dietz%usc-cse@USC-ECL@sri-unix (12/16/82)
Reply-to: dietz@USC-ECL
The following article appeared in New Scientist (Dec. 2, 1982, page 563):
"How to flatten spacetime"
The general assumption that an object in orbit around the Earth
experiences zero gravitational forces is not quite true. The orbital
motion precisely cancels the Earth's gravitational force only at the
object's centre of mass. At other points, there is a small residual
gravitational field know as the gravity gradient tide, which arises
from the gradual decrease of the Earth's gravitational force as
distance from the planet increases. Such forces can produce a force of
1.0E-7 m/sec^2, or 1.0E-8 of the force generated by the Earth's
gravitational field at sea level, at points only 3 cm from the
sattelite's centre of mass, according to Robert L. Forward of the
Hughes Research Laboratories in Malibu, California. In addition, the
mass of the object also produces its own gravitational field, producing
a "self gravity" of comparable magnitude.
These residual effects could make it impossible to achieve the perfect
"zero gravity" environment envisioned for space-based experiments on
gravitational measurements or materials processing. In practice,
residual forces are far above the theoretical minima, particularly in
manned spacecraft. Forward points out that "a sneeze by a crewmember
in a 100-tonne spacecraft will induce an acceleration of 1.0E-3m/sec^2,
or 1/10,000 of the force of gravity at the Earth's surface. Manned
spacecraft thus probably will not be suitable for experiments requiring
extremely low residual gravity. However, in a paper in @i(Physical
Review D) (vol. 26, p. 735) Forward proposes ways to reduce residual
effects in small volumes by as much as a factor or 1000 below what
would otherwise be the theoretical minimum in unmanned craft.
To reduce gravity-gradient tides, says Forward, a ring of mass would be
ideal, but from the practical standpoint, though, it is sufficient to
use six spheres in a ring that has a plane perpendicular to a line from
the central object to the centre of the Earth. Six 100-kg spheres
would produce a counter-tide that could reduce tidal accelerations by a
factor of 100. The affected volume would increase with distance from
the Earth, from the size of a box of bath powder in near-Earth orbit
(about 100 km altitude) to the size of a birthday cake (30 cm in
diameter and 20 cm thick) in geosynchronous orbit at 23000 km. Forward
developed the "gravity-gradient-compensator" for a science-fiction
novel he wrote, @i(Dragon's Egg), which was first published in 1980.
He needed the gravity-tide cancelling ring to let humans approach
within 400 km of a neutron star, inhabited in the novel by a
rapidly-evolving life-form. Without the compensation, the gravity-tide
forces close to the neutron star would have been enough literally to
tear the people apart.
In his paper in @i(Physical Review), Forward also proposes a different
approach to reducing self-gravity. For a disc shaped object, external
guard rings and guard caps could smooth out edge effects, the object
could be rotated, and two exterior massive spheres cuold serve to lower
the gravitational forces inside the disc. In one example, Forward
calculates that self-gravity should be reduced by a factor of 2000.
Any practical applications of the proposals are likely to be 15 or 20
years away. Gravitational fields a millionth or less than that of the
Earth should already be possible in free-orbiting unmanned sattelites,
and these are low enough for the first experiments in growing materials
and producing pharmaceuticals. However, Forward feels certain that
demands for higher-quality materials will eventually lead to a need for
reducing gravitational fields below the natural levels.
>From a theoretical standpoint, gravitational forces are the physical
manifestation of the curvature of spacetime, induced by the mass of
objects such as the Earth. Thus reducing gravitational forces is
tantamount to flattening spacetime, and Forward concludes: "One would
think that there would be a good scientific use for a chunk of flat
space the size of a hatbox, but except for gravitational clock
experiment, I have not thought of one [yet]."
REM@MIT-MC@sri-unix (12/16/82)
From: Robert Elton Maas <REM at MIT-MC> At the start of that article Mr. Forward is beating a dead horse. Not even network newscasters are referring to the STS environment as "zero-gee" now. They're referring to it as "micro-gravity". This is one case where Jules Bergman et al were one step ahead of yours truly! (I.e. now "everybody knows there's no such as zero gravity.") Mr. Forward is saying microgravity isn't enough, nanogravity is needed. Well, for many experiments microgravity is quite sufficient. When it isn't, his ideas for achieving nanogravity are interesting. But I'd prefer developing SEPS so we can go to deep space where nanogravity is the default rather than trying to cancel curvature in LEO. Also in deep space you can cancel nano-gravity to achieve pico-gravity if you have instruments able to measure the nano-gravity and algorithms&computers to compute the placement of the masses to cancel it. <Opinion by REM, a lay scientist, not professional in this area.> FROM:37'28N122'08W415-323-0720, about 3 miles from Stanford
karn (12/16/82)
There's one additional factor that disturbs the low gravity environment, at least aboard the shuttle: thruster firing. When the orbiter is commanded to hold a particular attitude, thruster firing can be quite frequent in order to counteract the orbiter's tendency to orient itself along the local gravity-gradient. Free-flying payloads, such as the Long Term Exposure facility, will probably be much better for experiments requiring minimal accelerations. Phil Karn