[meteorite-list] Fw: U.S. To Shoot Down Defunct Spy Satellite

[Sterling K. Webb]

Forwarding this to The List for:
----- Original Message ----- 
From: "Matson, Robert D." <ROBERT.D.MATSON at saic.com>
To: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>; 
<Meteorite-list at meteoritecentral.com>
Sent: Friday, February 15, 2008 4:54 PM
Subject: RE: [meteorite-list] U.S. To Shoot Down Defunct Spy Satellite


Hi Sterling (and List, if Sterling forwards for me),

> Rob pointed out that only a very small percentage of the debris
> will be directed into potentially dangerous orbits, but a "small
> percentage" of 100,000 is still a respectable number.

Agreed; however, due to conservation of momentum the most "wayward"
pieces will be the smallest pieces, and these will have the highest
drag coefficients.  So it will not take long before drag at perigee
reduces the new boosted apogees right back down again.

The longer they wait to intercept USA 193, the fewer fragments that
will survive one orbit.  For example, at 200-km altitude there is
about a 5-degree half-angle fan of post-impact velocity vectors
which produce fragments that survive beyond one orbit.

The "magic bullet" fragments are the ones that depart the point
of impact in nearly the same direction that the original satellite
was moving -- but with higher velocity.  Perigee stays the same,
but apogee (and orbital lifetime) gets boosted.  It doesn't take
a lot of extra velocity.  At 200-km perigee, a boost of 30 m/s
will put apogee at 300 km; 59 m/s puts apogee at 400 km; 221 m/s
would send apogee all the way up to 1000 km.  But the key is
that the velocity has to be in that narrow window of directions
that maintains perigee above 100 km.

For similar reasons, at least half the fragments are going to
decay within half an orbit since they will have velocities lower
than the satellite had prior to impact, resulting in new perihelions
below 100 km.  So they will certainly choose an impact time such
that the ground track of the satellite does not pass over populated
areas for at least 1/2 an orbit.  That pretty much means a
descending node pass over the north Pacific -- the further north,
the better such that the only land mass crossed is the extreme
southern tip of South America, and Africa is completely avoided.

By the way, don't miss the opportunity to see passes of USA 193
in the evenings right now!  For example, there are excellent
passes of USA 193 for the next 4 nights over the southern
California area, and there are passes for New York City starting
Saturday night, every night for a week!  --Rob