Although it is pretty clear that the Mars Polar Lander (and the two
microprobes, "Scott" and "Amundsen") are lost, I came across an article in
the NY Times online which mentioned that the microprobes were instrumented
with accelerometers which were to record the deceleration as they slammed
into the Martian surface and came to rest about 4 feet down.  Arrival speed
at moment of impact was expected to be about 400 mph.  Variations in the
deceleration force would contain information about differences in density
of layers in the first 4 feet of soil, which would later be broadcast back
to earth via the lander, or the MPL, or the Mars Global Surveyor, currently
in orbit.

This must have been tough gear.  If my math is right, 400 mph is about 600
ft/sec.  To decelerate from 600 ft/sec to 0 (assuming uniform deceleration)
means that the avg speed during deceleration is 300 ft/sec.  At 300 ft/sec
it takes about 14 msec to go 4 feet.  Therefore the deceleration must have
been 300 / 0.014 = 21,400 ft/sec/sec or about 670 G's.  The science team
was prepared to lose either probe if it hit a rocky surface (which would
result in about 10,000 G deceleration I'd guess).  But they tested the
units to thousands of G's on earth by firing them into the earth from
airborne cannons.  (Why didn't I go to work for NASA?)

Does anyone know any more about these accelerometers, the instrumentation,
and in particular what kind of sampling rate they wanted to get the fine
structure of the subsurface geology?  If you wanted resolution to 1 mm
you'd need about 1300 samples in 0.014 sec or about 92,000 samples/sec.
How do you design an accelerometer which will report 100,000 times per
second as it records accelerations between 0 and 1000 Gs?  Strong motion
indeed!

 Imagine little Lehman sensors in the probes!  Now imagine the
technicians on Earth performing the final assembly as one says to the
other, "Should these little magnets attract or repel each other?")  ;)


Regards, Ted



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