moon and NEAs

Hi John,

Whether or not a NEA may collide with us is a matter of the relative
trajectories of its orbit and ours. The amount of delta V required to
reach and return from an asteroid is a matter of relative velocity
change required. Accordingly, the fact that an asteroid may or may not
have similar orbital trajectories to Earth does not mean it needs less
delta V than one further away. The fact that any particular NEA might
hit us does not mean it necessarily needs a low delta V to reach it.

Best wishes, Andy.

John Frazer wrote:

> andy-nimmo wrote:
> (About required delta V for reaching the asteroids)
> > You will find though, that the vast majority require considerably
> greater delta V to reach and return from than does our Moon.
>
> Which asteroids are you considering? The Main belt? Certainly,
> these have high delta-V, but I thought the discussion was
> centering on NEAs (Near Earth Asteroids, or the Earth
> Crossers). Many of these orbit between .5AU - 1.5AU, and some
> even are more nearly circular between .8AU-1.2AU. These have
> very near 1 Earth-year orbits, and consequently, they can have
> extremely low delta-Vs. Lewis cites at least one with a return to
> Earth orbit deltaV of only 60 _meters_/sec.
> I'll hazard a guess that nearly a hundred of many types are
> known with less delta-V to reach them than our moon. In addition
> to their low return velocity, they have surface G of less than
> .0001G. Lifting off from them is vastly less demanding than from
> our Moon.
> Then there is Diemos, the outer moon of Mars (bookmarked
> article here)
>
> These offer great leverage, since a mass in LEO from the
> ground can send much more equipment to them than to Luna,
> many are dead-sure certain to have ices as well as metals of
> better concentration than our Moon. Crews can be exchanged
> every so often, so even with radio delays of up to 18 minutes,
> work can continue, and the _entire big rock_ can be brought
> back right to the factory with mass drivers or another rocket
> throwing away the slag.
> Mining & separating these materials is not difficult to do. See the
> relevant articles at permanent.com (the bookmarks link at left)
> The fact that the Moon has gravity doesn't make it more
> reassuringly familiar to us; gravity hinders many processes, and
> the Lunar 14 day night is very bad.
>
> > Al Globus wrote:
> >> A lot of asteroids are found these days because of concerns
> about a collision with earth. I haven't checked the data for
> awhile, but I'm sure many asteroids with very low delta-v have
> been found.
>
> I'm fairly sure I wrote this before, but Al is exactly right. The
> Earth-crossers include many of those with an elliptical orbit,
> some of which take them out to 2.5AU+. The term NEA is more
> specific to those which are not so elliptical, and spend more of
> their time very near 1AU. The very same fact that they have low
> delta-V requirements is what makes them so dangerous: They
> would enter our atmosphere with low relative velocity, and they
> would not build up a tremendous pressure wave of
> super-heated air. The higher the pressure of this bow-wave, the
> more likely that it will exceed the crush strength of the body, thus
> a lower velocity means it's more likely to hit the ground! Added to
> this, is the fact that a metallic or stony body has more mass to be
> released as it vaporises after hitting the ground.
>
> >> andy-nimmo wrote:
> >>> ...As I recall, (Back in the 70s) only two or three asteroids
> had been discovered that needed less delta V to get to and
> return than the Moon. I haven't heard of any new asteroids in that
> category having been discovered since, but then delta V is not
> what the various reporters report on these days. Perhaps it is
> time we got that changed back again? Problem is, most of those
> who report probably don't know what delta V is!
>
> Old data.
> SpaceWatch is starting out, with pathetically small funding, but
> they're reporting new ones all the time.
> Read John Lews: "Rain of Iron & Ice" and "Mining the Sky".
> See "Threat or a Promise"
> http://www.space-frontier.org/PROJECTS/ASTEROIDS/
>
> "As many as 900 large asteroids (a kilometer or greater) in the
> inner solar system... according to a new study published in the
> June edition of the journal 'Science'..."
>
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