Thinking about this some more, I must wonder if this Apollo 'rock skipping' analogy is real, or merely a simplification for the general public. To 'skip', the object must somehow get some lift or at least asymmetric fluid pressure on the vessel. I do remember that Apollo CMs (command modules) during reentry were not aimed with the blunt heat shield squarely forward, but rather were tilted upper-edge forward so that there might possibly be some push up. If hypersonic aerodynamics can do that, which isn't something I know anything about. The 'simplification' angle is that the CM was in a high energy highly elliptical orbit, with perigee aimed low enough to dip into the atmosphere just enough to drag it down on the first pass. Aim the perigee a bit too high, and it doesn't collide with enough air molecules to get dragged down on that first pass, thus getting (or needing) to go around again. In a high orbit taking weeks, not hours. (The opposite, aim too low, would crush the crew with excessive g forces.) No true skipping, merely inadequate drag to get slowed down before punching through and out the other side of the spherical shell of thin air. This later scenario is known to happen in real life, e.g the Daylight Fireball of 1972. Though this object was not in earth orbit, but both entered and exited faster than escape velocity. And my sister even witnessed it, though couldn't get back from the fields in time to notify the rest of us before the smoke trail dissipated. In short, I don't know the full story behind this feared Apollo 'skipping off the atmosphere' scenario.
That is the consensus. Initially the orbit is elliptical but as it goes lower, the higher drag makes it more circular and lower. Bob Wilson
Plot of orbit becoming more circular in recent days. Because eccentric is unbecoming. By way of comparison, Earth orbits Sun with eccentricity 0.0167.
... since then, 5.9 and 6.9 km. Latest forecasts show narrowing time windows: US-StratCom: 1 Apr 2018, 21:30 (±10 hours) Aerospace Corp: April 1st, 2018 16:15 UTC ± 9 hours. SatFlare: not updated?
Upper right panel in Tiangong-1 Reentry | The Aerospace Corporation Now shows sharp descent in both apogee and perigee at +20 hrs. But as that graph has been so flaky recently, it is hard to consume. Their other data suggest about 25 orbits now remain for TG-1, and thus that 158 km may be where TG-1 stubs its toe. If you say 160, I have no basis to disagree, but that number is wotrth knowing. Each year (month, week) other rocket-booster junk gets low and stub their toes and re-enter uncontrolled. TG-1 is recently big cf. Mir/2001 130 tons. More recent re entries are 1 or few tons I think about what it would be like to be in TG-! right now. Strong vibration at perigees? Starting to get hot inside the can?
That looks like the same downtick I was seeing back before you mentioned the uptick spotted yesterday. Both of which disappeared by the time we were discussing it. Absent better information, I'll write it off as something flaky in the graph. Back in that exercise about drag forces and orbital decay, I computed that when it gets to dropping 1 km/orbit, aero drag power will be roughly 10kW averaged around a full orbit. More at perigee, less at apogee. Right now, it is dropping roughly 0.5 km per orbit. So drag power is crudely averaging 5kW. Almost all of that should be materializing as heat, some on the forward skin of the vessel itself (including solar wings, probably taking much more than the station core), and some shed in the very thin but hypersonic plasma trail left behind. So it may very well be getting hot inside that can.
So I threw together a quick spreadsheet of projected viewing times this morning, March 31: area time 1 East Coast 04:09 AM 05:37 AM (EDT) 2 Mississippi Valley 04:37 AM 06:05 AM (CDT) 3 Midwest Arizona 05:05 AM 06:33 AM (dawn?) (MDT) 4 West Coast 04:05 AM 05:33 AM (PDT) I used crossing latitude 29 degrees, the Gulf Coast, and orbital period 88 minutes. These will be ascending passes from south to north. Lack of coffee, local dawn, and clouds may inhibit viewing. Close to perigee on these passes. Observations: First pass - late to reach observation point, one meteor and a redeye to the East. No satellite tracks which suggests too early for sunlight. Second pass - predicted 14 degrees maximum elevation to the North. Marginal but I'll give it a try. Full moon, haze to north, visible star at 30-45 degrees. Bob Wilson
Although it is easy to think we might be able to dodge incoming satellite parts, they have more in common with shrapnel: Bob Wilson
This site shows the next potentially visible pass date and time: TIANGONG 1 Satellite details 2011-053A NORAD 37820 If the "MAX ELEVATION" is close to 90 degrees, it is time to wear a tin foil, protective hat. Bob Wilson
And it is gone, again, now showing 24 hours of smooth decline. But not the last few orbits of a 28.47 hour life expectancy. But the graph still shows a difference between apogee and perigee of 11 to 12 km, while the numbers in a different frame show only a 3. 56 km difference. Heavens-Above does show an 11 km difference. I'm well north of the path right now, so haven't been wearing my tinfoil hat at all. There was a 24 degree pass this morning, in daylight, but my highest remaining pass is only 22 degrees,also in daylight.
Last night, the moon was in the path and high clouds to the west. Tonight, the moon was an hour later and out of the direct path. All I saw was an owl and ursula major. I skipped the 5AM pass because it was also going to be 'in the haze.' Bob Wilson
Barred owl perhaps? == About 6 orbits from now, we enter the range of TG-1 de-orbit predictions that span 4 orbits. Ignoring their stated uncertainty ranges. Then we can find something else to obsess over.
Phil Plait liked photos of this down-goer in 2016 Jan: Chinese rocket re-entry caught by Steve Cullen.
