Asteroid Capture

I think the excerpt from the summary you linked to illustrates the
nature of the problem, Brooks.

The computer simulation JPL ran (remember, this was a simulation based
on a theoretical model) indicates that over the course of 200 days of
simulated "gravity tugging" they achieved a velocity change in this
hypothetical asteroid of ~0.07 cm/sec. at a fuel expenditure of 0.05
kg/day.

Lets say, for the sake of argument, that we're very lucky and that this
asteroid needed a change in velocity of only 0.7 km/s to be redirected
into an eventual Earth-centered circular orbit somewhere in the general
vicinity of the ISS.

At a rate of change in velocity of 0.07 cm/s every 200 days, it would
take 200 million days (547,945 years) to achieve a 0.7 km/sec total
change in velocity. Also, a fuel expenditure of 0.05 kg/day for 200
million days results in a total fuel mass of 10 million kg.

Given that the mass of a 5 meter diameter asteroid would be less by a
factor of (140/5)^3 - (21,952), we can estimate that it would take about
25 years and about 455 kg of fuel to make this very small change in
velocity for this very small asteroid.

What we really need, as has been mentioned, is to pick out a known NEA
that seems a good candidate for materials and have someone familiar with
orbital mechanics and thrust calculations run the numbers (not me - I'm
not that smart). If we do this, then we might be able to start working
with some real numbers instead of simulations and extrapolated values.

Chris

--- In spacesettlers@yahoogroups.com, "brooksn" wrote:
>
> "In summary a 140 meter diameter equivalent, Hayabusa-shaped NEO, with
a rotation period of 6 hours was approached by the t-GT spacecraft with
the initial task of determining a precise orbit for the NEO. The solar
powered, 1150kg. t-GT spacecraft towing performance evaluation was
performed in a 200 day simulated operation in which the t-GT was
stationed 155 meters forward of the NEO center of mass along its
velocity vector. A very simple control law was employed to maintain the
spacecraft within a "box" centered on this nominal location. No control
problems or unusual excursions were experienced during the months of
towing which produced the desired velocity change of ~0.07cm/sec. During
towing a NEO acceleration of
> 0.22 microns/sec/day was achieved at a fuel expenditure of
approximately 0.05 kg/day. The study confirms that a t-GT spacecraft can
determine the orbit of an asteroid accurately enough to assess whether
or not it is on an impact course with Earth, even if that course must
pass through a relatively small keyhole first. Furthermore, towing
operations by such a spacecraft will work with a simple and robust
spacecraft design."
>
> http://www.b612foundation.org/papers/t-GT_summary.pdf
>
> So a 140 meter NEO is able to be tracked and nudged with pretty modest
craft and fuel requirements. Now at 5 meters how much more nudging are
we likely able to do? Also we currently have a 5 meter asteroid striking
Earth every year so having to nudge a candidate by 10s of thousands of
km to get where we want it to be should be less of a problem. Though we
have a few candidates now, I agree it will quickly become an issue
finding new tiny incoming candidates considering our current tracking
ability.