bc (12/23/82)
Disclaimer
The following is strictly tablecloth engineering, and may be the
fevered product of too much Sczechuan Beef. I request your best
attempts to shoot it down. It sounds as if it should work, but I just
don't know enough about celestial mechanics, etc, to say.
Proposal
That some space society or consortium of societies send up in the
shuttle a getaway special package which contains a probe to be sent to
one of the Lagrange points L4 or L5. The probe would be motivated by a
light sail, and powered either by solar cells or by a SNAP battery. It
would contain an instrument package which would study the environment
at the Lagrange points: dust and charged particle density, magnetic
fields and solar wind, presence of large rocks, etc.
Before I get involved in the technical discussion, I want to discuss
the motives for such a probe. It seems to me that NASA is not willing
to work seriously towards large scale space colonization or
industrialization, for fear of adverse publicity. They are trying to
stick closely to what they view as the "mainstream" of space
development which they view as development of Earth-directed services
such as weather observation and communications in LEO. They are hoping
that the private sector will vindicate their choice of projects by
slowly taking over the investment. The space colony and industrialism
proponents on the other hand, are building fairly involved scenarios
for the development of space, but lack hard information on the
environment in which development will take place, because they lack the
means to explore that environment. Conditions at the Lagrange points
including relative abundance of materials such as hydrogen and carbon,
dust density, numbers of large rocks, and precise placement relative to
the Earth's magnetopause would affect construction there. We (I'm a
proponent of space colonization too) keep hoping that NASA will develop
projects to explore these conditions, but it seems unlikely that they
will do so in the next decade, given their current emphasis.
The idea for this probe developed from a conversation about the optimum
payload for a non-NASA, non-aerospace-corporation developed experiment
to be carried by the shuttle. Such an experiment must be small in
order to fit into the space allotted to a getaway special,
self-contained, relatively inexpensive (assuming volunteer labor for
design and construction), and be intended to study an area which is not
likely to be studied by anyone else for awhile. An example of such a
payload is the series of OSCAR sattellites developed by the American
Radio Relay League for relaying amateur radio signals. NASA is
concentrating on LEO, with a few excursions to the planets, for the
next five years, and the corporations are concentrating on materials
research in LEO. Until someone finds a need to exploit cis-lunar space
beyond geosynchronous orbit, or decides to go into space construction
beyond LEO there won't be any call for NASA or the corporations to
explore out there. For that matter, there is not yet a reasonable way
to get large payloads beyond LEO, since the orbital tug does not yet
exist.
Rightly or wrongly, the space colonization movement has become
identified with the Lagrange points. It's not clear that L5 is the
best place for a colony, but that's the place that people think about
when space colonies are discussed. A probe to L5 (or L4, whichever
makes more sense from an astronautical point of view) would be an excellent
symbol of determination on the part of space colony proponents, a proof
that private exploration of space is possible, and a means of
investigating the Lagrange points to gather hard evidence on their
utility for construction sites.
On to the technical details.
The space and cost constraints seem to rule out the use of chemical
rockets to give the probe the impulse needed to get to L5. In
addition, the engine needs to be restartable, since the probe needs to
match velocities when it reaches L5 (considerably more useful
information can be gathered with a given mass of instruments if the
probe stays at its target rather than flies by). These considerations
favor a constant thrust engine: either an ion-engine or a sail. An
ion-engine requires a good deal of electrical power to operate,
increasing the cost, and probably decreasing the useful payload mass.
A sail requires mechanical parts to steer, and involves a technology
which has never really been tried in space before (which in itself may
be a reason to use it). The factor which determines a sail's
feasibility for this mission is the effective acceleration per unit
area which can be obtained, given the solar radiation pressure (and the
solar wind?) and the density of the best available sail material. I
don't know enough to evaluate this factor, but it sounds reasonable
that a solar constant of more than a kilojoule/sq. meter would provide
enough thrust to move a total instrument/navigation package of a few
tens of kilograms at a few thousandths of a g with a rasonable size
sail. Comments, anyone?
I envision the sail control as a set of piezoceramic benders which are
small, light, and use little power, with rachet and pawl mechanisms to
bend, move, and hold the sail in a given position relative to a
framework of plastic tubes. The tubes could be carried in the shuttle
coiled up flat, then deployed by inflation and stiffening with an
ultraviolet-cured epoxy coating. The curing would allow the tubes to
hold shape even if the gas used for inflation leaks away. The keel and
navigation reference could be a weight on a wire boom, tidal-locked to
the Earth. This would work at least out to GEO, but I don't know how
well it would work near L5. My guess is that it would work, because at
L5 the lunar and solar tides are approximately the same as on Earth,
but the Earth's tidal effect would be greater because L5 is fewer Earth
radii from Earth than lunar radii from the moon. A very long boom, and
possibly some sort of variable damping device, to compensate for the
change in resonant frequency with changing tidal force, might be necessary.
The payload would consist of an three parts: an astrogation package, an
instrumentation package, and a communication package. The astrogation
package would contain some sort of sensors, a microcomputer, and the
drivers for the mechanism which controls the sail. The instrumentation
package would contain the instrumentation for the experiments, a clock,
some sort of recorder (magnetic bubbles perhaps), and a controller for
the experiments. The communication package would consist of a
transmitter to send recorded experimental data to Earth (this could be very
small if the NASA deep space net were used for reception on Earth), and
(optionally) a receiver for commands from Earth. If the transmitter and
receiver were configured to operate as a transponder, they could be
used to track distance from Earth to the probe, and power consumption
on the probe could be reduced, since the transmitter would only be
operating when the receiver on Earth was ready for it. Alternately,
tracking could be done by mounting a corner reflector on the probe, and
bouncing radar or laser beams from Earth off of it.
I can think of a few experiments off the top of my head. Charged
particle density could be measured with a Faraday cup (or use the sail
as a collector?) and charge detector. Dust density could be measured
with a micrometor detector, or by detecting changes in amplitude and
polarization of the radio signal returned to Earth from the probe (I
don't know what lower limit of density is detectable by this method).
The stability of the Lagrange point could be determined by measurements
of perturbations in the probe's orbit (the sail may have to be dumped
when the probe reaches L5). The local magnetic field could be measured
with any standard magnetometer design. Large rocks in the neighborhood
could be detected by an on-board radar, or by visual observation
through a TV camera. If a large rock is detected, it might be possible
to maneuver alongside it for a close look.
Conclusion:
The mission looks feasible to me, and the probe may or may not be
technically possible. In my own area of expertise (the
instrumentation electronics and the computers) I know that what I've
described can be done simply and (relatively) cheaply, but I am not
sure about things like the sail deployment and control, power source,
etc. These things need to be investigated by experts in the
respective fields, many of whom, I hope, are reading this article.
I welcome net mail or articles to net.space or the ARPA space digest
either attacking or defending the feasibility of this proposal.
I think the primary benefits of this mission would not be from the
information it returned, but from the fact of its completion. It would
prove that the technological base established by NASA in the last 20
years is now usable by groups other than governments and large
corporations, and it would prove that the space exploitation movement
has the determination and expertise to carry out the first steps in
implementing its own proposals.
Bruce Cohen
Intel
USENET: ...{pur-ee,hplabs}!intelqa!omsvax!bc
ARPA: ... hplabs!intelqa!omsvax!bc@UCB-C70REM@MIT-MC (12/28/82)
From: Robert Elton Maas <REM @ MIT-MC> I like your idea but may I suggest that we first test the solar-sail system by itself and get it really working before we use it for the fully-instrumented L-4 or L-5 mission? This is analagous to testing a rocket engine on Earth in a test rig, then perhaps in test flights, before applying it to a really serious use where lots of other equipment is at stake. I propose we design a solar-sail-test experiment. We'd have just the solar-sail equipment and the communications system using an omni-directional antenna. We'd have it let loose from the shuttle, it'd sail around the shuttle without really getting far from it, then perhaps we'd have it fly back to the shuttle for return to Earth (perhaps we'd want to check wear&tear on the sail, or we might actually want to re-use it rather than build a new one next time). If it was very successful and by the end of the shuttle mission we had enough confidence to put it into a longer test flight, we'd have it move far away from the shuttle to avoid damage from de-orbit burns of the shuttle. If after much testing we found it to be capable of going to L-4 or L-5, we might send it there, with no special instruments, just see if it can get there at all, and if so use radio attenuation as it flies behind L-4 and/or L-5 to get a crude first approximation to the amount of material there. As its final act, it could crash thru L-4 or L-5 to get another estimate of material there (if it hits something and gets destroyed, that's useful info), or go into orbit of the Moon to provide occultation timing information, or return to LEO for recovery by a later shuttle flight. More likely it won't be totally successful and we'll just keep it in LEO the whole time and build the next one better. When we get one that works, we'll send it on the above-described very-preliminary L-4&5 mission.
henry (01/01/83)
Don't forget that you cannot use a Getaway Special to launch something like this: one of the restrictions on Getaway Specials is that they are forbidden to vent, expel, or eject *anything*.