A while back we had a debate regarding the safety of mass driver
propulsion, such as has been proposed for the "tug" in the O'Neill
architecture. Under that architecture, bucket-loads of lunar material
are caught by the tug at an Earth-Moon Lagrangean point. The tug
periodically moves the accumulated load from that Lagrangean point to the
processing site (elsewhere) using mass driver propulsion, firing pellets
off into the void.
was extremely unlikely that any of the pellets would return and become a
hazard at some later point in time.
My question is,
What say you now after reading the article below?
Ron
baalke@...
09/19/02 04:09 PM
Please respond to neo-owner
New Clues in the Continuing Detective Story of J002E3
Paul Chodas and Steve Chesley
NASA's Near-Earth Object Program Office
September 19, 2002
The precision of our orbit solution for the unusual Earth-orbiting
object J002E3 has improved considerably since last week, as
astronomers around the world have continued to track this
interesting body and provide measurements of its position.
Even though the object has been tracked for only 15 days and
traveled only about one sixth of the way around its orbit since
discovery, it is now possible to draw more precise conclusions
about its origin and future destinations.
While it was strongly suspected a week ago that J002E3 had been
captured by the Earth in April of this year, it was not known
how long the object had been orbiting the Sun prior to capture.
Additional observations have now confirmed that the object was
indeed captured from a solar orbit earlier this year, and they
have also made clear that it escaped the Earth-Moon system in
March 1971. The mechanics of the escape are much like those of
the capture in reverse: in both cases, the object passes slowly
through a portal separating the regions of space controlled by
the Earth and Sun. The portal is located at the L1 Lagrange
point, about 1.5 million kilometers from Earth on a line
towards the Sun.
The timing of the object's escape is consistent with our
theory that this object is the Apollo 12 S-IVB third stage,
which was left in a distant Earth orbit after it was launched
on November 14, 1969 and passed the Moon four days later. We
theorize that the spent rocket orbited the Earth chaotically for
15 months before finding the exit pathway through the L1 portal.
The excellent match between the intrinsic brightness of J002E3
and that expected for a rocket stage of the S-IVB's size also
supports this theory. The other four S-IVB stages still flying
(those from Apollos 8 through 11) have been dismissed as
suspects because they entered solar orbit much earlier than
March 1971.
Other possibilities for the true identity of J002E3 have been
suggested. The object could be one of the Spacecraft-Lunar
Module Adapter panels which enclosed the Lunar Module during
Apollo launches. Or, the object might be a rocket stage from
an unmanned lunar probe from that era. None of these, however,
were launched at the right time or are known to have entered
the sort of distant orbit from which escape is possible.
Furthermore, these alternative candidates seem too small to
explain the current brightness of J002E3. We conclude that the
Apollo 12 S-IVB stage is the most likely identity of the object.
Our improved orbital knowledge for J002E3 is also allowing
more precise predictions for its future motion. The likelihood
that the object will impact the Moon next year has decreased
to less than one percent. This new conclusion follows from
the fact that the range of possible motion in 2003 is now more
tightly constrained and barely intersects the Moon. The
possibility of collision with the Earth has also decreased,
down to well less than one percent. (Even if it should hit our
planet, the object is too small to be considered hazardous.)
It now appears likely that the object will escape back into
solar orbit in June 2003 after its brief six-orbit visit to
our planet. In 30 years time the Earth may once again capture
J002E3 for another brief tour around its home planet.
Message A while back I made the comment on a space newsgroup that one wouldn't have to worry about the pellets coming back because the exhaust velocity would be in excess of solar escape velocity. Someone posted to say that was crazy, and that there'd be no reason to make the enormous investment in energy necessary to accelerate the reaction mass to solar escape. I don't know that they were looking at the specific exhaust velocityproposed in the High Frontier studies that got published or just expressing their own opinion. I've noticed thetendency of many to substitute their own opinions for many of the recommendations published in the NASA-sponsored studies. I posted to say that I was working off of memory, and allowed that I might have misremembered what I read and perhaps it was only stated that the exhaust velocity was in excess of Earth escape. That might satisfy many, as at least the pellets would leave cislunar space. But if the exhaust was only just barely in excess of Earth escape, I suppose the scenario you raise could come about. Does anybody know the specifics of the mass-driver tug exhaust velocity?
Mike Combs
> Does anybody know the specifics of the mass-driver tug exhaust velocity?
>
http://lifesci3.arc.nasa.gov/SpaceSettlement/spaceres/III-3.html , by
O'Neil himself et al. , it seems (from the graphs) that they were
considering exhaust velocities ranging from 100 m/s to 100 Km/s (!).
Well, at 100 Km/s you certainly would throw anything out of the Solar
System regardless of the direction. But it seems that other projects
using mass-driver tugs, like
http://lifesci3.arc.nasa.gov/SpaceSettlement/spaceres/III-3.html , are
considering "modest" exhaust velocities of 4-10 Km/s - and that would
turn the pellets into Solar-System trapped objects.
> Regards,
>
Regards,
> Mike Combs
Lucio Coelho
Well, at 100 Km/s you certainly would throw anything out of the Solar
System regardless of the direction. But it seems that other projects
using mass-driver tugs, like
http://lifesci3.arc.nasa.gov/SpaceSettlement/spaceres/III-3.html , are
considering "modest" exhaust velocities of 4-10 Km/s - and that would
turn the pellets into Solar-System trapped objects. And more to the point, they would remain in cislunar space. Anything ejected in excess of 11 Km/sec will leave cislunar space, but I think Ron's point was that "well in excess" of Earth escape was preferable to "just barely in excess" of Earth escape, as the recapture of that Apollo 3rd stage attests. But I think ultimately, the argument was, "Micro-meteors are already whizzing through space everywhere in the solar system, so the only reasonable concern is: Will the mass-driver reaction mass significantly increase the already-present risk?" And the answer was no, it would make an insignificant contribution to the problem. But all the same it was recommended that,shortly after the ore refineries were producing oxygen, to switch the mass-driver tugs over to liquid oxygen for reaction mass, as we would be producing more oxygen than we needed anyway, and the oxygen would vaporize on release. So the stages where the mass-driver tugs would initially be using pelletized Space Shuttle ETs, and then later raw lunar or asteroidal soil, would be early and temporary stages of the operation.
Mike Combs