SuperSalad]

TenPenny,

Yes, but if it does come down in the first week of April, the Aerospace Corporation will appear to be brilliant with their "more accurate" prediction, whereas if it comes down in the larger window of time they both are actually predicting, they'll be equally as correct as the ESA​

Andrew H.

http://www.mototribology.com

 

[IRstuff]

In case anyone is curious, here's the output from STK for one orbit of TianGong-1. STK is absolutely free for doing this sort of thing right out of the box. You can pay to do more, but it's pretty awesome as a free tool. Much more fun that some other programs that are free.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[cranky108]

The question is, how many big pieces won't burn up on reentry?

Another is how many small space garbage is it collecting, vs how much is it depositing?

Is there a positive here, or is it all negative?

 

[JStephen]

Remind me to carry my umbrella, Wiley Coyote-style.

 

[IRstuff]

"Another is how many small space garbage is it collecting, vs how much is it depositing?"

The stuff it is collecting is essentially irrelevant, as they are extremely small. Whatever it winds up generating will virtually all fall back to Earth, since they'll be in too low an orbit to be sustainable. Anything big that it hits will likely result in a spray of particles and objects, and would be a losing proposition.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[VEBill]

"...it is impossible to [know] where module will re-enter..."

Wiki says: "Orbital inclination: 42.8 degrees"

Anyone north of about 43°N or south of about 43°S is probably safe. At least from this particular risk.

_{Checks location, confirms 45°N. Phew!}

Fairly good odds (50-50) it'll hit the Pacific... ...maybe.

 

[cranky108]

Is it hard to predict because it might start tumbling?

 

[VEBill]

Reportedly, the upper atmosphere moves up and down depending on the Sun's mood and other factors. Effective upper atmospheric height is a very critical factor due to the high order positive feedback, as it rapidly cascades into re-entry. Since 45 minutes is halfway around the world, and the inclined orbit shifts with each pass, it's all a bit semi-random day to day. That's presumably the most influential factor.

Re-entry, start to finish, can cover a couple thousand miles. But once it starts, any drag or tumbling in the atmosphere would be closely followed by almost immediate disintegration into bits, so that tumbling would merely adjust the outcome within a small region. Drag while still in the very wispy upper reaches would be affected by the station's aspect angles relative to the movement; so you'd have a good point there.

Tidbit: The Apollo CM had manoeuvring capabilities during re-entry, so they could be steered to land a few miles from the recovery fleet, if they were skilled and lucky on the day. The CM actually went back up (400,000 feet ?) to cool off, and then fall back down again.

 

[cranky108]

I remember that back up in the charts, but it did not understand enough to ask (besides who would I ask).

I also assume the orbit is a little out of round, so it would have some high and low points in the orbit.

 

[VEBill]

Not very eccentric. But most of the action will happen at perigee, due to the high order positive feedback.

The orbit data is extracted from the following two-line orbital elements,
1 37820U 11053A 18066.28711319 .00173836 22757-4 17260-3 0 9997
2 37820 42.7506 1.1197 0016400 192.5561 300.3591 16.10185824369864
Epoch (UTC): 07 March 2018 06:53:26
[highlight #FCE94F]Eccentricity: 0.0016400 [/highlight]
inclination: 42.7506°
[highlight #FCE94F]perigee height: 235 km
apogee height: 257 km [/highlight]
right ascension of ascending node: 1.1197°
argument of perigee: 192.5561°
revolutions per day: 16.10185824
mean anomaly at epoch: 300.3591°
orbit number at epoch: 36986

Ref:

http://www.heavens-above.com/orbit.aspx?satid=37820

 

[btrueblood]

Apollo and similar also had a spacecraft that was command to fire braking thrusters to control time, position, and angle of re-entry. The Chinese station is reportedly no longer under ground control, and so is going to re-enter at some random combination of those factors.

For comsats in geostationary orbits, there is a protocol that requires spacecraft to reserve enough fuel for them to be put into parking orbits away from useful slots, to prevent collisions when replacements are sent up. And at least Iridium, if not others, using low orbits, has (had?) planned for de-orbiting fuel reserves, i.e. dropping their spacecraft back into the oceans, to similarly keep their orbit stations clear and also to drop them into less populated areas.

 

[VEBill]

Somewhere on YouTube is an old (1960s) film (now video), about 20-30 minutes in length, where the whole CM re-entry process is explained. It's much more complicated than one might assume.

The Apollo CM had to be steered into a fairly narrow re-entry corridor, to avoid burning up or skipping back out. So the CSM (CM + SM still attached) was steered into the approach corridor (using thrusters) before the SM section was jettisoned. Once the CM was on its own [and in the atmosphere], they could use aerodynamic steering by rotating the CM on its yaw (?) axis, using little thrusters to turn, to have the center of mass above or below the center of pressure. It actually went back up, trading speed for huge altitude gain (for cooling). One benefit of this steering was that it widened the allowable corridor from something like 2°, which was impractically narrow and thus dangerous, to a more reasonable 8° (these numbers from memory, so might be incorrect).

 

[btrueblood]

part 1:

https://www.youtube.com/watch?v=G-6VQsVoc1I

part 2:

https://www.youtube.com/watch?v=cOiVKUI07G0

 

[Wallache]

Great videos! Those old films are so much better at conveying a subject than the quarter-assed PowerPoint presentations that pass as training these days.

 

[btrueblood]

Um, well, thanks for the stars, but VE's the one who pointed me towards them, so gave him one from me...

The YouTube channel that hosts those videos does have a bunch of other old training films (from WW2 onwards), and yes, the level of information and use of humor make for good watching.

 

[Sparweb]

Mission reports from all Mercury, Gemini, Apollo missions plus many other subjects:
Apogee Books:

http://www.cgpublishing.com/

STF

 

[Sparweb]

Oh, and how could I forget, on a thread about a space station with a decaying orbit:
Skylab


STF