From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech,sci.space.science
Subject: Re: Where do 'near Earth' asteroids come from?
Date: Sun, 5 Mar 2000 03:25:22 GMT

In article <89rfsd$gh9$1@nnrp1.deja.com>,
Andrew Thompson  <amthompson64@my-deja.com> wrote:
>As I understand, NEA have highly elliptical orbits and
>originate from the the zone between Mars and Jupiter.  If this is
>correct it would mean that when the asteroids are 'near' Earth they
>would be travelling at a high velocity relative to Earth.

No, not all of them.  While that is probably how their orbits *start out*,
then are then modified repeatedly by encounters with the inner planets.
The result is that some of their orbits are still quite eccentric, but
others are nearly circular, and some make encounters with Earth at fairly
low relative velocity.

>It would follow that even though they're close, the fuel required to
>*land* on NEA's (as opposed to doing a fly-by) would be greater than
>that necessary to get to Mars.

In general, reaching a well-chosen NEA is easier than reaching Mars orbit.
It's *much* easier than reaching and returning from Mars's surface.

>Am I correct in assuming NEA's originate from the main Mars-Jupiter
>belt?

While it is not absolutely proven, the current consensus theory is that
the NEAs are the result of main-belt asteroids being driven out of the
belt by (mostly) resonances with Jupiter.  There are various small
disturbances which cause asteroid orbits to shift a bit sometimes, and if
they shift into a resonant relationship with Jupiter, the orbit can then
change a great deal in (astronomically) a very short time.
--
Computer disaster in February?  Oh, you |  Henry Spencer   henry@spsystems.net
must mean the release of Windows 2000.  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Mars
Date: Wed, 8 Mar 2000 14:30:28 GMT

In article <8a4vub$2h7$1@nnrp1.deja.com>,  <pooua@aol.com> wrote:
>> ...as neutron bombs aren't effective if you can put 8 feet of
>> relatively dense material (dirt will do) between you and the bomb...
>
>At what point in the history of development do you
>expect to see orbital space stations w/ 8-foot thick
>walls? Not right away, I would guess.

Fairly early, once one starts talking about long-term living in space.
Shielding approaching that is highly desirable to bring cosmic-ray dose
down to healthy levels.  Not worthwhile for short flights, but when you
start spending decades in space, it's a significant issue.

>...Whereas our
>knowledge of asteroids so far shows that at least some
>of them are actually rubble piles held together
>gravitationally...

Careful here:  that's only one theory -- although its advocates tend to
present it as established fact -- for the strangely low density of some
asteroids.  There is at least one credible alternative (ice under a
blanket of regolith), plus the obvious possibility of surprises.  There
is *no* evidence specifically pointing to that theory in particular.
--
Computer disaster in February?  Oh, you |  Henry Spencer   henry@spsystems.net
must mean the release of Windows 2000.  |      (aka henry@zoo.toronto.edu)