Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.space
Subject: ultralight probes
Message-ID: <5630@utzoo.UUCP>
Date: Sat, 25-May-85 21:06:15 EDT
Article-I.D.: utzoo.5630
Posted: Sat May 25 21:06:15 1985
Date-Received: Sat, 25-May-85 21:06:15 EDT
Organization: U of Toronto Zoology
Lines: 76

In response to Jim Houser's query for ideas about a near-term interstellar
probe (which I've unfortunately had to shoot down in a separate article)...
Here is another interesting notion which is a bit closer to reality, and
which could use some ideas.

The highest-performing solar sails that we know how to build are Eric
Drexler's aluminum sails.  His "baseline" design is 10 km across and has
high performance with a 20-ton payload.  Building a 10-km object in space
(Drexler sails are rigid and cannot be folded for launch) is a bit on the
ambitious side just yet, especially since air-drag problems may rule out
construction in the very low orbits that give maximum shuttle payload.

Consider a scaled-down version:  100 m across, hauling a payload of 2 kg.
Obviously this cannot be manned, but it could still be very useful for
comet rendezvous missions, surveying the asteriod belt for resources, etc.
The big question is, can we build a complete control and communications
system, with useful instruments, within 2 kg??  Some weight growth is
acceptable, although it can't be an order of magnitude or performance
will suffer.  (The 100-m sail weighs only 2 kg itself, so a 20-kg load
would be a huge increase in total vehicle mass.)  Long life and near-total
absence of moving parts are major virtues, since this thing will spend a
long time in space.  (Some of this might even be relevant to Houser's
proposal...)

Power and communications will obviously be major issues.  Solar cells are
the obvious choice for power, but very lightweight construction	will be
vital.  Can we get away with a rigidly-mounted solar array, bearing in
mind that the sail constrains the sun angle substantially?  Communications
will probably require a steerable antenna, alas.  Would optical communication
be better?

Basic control will probably be by tiny winches on the sail shrouds, which
will have the net effect of shifting the payload's position with respect
to the sail.  Is this sufficient?  Note that the sail spins slowly to
maintain its rigidity; can clever design of communications, guidance
sensors, and instrumentation avoid the need for a despun platform?  If
not, perhaps the spinning and despun sections should have independent
solar arrays and computer systems, communicating optically, to avoid the
reliability problems of carrying power through rotating bearings.

Substantial onboard computing is needed, since this vehicle is "under
power" at all times, albeit at very low acceleration.  Speed-of-light
lags impose the usual limitations on human intervention.  Close approaches
to asteroids would be particular tricky.  Can we get radiation-resistant
chips with enough computing power and sufficiently-low current drain?

What sort of instrumentation would be practical?  Imaging is obviously
a high priority.  We could use the sail spin for one dimension of scan,
although this will mean slow imaging because the sail spin isn't quick.
Limited communications power may impose serious constraints on imaging
data rates anyway.  How much steerability do we need for useful imaging?
Filter wheels are troublesome; can we get away with multiple image
sensors with fixed filters?  If we do use an ordinary 2-dimensional
imaging sensor, what about a pattern of filter stripes across the image?

Obviously navigation wants a sun sensor; can the imaging system be used
for this?  If not, can it at least be used for the rest of the guidance
requirements?  Note that real-time attitude control requires on-board
interpretation of sensor data.

Can we measure micrometeoroid density by measuring perforations of the
sail?  The resulting "collecting area" is orders of magnitude larger
than anything we can do with a separate sensor, but scanning it for
small perforations isn't trivial.  If we use optical communications via
solid-state laser, could the communications system's laser and receiver
be used to scan the sail?

Solid-state radiation sensors are an obvious possibility.  How detailed
can we make their data return within severe weight constraints?  Can we
get enough sensitivity for things like gamma-ray spectrometry, to examine
asteroids and comets for volatiles?

Ideas welcome...
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry