Russian and US satellites crash in space at 25,000mph

In article ,
says...
A New Scientist article quotes someone from Iridium in 2007 saying that they
were getting 400 close approach (5km) warnings /per week/
http://www.newscientist.com/article/...certainty.html

Thank you for that link, which provides a comprehensive answer to my
original question. I'd wrongly guessed that orbits (at least for
sizeable satellites in other than the lowest orbits) would be better
known than they appear to be.

--
Cheers,
John

John Aldridge wrote in message
.. .
In article ,
says...
A New Scientist article quotes someone from Iridium in 2007 saying that
they
were getting 400 close approach (5km) warnings /per week/

http://www.newscientist.com/article/...ediction-is-pl
agued-with-uncertainty.html

Thank you for that link, which provides a comprehensive answer to my
original question. I'd wrongly guessed that orbits (at least for
sizeable satellites in other than the lowest orbits) would be better
known than they appear to be.

--
Cheers,
John


Interesting comments in th eNS piece. I imagine an important perterber of
orbits is the micrometeorites, flakes of paint etc , less than 4cm but still
pack a "conservation of momentum", proportionally bigger punch, to the far
more numerous small objects towards the 4cm size , but still perhaps
measurable effects on the big things.

400 notifications per week for a 5km sphere of space around iridium
satellites. I make that about 400 million years before 2 off 3x3x3m objects
are in the same bit of space within that 5km radius sphere. So something not
tallying there.


--
General electronic repairs, other than TVs and PCs
http://www.divdev.fsnet.co.uk/repairs.htm

Diverse Devices, Southampton, England

In uk.sci.astronomy message , Sat, 14
Feb 2009 22:46:42, N_Cook posted:

When you see orbital data quoting numbers like 14.32045077817866
and an earthquake induced 2.7uS variation to a daylength is 0.00000000032---
of a day, to the same degree of precision, you start to see a problem

One also sees a problem when one sees a duration quoted in micro-Siemens
- which are reciprocal megohms. The SI symbol for seconds is s.

--
(c) John Stockton, near London.
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
Correct = 4-line sig. separator as above, a line precisely "-- " (SoRFC1036)
Do not Mail News to me. Before a reply, quote with "" or " " (SoRFC1036)

In uk.sci.astronomy message , Sun, 15
Feb 2009 10:10:36, N_Cook posted:

400 notifications per week for a 5km sphere of space around iridium
satellites. I make that about 400 million years before 2 off 3x3x3m objects
are in the same bit of space within that 5km radius sphere. So something not
tallying there.

Agreed. I don't see what arithmetic you've done,

What matters is that, measuring from one satellite, the path of the
centre of the other comes within a disc of 5000 m radius at a frequency
amounting to 400 times a week for all attacking satellites plus junk.
There will be a collision if it comes within 5 m, the approximate
average sum-of-radii. That will happen a thousand squared times less
often.

1/(52*400)* Math.pow(5000/5, 2) = about 48 (years).

Satellites generally have solar panels, and being swatted by a solar
panel will generally kill the swattee, though the swatter *might* just
end up spinning and with a missing panel. Allow for the shape and area
of the panels, and that 48 will be, I guess, at least halved, maybe
quartered.

That means that of the order of one collision per decade should be
expected (if you mean 5 km radius sphere); and that we should not have
been surprised.

--
(c) John Stockton, nr London, UK. Turnpike v6.05 MIME.
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

N_Cook wrote:
400 notifications per week for a 5km sphere of space around iridium
satellites. I make that about 400 million years before 2 off 3x3x3m objects
are in the same bit of space within that 5km radius sphere. So something not
tallying there.

It's a cross-section problem rather than a volume problem, so
I make it (5000/3)^2 / 400 weeks, or somewhat over 130 years. On that
basis [alone], it's unlikely but not *that* unlikely, given that we are
now some decades into the satellite era. But of course the "400/week"
was for Iridium satellites only, so *some* sort of major collision was
rather likely over two or three decades.

--
Andy Walker
Nottingham

Andy Walker wrote in message
...
N_Cook wrote:
400 notifications per week for a 5km sphere of space around iridium
satellites. I make that about 400 million years before 2 off 3x3x3m
objects
are in the same bit of space within that 5km radius sphere. So something
not
tallying there.

It's a cross-section problem rather than a volume problem, so
I make it (5000/3)^2 / 400 weeks, or somewhat over 130 years. On that
basis [alone], it's unlikely but not *that* unlikely, given that we are
now some decades into the satellite era. But of course the "400/week"
was for Iridium satellites only, so *some* sort of major collision was
rather likely over two or three decades.

--
Andy Walker
Nottingham


I was assuming that most of these 400 objects per week were down nearer the
4cm sort of dimensions hitting a 3x3x3m sort of object.

I wonder what the ranking is of orbit perterbation from
1/ solar flux, normal and mass ejection push and pull on solar panels etc
2/ micrometeorite and other less than 4cm stuff hitting other small stuff
2b/ similar but hitting big stuff
3/earthquakes
4/"astrological" effects if I can call it that, other planet affecting the
orbits

"Mark McIntyre" wrote in message
...
OG wrote:

Changes in the Earth's spin (for whatever reason) only affect the paths
relative to the ground; the satellite orbits wouldn't change in
themselves.

Not 100% correct - the earth's magnetic and gravitational fields will
shift, affecting any orbiting bodies slightly.

Do you think so? I doubt there would be any measurable effect from the
changes to the gravitational or magnetic fields following events such as the
2004 Tsunami . I'm quite happy to be surprised though; so is anyone up for a
back of the envelope estimate for either?

"N_Cook" wrote in message
...
Andy Walker wrote in message
...
N_Cook wrote:
400 notifications per week for a 5km sphere of space around iridium
satellites. I make that about 400 million years before 2 off 3x3x3m
objects
are in the same bit of space within that 5km radius sphere. So
something
not
tallying there.

It's a cross-section problem rather than a volume problem, so
I make it (5000/3)^2 / 400 weeks, or somewhat over 130 years. On that
basis [alone], it's unlikely but not *that* unlikely, given that we are
now some decades into the satellite era. But of course the "400/week"
was for Iridium satellites only, so *some* sort of major collision was
rather likely over two or three decades.

--
Andy Walker
Nottingham


I was assuming that most of these 400 objects per week were down nearer
the
4cm sort of dimensions hitting a 3x3x3m sort of object.

I wonder what the ranking is of orbit perterbation from
1/ solar flux, normal and mass ejection push and pull on solar panels etc
2/ micrometeorite and other less than 4cm stuff hitting other small stuff
2b/ similar but hitting big stuff
3/earthquakes
4/"astrological" effects if I can call it that, other planet affecting the
orbits






Add "complicated gravitational figure of the Earth" and "varying atmospheric
drag" to that list. I think these are dominant and add unknowns to the
orbit determination that gets worse over time--hence the need to monitor
continuously. Otherwise your number 1 is highest on the list, mainly
because orientation of the panels and spacecraft (especially if defunct) is
difficult to predict.

Another important effect is infrared radiation pressure from the Earth,
though this is relatively predictable.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)