Not that anyone asked, but atmospheric density in the zone of interest is about 2x10^-10 kg/m3 2x10^-13 in the (water=1) units above. That ain't much. But our contestants are moving at nearly 8 km/second which means lots of little bumps. ISS (400 km up) has two orders of magnitude less bumps but still needs a re boost every few months.
Do note that for falling satellites, it is cross sectional density that matters, not the volume density we earthlings normally use. Kg/m^2, not kg/m^3. So for objects of the same volume density, size still matters. For a sphere like HS, figuring this is easy. But for a large station with big flappy solar wings, and tumbling uncontrollably, this is difficult to figure. Posted via the PriusChat mobile app.
Certainly true, and varying perhaps 10--fold with orientation: Supposing sturdy bit that may reach ground (water) are docking ring (near end) and largest rocket motor (far end). Plus hydrazine tanks that are stainless steel balls, probably. I do wonder whether it 'weathervanes' near perigee it if happens to get there sideways. Solar flappies seem unlikely to survive until the very last orbit. Not built to withstand any sort of wind load. This art suggests otherwise:
I don't recall any discussions of previous satellites weathervaning prior to reentry. But plenty have tumbled out of control, often with very significant spin rates that intuitively seem too high for weathervaning stabilization. And the highly variable aero cross section makes mincemeat of any attempted precise reentry forecasts. ================= How much wind force should we guess that it takes to snap off those flappy wings? Just making a swag here, without my handy old equation set: if it takes 100 pounds-force, then by the time they finally break off, they have already slowed this 8.5 ton craft enough that it isn't making another full orbit.
Sounds good to me. On Heavens-Above, TG1 perigee is latest reported 210 km. Humanity Star has got down to 171. When do things really fall apart? Have seen range of 160 to 120 km for that, and might HS fail earlier because of its fragility?
Since this was hoped to be a teaching thread, let me try an exercise and simple lesson in basic orbital mechanics: How much drag does it take to drop this satellite 1km during a single orbit? To do this, the potential energy must be reduced by the same amount as lowering this satellite's mass by 1 km. In reality (an illustration for a future exercise), only half of this energy loss must be removed by rockets or drag, because the other half is converted to kinetic energy (the lower orbit requires a higher speed). This balanced 50-50 split is quite convenient when making swags. Round TG1's mass up to 10 metric tons. Local 'g' at this low earth orbit is little different than at the earth's surface, so round 'g' up to 10 m/s^2. Thus, lowering TG1 by 1000 meters means a potential energy loss of (10,000 kg) * (10 m/s/s) * (1000 m) = 10E8 joules (J). Only half of this (5E7 J) represents friction drag loss, the other half goes into increasing orbital speed. Just as energy = force * distance, power = force * speed. Low orbital speed is very crudely the earth's circumference (~40,000 km) in 90 minutes, or 7400 m/s. So to drop 1 km in one trip around, this craft must shed 5E7 (or 50,000,000) joules in 5400 seconds (90 minutes), an average power rate of 10,000 watts (rounded). So the force to do this is Force = power/speed, or (10,000 watts) / (7400 m/s) = 1.35 newton. How much is a newton? A little under a quarter of a pound(-force). Then, put about 100 pounds-force on those solar wings. Keep it up for the perigee section portion of the orbit, let's say lasting about a quarter away around Earth. In a mere 20-something minutes, we have lowered the orbit by ~100 km. At this point, it's life expectancy is minutes, not hours. It won't be coming around for another full orbit. ==================== This is all subject to review for sensibility, so I could be back to delete or update it if blunders are spotted. And feel free to plug in real un-rounded numbers.
The HS website tracking still has it orbiting, as of when I type this. I went out this morning at 5:54am to try to see it as it was scheduled to go overhead. Clear skies. Didn't spot it. Did see other satellites going every other which way though.
OTOH, Satview.org (post #55) has a predicted re-entry time of 14 hours ago. But the site hasn't been updated to reflect that expected demise. Note also that the HS tracking site shows an altitude of 290+ km as I view it. Two other sites show it more than 100 km lower this morning. I.e. HS is using very obsolete orbital elements.
Or... might be why I didn't see it. I was actually hoping to spot the Humanity FireStar as it de-orbited over my house. More than that, maybe a partial scrap of a mirrored side to collect from my backyard. Will have another chance in a week as the Chinese station de-orbits. Perhaps I can score a melted hatch handle or some other remnant. If I do, I'll be sure to post pictures.
Heavens-Above for both of these falling birds has most recently been slow to update 'epochs'. Can't hate on Chris Peat with a free website! Yet it would be nice to know how things have developed.
Did see other satellites@68. This is fun but strongly dependent on how dark your sky is. I went out this early AM to scout my best nearby location. No clouds at all but (after eyes got adapted) I could only see 15 stars. More with binoculars of course, but here's an approximate guide. Eyeballed stars Magnitude threshold 10 - 1.2 20 - 1.9 30 - 2.1 50 - 2.6 Remember that small magnitudes are brighter and the scale goes negative. An ISS overpass can be -1.7 or brighter and even visible through thin clouds. I infer that DavidA seeing sats every which way means he has a darker sky and sees enough stars that it would be tough to count them. For many city dwellers this is a problem. Very few sat overpasses are brighter than mag 2. ISS and TG1 (for another week or so) are. Most rocket bodies and other sats are dimmer. So, go to some darker spot, especially after using Heavens-Above or one of the other websites to choose a good ISS overpass. Or just go anytime. Fun begins about 2 hours before dawn and second act ends about 2 hours after dusk. If you can see a bunch of stars, you will see sat overpasses. If you want to know who's who afterwards, note the time when each one is highest in the sky. Then use the web.
Hehe. Oh, I wish. I am in the suburbs of Chicago! Seeing is near zero, on a good night sky. The "every which way" is somewhat understandable, as there are ~1,000 viewable objects up there to see. Just before daylight and after sunset, one can't help but to see satellites zinging by once you begin looking for them.
What I consider great fun is looking up an Iridium flare prediction on Heavens Above for a clear sky night when I will perhaps be outside with a group. There was one that worked out a couple years ago with a group of students in a parking lot. It was a bright one too, -8.0. Only a second or so, but it gives you that exhilaration similar to experiencing an eclipse. That you are somehow part of a larger clockwork that is constantly turning. A great natural effect that could be used for emphasis if you time it just right.
Local TG1 overpass 0617 hr tomorrow. Probably my last look because she's a'sinkin'. Presently, too much cloud. So we'll see/not see.
Not seen. But now perigee is 199 km, decaying more steeply, and with re-entry predictions going to April 1. == For perspective, re-entry of orbital hardware is not rare. Actually it is about one ton per week over that time I examined. TG-1 is merely a few months' worth, promptly. Not unique. Seems a good thing that such decays routinely occur, or things would get all Kessler-clogged up there. You will not get hit on the head. You will not find a souvenir. You might, slimmest of maybes, see a streak of bright light.
Nice chart of the station's altitude over the past year: Chinese space lab Tiangong-1 to fall to Earth within days - CNN China Manned Space agency, daily updates of average (not perigee) altitude. When we were talking about this earlier this month, it was falling about 1 km/day. Now, after several days of 3km/day, it has fallen 4.2 km in a single day: CHINA MANNED SPACE
by Heavens-above, descent has been 8 km in latest 1.6 days. Clearly in the end game. Perhaps not fair comparison but the last epoch for Humanity star there was 171 km perigee and TG-1 is now 189. It may beat the April 1 prediction.
CMS now says: "The estimated reentry window is between 31 March and 2 April, Beijing time." Daily drops from CMS (oldest ones from memory, as the website shows only the latest four days) are 3.0, 3.0, 3.x, 4.2, 4.3, and 5.4 km. I don't know what time of day this is updated, Heavens-Above could more fresher.
