RE: shoot down questions

From: Ted Molczan (
Date: Fri Feb 15 2008 - 08:38:23 UTC

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    Dale Ireland wrote:
    > If the satellite is hit wouldn't it actually reenter sooner 
    > as small high drag pieces would come down faster. 
    Yes, it would. Prior to the fragmentation of 79017A / 11278 (aka P78-1 or
    Solwind) by an ASAT, it had been estimated to remain in orbit for 40 years
    (source: The RAE Table of Earth Satellites, 1957-1980). Of the 284 debris
    fragments catalogued after the ASAT test, 90 percent decayed within 4 years:
    > It wouldn't launch many fragments into higher orbits would it?
    Many fragments will have a somewhat greater apogee than prior to the impact, but
    in most cases, their perigee will also have decreased, which will tend to hasten
    decay, especially for the fragments with the highest ballistic coefficients,
    i.e. area to mass ratio. A few fragments may have apogee far greater than that
    of the original orbit. 
    At the time Solwind was destroyed, its orbit was 513 X 543 km. One of its
    fragments, 79017Z / 16071, entered a 510 x 1765 km orbit. Its orbital history is
    available here:
    Note that despite its boost in apogee, it decayed within 1.83 years of the
    attack, due to its very high ballistic coefficient. If that fragment had been
    generated at the altitude at which the U.S. DoD intends to destroy USA 193,
    reportedly about 130 n.m. = 241 km, then it would have decayed far more quickly,
    due to the vastly greater atmospheric density at 241 km, compared with 510 km.
    At the present typical solar flux and geomagnetic activity, the MSIS-E-90
    Atmosphere Model reveals about a 400 times greater atmospheric density at 240 km
    than at 510 km, which would reduce the time to decay from 1.8 years to several
    Of course, objects having a much lower ballistic coefficient might remain in
    orbit for weeks, even a few months, but they would likely be few in number;
    therefore, not considered to have materially increased the existing risk of
    debris collision to operational spacecraft.
    The remainder of Dale's post strayed off topic for this list.
    Ted Molczan
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