Space Forum / Space Flight / July 2005

ESA's Mars Express mission includes MARSIS. It's hoped MARSIS will send
radar as deep as 5 kilometers into Mars' surface. One of the things it
would look for is subsurface liquid water.

So far as I know, MARSIS is still unfolded :(.

If they work the bugs out of MARSIS, perhaps this design would be useful
for asteroid prospecting.

If you wanted to get serious about asteroid prospecting you'd need to
make a bunch of probes. This would lower the unit cost for each probe.

But incoming data from an army of prospecting probes would likely be
more than the Deep Space dish network could handle.

Depends a little on how ambitious it is.  For remote sensing, there
definitely is no particular reason to send people.  For major surface
activities like deep drilling, we are not yet up to operating without
humans on hand to troubleshoot and do repairs -- we may be able to do it
eventually but we aren't there now.  In between, it depends somewhat on
how quickly you want results -- just roaming around looking at things can
be done remotely, but the MERs are several orders of magnitude slower than
a human field geologist.

Depends a little on details like integration time and distance:  such
spectrometers sense very faint emissions, and get a better picture if you
give them more collecting time and move in closer so the asteroid fills
more of the sky.  (Another reason for moving in close is that they are not
very directional, so the surface resolution achieved roughly equals the
altitude.)  However, they go a few meters deep at most.

Nope, wouldn't help.  There's too much mass in the way.

There *are* some sensors that will work to greater depths.  They generally
don't tell you much about chemical composition, although you can learn
things about the interior structure from which you might be able to infer
likely composition.

One very good choice for asteroid inspection is penetrating radar, which
in favorable circumstances -- dry regolith like the Moon's -- can work to
a kilometer or more.  You'll hear more about this shortly, because Mars
Express has just finished finally deploying the antennas for its radar
instrument.

Gravity mapping (via precision spacecraft tracking or an on-board gravity
gradiometer) has limited performance in such a weak gravitational field,
and its vision blurs as you go deeper, but it is inherently very
penetrating.

Magnetometers can reveal deep internal structure, *if* there's a magnetic
field for them to sense.  There are indications that some asteroids have
at least a little bit of a field, but data is very limited.  Eros, the one
asteroid to get lengthy study from a spacecraft with a magnetometer, has
essentially no field at all -- the magnetometer results can be summed up
fairly well as "nothing".

If you're down at close range, you may be able to learn something by doing
mass spectrometry or even neutral-atom imaging on outgassing -- atoms
escaping from the asteroid.  Asteroids with ice content, in particular,
should be outgassing some unless they're a long way out from the Sun.

Finally, there's a form of semi-remote sensing:  seismology.  Drop
seismometer packages and explosive charges, and you can in principle map
the interior in three dimensions.  There are a number of practical
problems, and nobody's yet sure how well this would work on a small body.

At the moment, the only way to get direct information on interior
composition is deep drilling.