NEA trajectory / relative velocity

andy-nimmo wrote:
> 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.

Quite the contrary: Its orbit is very like ours (definition of NEA).
Similar mean distance from the Sun necessarily means that its
velocity around the Sun is similar to ours around the Sun. Hence,
low Delta-V requirement.

> The fact that any particular NEA might hit us does not mean it
necessarily needs a low delta V to reach it.

The 2 are interconnected -one factor depends on another. If it
didn't have an orbit very similar to ours (which would make it less
of a threat) then it would have a larger velocity difference (which
would make it a poor prospect for mining).

The other class of threatening asteroids, the Earth Crossers,
are not such good prospects. The fact that they have more
elliptical orbits, and that they go farther out from the Sun, means
that when they are in closer (and nearer to us) they are moving at
their fastest (Back to Kepler: when an elliptically orbiting body is
closest to its primary, it moves faster). Incidently, this also
means that it would hit Earth's atmosphere with higher velocity,
thus less likely to survive aeroentry (this doesn't apply to the big
ones, though, which makes these Earth crosser elliptical orbit
asteroids very dangerous).

It has been suggested that since many of these go out to 2 or 3
AU, and are sweeping through the main belt, it would be helpful
to hitch a ride on one out to the main belt.
This does not follow. While they are close to us, they are moving
fast, so we'd need a lot of delta-V to match them. If you've got the
delta-V to do this, you can get out to the belt on your own.
Similarly, if you did ride one out to the belt, you would be moving
at the same speed as your taxi: too slow for that orbital distance.
You would fall back in if you didn't expend propellants to
circularise at that distance from the Sun.
The only possible help is that they could supply volatiles to the
ship during the trip out -either for life support or for refueling.
In such a case, it doesn't make sense to go out to the belt, since
your taxi has what you need -unless you were going to the belt
anyway, and you had the rockets to do the required velocity
changes, but for some reason needed to resupply on the way.

Another option is for later.
EM rails could be attached to the taxi, so that a properly
equipped ship could engage these rails, to get boosted to the
asteroid's speed as it swept past. Similarly, the rails could shoot
you off the asteroid taxi with the required speed to let you stay out
there. Tethers could also do the same thing.
In this case, you're adopting the 'Cycling Station" concept.
Entirely likely, and useful, if the needed EM rail or tether
capability exists.
Note that this follows along with Moravec's concept for rotating
tether momentum exchange stations throughout the solar
system. Sheffield & Clarke also used this in fiction.

>> 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.
>>
>> John F. wrote:
>>... 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!