Re: Deep Impact

Dan Clemmensen (
Sat, 14 Mar 1998 15:56:02 -0500

Michael Lorrey wrote:

> Anton Sherwood wrote:
> > > Michael Lorrey <> writes:
> > > > ... since its alread in near earth orbit, simply put a
> > > > small mass driver or plasma thruster and some solar panels on it. Use a
> > > > portion of its own mass as reaction mass, with solar power for driver
> > > > energy.
> >
> > Anders Sandberg wrote:
> > > This will take a while, but would be very nice for a Toutatis-type
> > > asteroid in an orbit close to the Earth's. The problem is the quick
> > > ones (like new comets) that might sweep through the Earth-moon system
> > > once in a while, we have less time to do something about them after
> > > discovery than the Toutatis NEOs and they have a much larger velocity
> > > vector. More heroic measures may be needed, since we need a much
> > > larger isp.
> >
> > Here's a wacky idea:
> > Suppose we've already got a couple of dozen asteroids captured, orbiting
> > Earth somewhere outside geosynch. If the motors that put them there
> > haven't been dismantled, could one of these rocks be shifted into the
> > path of a killer comet, to deflect it and/or break it up?
> Comets have random jettings of gas, which causes their courses to be rather
> erratic. A large lumbering asteroid being driven by a small mass driver would
> not have the capability. However, if we captured some comets, and kept the
> comets sheilded in earth orbit, then we could use the comets mass (the ices
> being much more volatile than rock) for much more rapid maneuvering.

OK, why not keep a bunch of caputred, paired asteroids? each pair
will be coupled by cable and spun about their common center of mass.
you can spin the pair up using mass drivers or ion engines. When a
comet is detected, simply pick an asteroid and cut the cable at the
proper point. This will launch an asteroid on a trajectory to intercapt
the comet. (Please be careful to compute the trajectory of the other
member of the pair!)

Recall the statistics: about 90 percent of the danger is from NEO asteroids.
the remaining 10 percent is from comets. We have the technology to
find and deflect NEO asteroids already, because they come from a
fairly near and bounded space that can be completely searched, and because
we will generally have years or decades to take corrective action.

Comet detection is a lot harder, and deflection is harder still. Detection is
harder because the space in unbounded, and detection must occur on the
incoming impact trajetory. By contrast, NEO detection may occur on
any previous orbit. deflection is harder because the response must be
immediate (unually less thatn one year), and because you have to
intercept. It's not feasible to macth orbits and use the comet's mass.

So, a $10M/year program would probably let us find essentially all
dangerous NEO asteroids in about 3 years, and a $1M/year continuing
prpgram would probably keep the NEO census current. This should
fund a few small (1 meter) 'scopes.

By contrast, I speculate (NO data!) that we will need a $100M/year
continuing program to look for comets. Basically , you need lots of
big scopes to maintain a full comet sky survey.

Bottom line: I think we should immediately fund the NEO survey using
exiting technology. This has a very high return on investment.
I think we need to figure out a cheaper way to do a comet
survey rather than attempting to use current technology immediately.