Sanity check:

If there were no atmosphere/drag, a satellite at 163km that is going at
28,080kmh and reduces it speed by 1kmh, would simply drop in altitude
and now be at a lower orbit, correct?

Theoretically, it could progressively reduce its speed and drop in
altitude and end up going 28,440km in a orbit 100m above surface. Correct?

Now, let bring is a purely theoretical chinese boosters called CZ-5B
that is uncontrolled in a 163*301km orbit and bring back atmosphere.

With every perigee having a bit of drag how is the orbit changed? Is
only Apogee loweree? Both are changed equally? or does Apogee lose more
altitide than perigee with tendency to circularize?

If it circularizes, is it correct to state that the length of time spent
at/near perigee altitude (and thus getung drag) increases?

Since in an elliptical orbit, the object goes faster than required at
perigee (hence having enough "oumph" to climb back up to apogee), I am
curious at how the object eventually comes to a point where drag lasts
long enough to bleed enough speed to turn off orbital mechanics and turn
on ballustic re-entry.

Is there a magic altitude below which atmpsphere is instantly much more
dense, so the minute perigee drops below that altitude, it bleeds too
much speed to climb out of altitude that has drag and it is sayonara?

Or is re-entry much more subtle and gradual?

JF Mezei asserted that:
> Sanity check:
>
> If there were no atmosphere/drag, a satellite at 163km that is going at
> 28,080kmh and reduces it speed by 1kmh, would simply drop in altitude
> and now be at a lower orbit, correct?
>
> Theoretically, it could progressively reduce its speed and drop in
> altitude and end up going 28,440km in a orbit 100m above surface. Correct?
>
>
> Now, let bring is a purely theoretical chinese boosters called CZ-5B
> that is uncontrolled in a 163*301km orbit and bring back atmosphere.
>
> With every perigee having a bit of drag how is the orbit changed? Is
> only Apogee loweree? Both are changed equally? or does Apogee lose more
> altitide than perigee with tendency to circularize?
>
> If it circularizes, is it correct to state that the length of time spent
> at/near perigee altitude (and thus getung drag) increases?
>
> Since in an elliptical orbit, the object goes faster than required at
> perigee (hence having enough "oumph" to climb back up to apogee), I am
> curious at how the object eventually comes to a point where drag lasts
> long enough to bleed enough speed to turn off orbital mechanics and turn
> on ballustic re-entry.
>
> Is there a magic altitude below which atmpsphere is instantly much more
> dense, so the minute perigee drops below that altitude, it bleeds too
> much speed to climb out of altitude that has drag and it is sayonara?
>
> Or is re-entry much more subtle and gradual?

The rule of thumb is that to raise one extremum of the orbit, you do
the burn at the other extremum. Atmospheric drag at perigee is the
equivalent of a retrograde burn, so I'd expect apogee to be lowered.

As for the speed, remember that drag increases with speed. It's a nice
differential equation to figure out the relative, um, impacts of
short-fast drag events vs long-slow drag events. You can probably do
it quickly in Wolfram, but I'm not a Wolfram user.

The CZ-58 booster's perigee is well below the orbit of the ISS, but
IIRC from shuttle days the apogee is still within the bounds of
measurable drag. You might also consider the orbit of the Falcon 9
2nd stage recently retrieved near Seattle; I suspect it was a lower
apogee, especially since it was a Starlink launch. Hmmm, I see shell 1
is being loaded at 550 km/340 mi, but I haven't yet found the 2nd stage
apogee; maybe I'll replay some of the launch videos and scan the
telemetry display.

/dps

--
"I am not given to exaggeration, and when I say a thing I mean it"
_Roughing It_, Mark Twain

Snidely noted that:
> JF Mezei asserted that:
>> Sanity check:
>>
>> If there were no atmosphere/drag, a satellite at 163km that is going at
>> 28,080kmh and reduces it speed by 1kmh, would simply drop in altitude
>> and now be at a lower orbit, correct?
>>
>> Theoretically, it could progressively reduce its speed and drop in
>> altitude and end up going 28,440km in a orbit 100m above surface. Correct?
>>
>>
>> Now, let bring is a purely theoretical chinese boosters called CZ-5B
>> that is uncontrolled in a 163*301km orbit and bring back atmosphere.
>>
>> With every perigee having a bit of drag how is the orbit changed? Is
>> only Apogee loweree? Both are changed equally? or does Apogee lose more
>> altitide than perigee with tendency to circularize?
>>
>> If it circularizes, is it correct to state that the length of time spent
>> at/near perigee altitude (and thus getung drag) increases?
>>
>> Since in an elliptical orbit, the object goes faster than required at
>> perigee (hence having enough "oumph" to climb back up to apogee), I am
>> curious at how the object eventually comes to a point where drag lasts
>> long enough to bleed enough speed to turn off orbital mechanics and turn
>> on ballustic re-entry.
>>
>> Is there a magic altitude below which atmpsphere is instantly much more
>> dense, so the minute perigee drops below that altitude, it bleeds too
>> much speed to climb out of altitude that has drag and it is sayonara?
>>
>> Or is re-entry much more subtle and gradual?
>
> The rule of thumb is that to raise one extremum of the orbit, you do the burn
> at the other extremum. Atmospheric drag at perigee is the equivalent of a
> retrograde burn, so I'd expect apogee to be lowered.
>
> As for the speed, remember that drag increases with speed. It's a nice
> differential equation to figure out the relative, um, impacts of short-fast
> drag events vs long-slow drag events. You can probably do it quickly in
> Wolfram, but I'm not a Wolfram user.
>
> The CZ-58 booster's perigee is well below the orbit of the ISS, but IIRC from
> shuttle days the apogee is still within the bounds of measurable drag. You
> might also consider the orbit of the Falcon 9 2nd stage recently retrieved
> near Seattle; I suspect it was a lower apogee, especially since it was a
> Starlink launch. Hmmm, I see shell 1 is being loaded at 550 km/340 mi, but I
> haven't yet found the 2nd stage apogee; maybe I'll replay some of the launch
> videos and scan the telemetry display.

Today's launch was orbiting at 208 km at T+22 and 293 km at T+65
(deploy).

/dps

--
The presence of this syntax results from the fact that SQLite is really
a Tcl extension that has escaped into the wild.
<http://www.sqlite.org/lang_expr.html>

On 05-May-21 4:31 pm, JF Mezei wrote:
> Sanity check:
>
> If there were no atmosphere/drag, a satellite at 163km that is going at
> 28,080kmh and reduces it speed by 1kmh, would simply drop in altitude
> and now be at a lower orbit, correct?
>
> Theoretically, it could progressively reduce its speed and drop in
> altitude and end up going 28,440km in a orbit 100m above surface. Correct?
>
>
> Now, let bring is a purely theoretical chinese boosters called CZ-5B
> that is uncontrolled in a 163*301km orbit and bring back atmosphere.
>
> With every perigee having a bit of drag how is the orbit changed? Is
> only Apogee loweree? Both are changed equally? or does Apogee lose more
> altitide than perigee with tendency to circularize?
>
> If it circularizes, is it correct to state that the length of time spent
> at/near perigee altitude (and thus getung drag) increases?
>
> Since in an elliptical orbit, the object goes faster than required at
> perigee (hence having enough "oumph" to climb back up to apogee), I am
> curious at how the object eventually comes to a point where drag lasts
> long enough to bleed enough speed to turn off orbital mechanics and turn
> on ballustic re-entry.
>
> Is there a magic altitude below which atmpsphere is instantly much more
> dense, so the minute perigee drops below that altitude, it bleeds too
> much speed to climb out of altitude that has drag and it is sayonara?
>
> Or is re-entry much more subtle and gradual?
>
One way of thinking about this to remember that the object is subject to
drag all around its orbit. The drag at apogee reduces the perigee, and
the drag at perigee reduces the apogee.

Just that for non-circular orbit, the drag at apogee is very small
compare with the drag at perigee, so the apogee is reducing while the
perigee stays pretty much unchanged.

Since the apogee is changing, it's not strictly speaking true to say
that the orbit is elliptical, and it's also not strictly speaking true
to say that it becomes circular. Rather, the orbit becomes a spiral,
with the orbital height continuously reducing. The rate of reduction
itself increases with time (though things like sun spots affect the
atmosphere on Earth, which can cause some variation). Eventually the
rate of reduction becomes so high that it is not possible for the object
to complete an orbit, and you have your ballistic reentry.

Sylvia.

On May/5/2021 at 04:28, Sylvia Else wrote :
> On 05-May-21 4:31 pm, JF Mezei wrote:
>> Sanity check:
>>
>> If there were no atmosphere/drag, a satellite at 163km that is going at
>> 28,080kmh and reduces it speed by 1kmh, would simply drop in altitude
>> and now be at a lower orbit, correct?
>>
>> Theoretically, it could progressively reduce its speed and drop in
>> altitude and end up going 28,440km in a orbit 100m above surface.
>> Correct?
>>
>>
>> Now, let bring is a purely theoretical chinese boosters called CZ-5B
>> that is uncontrolled in a 163*301km orbit and bring back atmosphere.
>>
>> With every perigee having a bit of drag how is the orbit changed? Is
>> only Apogee loweree? Both are changed equally? or does Apogee lose more
>> altitide than perigee with tendency to circularize?
>>
>> If it circularizes, is it correct to state that the length of time spent
>> at/near perigee altitude (and thus getung drag) increases?
>>
>> Since in an elliptical orbit, the object goes faster than required at
>> perigee (hence having enough "oumph" to climb back up to apogee), I am
>> curious at how the object eventually comes to a point where  drag lasts
>> long enough to bleed enough speed to turn off orbital mechanics and turn
>> on ballustic re-entry.
>>
>> Is there a magic altitude below which atmpsphere is instantly much more
>> dense, so the minute perigee drops below that altitude, it bleeds too
>> much speed to climb out of altitude that has drag and it is sayonara?
>>
>> Or is re-entry much more subtle and gradual?
>>
> One way of thinking about this to remember that the object is subject to
> drag all around its orbit. The drag at apogee reduces the perigee, and
> the drag at perigee reduces the apogee.
>
> Just that for non-circular orbit, the drag at apogee is very small
> compare with the drag at perigee, so the apogee is reducing while the
> perigee stays pretty much unchanged.
>
> Since the apogee is changing, it's not strictly speaking true to say
> that the orbit is elliptical, and it's also not strictly speaking true
> to say that it becomes circular. Rather, the orbit becomes a spiral,
> with the orbital height continuously reducing. The rate of reduction
> itself increases with time (though things like sun spots affect the
> atmosphere on Earth, which can cause some variation). Eventually the
> rate of reduction becomes so high that it is not possible for the object
> to complete an orbit, and you have your ballistic reentry.

True. I would add that even if the drag at apogee is negligeable, you
still don't only reduce apogee. The perigee is a point, the drag in the
lower part of the orbit isn't only at perigee but on the part of the
orbit near perigee. So even if the drag at perigee only lowers apogee,
the drag five minutes before or after perigee lowers mostly apogee but
it also lowers lowers perigee a little.

Alain Fournier

On 2021-05-05 02:31, JF Mezei wrote:

> Now, let bring is a purely theoretical chinese boosters called CZ-5B
> that is uncontrolled in a 163*301km orbit and bring back atmosphere.

That was on May 5th.

This morning, the orbit has decayed to 146 * 218km.

So the apogee has dropped 83km while perigee only dropped 17km.

The TLEs were updated 2 hours prior to posting this.

From a public safety point of view, on the day of, who is in charge of
tracking the debris and updating predictions of when/where it falls ?
NORAD? Trump's Space Force?

I assume they use radar to get its position and update TLEs from that?
Is there a worldwide network to track the object during its last hours
to give better warning to where it might fall?

Or it it all done by math from the last TLE and factoring in drag to
calculate on which orbit the perigee will be "fatal" to the orbit?

Say the final orbit passes over panama, then east coast USA, then a
number of African nations, some Indian ocean islands and Australia, does
NORAD have a red phone to each country to warn them, or does it go up
the chain via state department to embassies etc?

Or is this a case of since there is nothing anyone can do to prepare,
there is no point in warning anyone an calculations are done more for
search parties to know where to look for debris?

On 08-May-21 5:47 pm, JF Mezei wrote:

>
> Or is this a case of since there is nothing anyone can do to prepare,
> there is no point in warning anyone an calculations are done more for
> search parties to know where to look for debris?
>

I think that's right, especially as the uncertainty is never reduced to
the point where an evacuation order would make sense even if there were
time to implement it.

I live in Sydney, which is within the possible landing area. Yet I'm not
worried. The chance of me and mine being impacted is so low it's not
worth being concerned about.

Sylvia.

On 2021-05-08 19:25, Sylvia Else wrote:

> I live in Sydney, which is within the possible landing area. Yet I'm not
> worried. The chance of me and mine being impacted is so low it's not
> worth being concerned about.

Apparently, the Chinese rocket decided to re-enter 24 minutes early, so
this measn into the Pacific instead of Atlantic. So a but earlier and
it might have sent debris on the Nullarnor and hit the museum that has
the recovered pioeces from Skylab !

As I type this, the web site say it has apparently re-entered.

How is confirmation obtained? lack iof radar echo where it should be?
Or do they just write it off the database and assume it re-entered?

BTW, China stated it dropped in the Indian ocean.

The predictiosn on the web pointed initially to re-entry over Atlantic
(so debris over spain or mediteranean) and later re-entry over
mediteranean withd ebris over saudi arabia or egypt.

The initial "it has dropped" had it drop 24 minutes early in the pacific
(so possible debris over central america or haiti).

So some questions here:

If China lost comms with its rocket, how could it know when it ceased to
"exist" ? How could it know it re-entered over Indian ocean?
(which majkes for interesting question: did it re-entery earlier than
than public "24 minutes earlier than predicted" or later?)

It it plausible that the rocket was still broadcasting and its signals
were still veing received by China, but that commands sent to it weren't
received or processed?

I note that for the Starlink test, despite the craft being "out of
wack", they were still able to send commands to it (assume when its
rotation had the antenna pointing right direction).

Secondly, do the real people like Norad have much mroe accurate
predictions on re-entry, or do those web sites have the "state of the
art" when it comes to prediction of re-entry time?

In article <ifokutF3fo1U1@mid.individual.net>, sylvia@email.invalid
says...
>
> On 08-May-21 5:47 pm, JF Mezei wrote:
>
> >
> > Or is this a case of since there is nothing anyone can do to prepare,
> > there is no point in warning anyone an calculations are done more for
> > search parties to know where to look for debris?
> >
>
> I think that's right, especially as the uncertainty is never reduced to
> the point where an evacuation order would make sense even if there were
> time to implement it.
>
> I live in Sydney, which is within the possible landing area. Yet I'm not
> worried. The chance of me and mine being impacted is so low it's not
> worth being concerned about.

It fell into the Indian Ocean.

https://spacenews.com/long-march-5b-falls-into-indian-ocean-after-world-
follows-rocket-reentry/

Jeff
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