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-From: bbbehr@sunspot.noao.edu (Bradford B. Behr)
-Subject: Re: Smith-Tuttle Comet a threat to earth?
-Date: 27 Oct 92 18:37:05 GMT
-Organization: National Solar Observatory/SP, Sunspot NM, USA

->One can extrapolate on this one since the shuttle is rougly doing 
->28,000 km/hr, is NASA taking into account the time distortion  between
->the crew and JSC?

-Shuttle velocity = 28000 km/hr = 7.78e3 m/s
-Lightspeed = 3.00e8 m/s
-v/c = 2.60e-5
-Time dilation formula:
-T = T0 / sqrt(1 - (v/c)^2)
-so dilation factor = 1 - 3.37e-10
-Mission duration = 10 days = 8.64e5 sec
-so Shuttle clocks will lag JSC clocks by 2.91e-4 sec at mission end.

-A third of a millisecond? Significant? Some timing links might
-conceivably get thrown off by a deviation of this magnitude. But
-presumably they resync throughout the mission...

Since the Shuttle is moving in a circular path, you probably have to use
general relativity to get a more accurate answer. Also, the fact that
the Shuttle is less deep in Earth's gravity well than the surface of the
Earth would cause the difference to be less than you would otherwise think.
(Time passes more slowly in a gravity well than in "free space".)

Atomic clocks have been placed in high-altitude aircraft, and a time
difference has been measured. If NASA wants their measurements tagged
with a 100-nanosecond timestamp like we use here, then they have to take
the difference into account.

John Roberts
roberts@cmr.ncsl.nist.gov