4 ms·

interesting that they have pretty much no idea where it will end up, i thought they'd be able to estimate that a bit.

by jgh 9y ago

deleted 9y ago
[deleted]
saagarjha 9y ago
Probably because it's hard to predict where it will reach an altitude after which it starts rapid reentry. This could basically happen anywhere in its orbit, since the station is uncontrolled, so there's not a whole lot of confidence in where it will end up.
- Turing_Machine 9y ago
  > Probably because it's hard to predict where it will reach an altitude after which it starts rapid reentry. I'd think there would be significant variation in that simply due to weather. If I remember right, the atmosphere expands quite a bit on a sunny day.
  - jvm_ 9y ago
    But every day is a sunny day in space? Or does the ground vs. clouds make a difference.
    - teraflop 9y ago
      "Space weather" is actually a thing. It doesn't really have anything to do with clouds, though. Most of the influence on the extreme upper atmosphere comes from solar phenomena, and their interactions with the earth's magnetic field. Solar flares and other similar events happen all the time. They're not really noticeable at ground level, because they don't appreciably affect the sun's visible brightness, and the density of matter involved is tiny compared to the atmosphere at ground level. But at the heights where satellites orbit -- where the atmosphere is already extremely tenuous -- solar and geomagnetic phenomena can have a huge relative effect.
      - Turing_Machine 9y ago
        > It doesn't really have anything to do with clouds, though. The expansion of the earth's atmosphere due to heating effects has very much to do with clouds.
    - Turing_Machine 9y ago
      But we're not talking about space. We're talking about the earth's atmosphere. The proportion of reflected light is called the "albedo", which ranges from a theoretical maximum of 1 (a perfect mirror, or a theoretical white surface with no absorption) to 0 (a theoretical "black body"). White clouds reflect a lot of the incoming sunlight (have a larger albedo). Light that strikes the ground from a clear sky causes more ground heating, which causes more air heating, which causes the atmosphere to expand. ATW, the Earth's albedo can range from about 0.9 (fresh snow on the ground) to about 0.06 (open ocean). White clouds can approach about 0.8. Edit: the height of the tropopause (the boundary between the troposphere and the stratosphere) varies from about 9 km (over the poles) to 17 km (over the equator) due largely to the greater temperature in equatorial regions. So that layer of the atmosphere, at least, is nearly twice as thick in warm regions than it is in cold regions. See: https://en.wikipedia.org/wiki/Tropopause https://en.wikipedia.org/wiki/Tropopause
greglindahl 9y ago
It's a timescale problem. They can figure out when it might fall to within a couple of days, but, it orbits once every 90 minutes. As the descent becomes steeper the error bars become smaller than a day, but it's still orbiting about once every 90 minutes.
simonh 9y ago
To estimate that they’d need very precise information about the atmospheric drag on the station. That depends on many factors including the precise geometry of the station and all it’s peripheral parts and attachments and their relationships to each other, it’s attitude, It’s rotation and angular momentum in every axis, it’s precise weight distribution, then there’s the atmospheric characteristics at every point through its trajectory across multiple latitudes, over sea and land and with varying underlying weather and temperatures. Then there is varying gravitational strength due to geological factors, and possibly even the influence of earths magnetic field on the structure. Bear in mind the vehicle is grazing through the atmosphere at an extremely shallow angle.
inopinatus 9y ago
The possible trajectories are quite specific: https://twitter.com/Marco_Langbroek/status/979034097812111361/photo/1 https://twitter.com/Marco_Langbroek/status/97903409781211136... because it will re-enter basically over its own ground track. So although we don't know exactly where this will happen, we have a very good picture of where it won't happen (and that is most of the planet). The alternatives occur because it is losing speed only very gradually with each orbit through the upper atmosphere. Tiangong-1 goes around the planet every 88 minutes, and one of those orbits will lose enough speed and altitude that the vehicle disintegrates due to heat or drag. It's just that due to variations in the upper atmosphere, we don't know which orbit exactly will finally tip the balance. This is in contrast to a controlled re-entry vehicle e.g. Soyuz that washes off something like 150m/sec in a single engine burn to place it on an known and specific sub-orbital trajectory to get home.
sephamorr 9y ago
I think the best analogy is a roulette wheel. You know about when it's going to stop, but that doesn't help much to analyze where.
- Lozzer 9y ago
  I think you'd enjoy the book "The Newtonian Casino".

interesting that they have pretty much no idea where it will end up, i thought they'd be able to estimate that a bit.

by jgh 9y ago

interesting that they have pretty much no idea where it will end up, i thought they'd be able to estimate that a bit.

deleted 9y ago

[deleted]

saagarjha 9y ago

Probably because it's hard to predict where it will reach an altitude after which it starts rapid reentry. This could basically happen anywhere in its orbit, since the station is uncontrolled, so there's not a whole lot of confidence in where it will end up.

Turing_Machine 9y ago
> Probably because it's hard to predict where it will reach an altitude after which it starts rapid reentry. I'd think there would be significant variation in that simply due to weather. If I remember right, the atmosphere expands quite a bit on a sunny day.
- jvm_ 9y ago
  But every day is a sunny day in space? Or does the ground vs. clouds make a difference.
  - teraflop 9y ago
    "Space weather" is actually a thing. It doesn't really have anything to do with clouds, though. Most of the influence on the extreme upper atmosphere comes from solar phenomena, and their interactions with the earth's magnetic field. Solar flares and other similar events happen all the time. They're not really noticeable at ground level, because they don't appreciably affect the sun's visible brightness, and the density of matter involved is tiny compared to the atmosphere at ground level. But at the heights where satellites orbit -- where the atmosphere is already extremely tenuous -- solar and geomagnetic phenomena can have a huge relative effect.
    - Turing_Machine 9y ago
      > It doesn't really have anything to do with clouds, though. The expansion of the earth's atmosphere due to heating effects has very much to do with clouds.
  - Turing_Machine 9y ago
    But we're not talking about space. We're talking about the earth's atmosphere. The proportion of reflected light is called the "albedo", which ranges from a theoretical maximum of 1 (a perfect mirror, or a theoretical white surface with no absorption) to 0 (a theoretical "black body"). White clouds reflect a lot of the incoming sunlight (have a larger albedo). Light that strikes the ground from a clear sky causes more ground heating, which causes more air heating, which causes the atmosphere to expand. ATW, the Earth's albedo can range from about 0.9 (fresh snow on the ground) to about 0.06 (open ocean). White clouds can approach about 0.8. Edit: the height of the tropopause (the boundary between the troposphere and the stratosphere) varies from about 9 km (over the poles) to 17 km (over the equator) due largely to the greater temperature in equatorial regions. So that layer of the atmosphere, at least, is nearly twice as thick in warm regions than it is in cold regions. See: https://en.wikipedia.org/wiki/Tropopause https://en.wikipedia.org/wiki/Tropopause

greglindahl 9y ago

It's a timescale problem. They can figure out when it might fall to within a couple of days, but, it orbits once every 90 minutes. As the descent becomes steeper the error bars become smaller than a day, but it's still orbiting about once every 90 minutes.

simonh 9y ago

To estimate that they’d need very precise information about the atmospheric drag on the station. That depends on many factors including the precise geometry of the station and all it’s peripheral parts and attachments and their relationships to each other, it’s attitude, It’s rotation and angular momentum in every axis, it’s precise weight distribution, then there’s the atmospheric characteristics at every point through its trajectory across multiple latitudes, over sea and land and with varying underlying weather and temperatures. Then there is varying gravitational strength due to geological factors, and possibly even the influence of earths magnetic field on the structure. Bear in mind the vehicle is grazing through the atmosphere at an extremely shallow angle.

inopinatus 9y ago

The possible trajectories are quite specific: https://twitter.com/Marco_Langbroek/status/979034097812111361/photo/1 https://twitter.com/Marco_Langbroek/status/97903409781211136... because it will re-enter basically over its own ground track. So although we don't know exactly where this will happen, we have a very good picture of where it won't happen (and that is most of the planet). The alternatives occur because it is losing speed only very gradually with each orbit through the upper atmosphere. Tiangong-1 goes around the planet every 88 minutes, and one of those orbits will lose enough speed and altitude that the vehicle disintegrates due to heat or drag. It's just that due to variations in the upper atmosphere, we don't know which orbit exactly will finally tip the balance. This is in contrast to a controlled re-entry vehicle e.g. Soyuz that washes off something like 150m/sec in a single engine burn to place it on an known and specific sub-orbital trajectory to get home.

sephamorr 9y ago

I think the best analogy is a roulette wheel. You know about when it's going to stop, but that doesn't help much to analyze where.

Lozzer 9y ago
I think you'd enjoy the book "The Newtonian Casino".