Subject: New Mitigation Strategy Minimizes Risk Of Asteroid Collisions
Date: 5 Apr 1998 23:33 UT
Organization: Jet Propulsion Laboratory
Lines: 70

News Bureau
University of Illinois at Urbana-Champaign
807 S. Wright St., Suite 520 East
Champaign, IL 61820-6219
(217) 333-1085   fax (217) 244-0161
e-mail: uinews@uiuc.edu

CONTACT: James E. Kloeppel, Physical Sciences Editor (217) 244-1073
E-mail: kloeppel@uiuc.edu

April 1998

New mitigation strategy minimizes risk of asteroid collisions

CHAMPAIGN, Ill. -- The spectacular plunge of Comet Shoemaker-Levy 9 into
Jupiter in July 1994 and recent concern about the projected "near miss" of
Asteroid 1997 XF11 with Earth in October 2028 brought renewed awareness that
collision events do occur within our solar system -- and the next one could
involve our planet. In fact, such a collision may be long overdue, and steps
should be taken to alleviate the risk, a University of Illinois researcher
says.

"If faced with this kind of danger, we would want to send a spacecraft to
intercept the object as far from Earth as possible," said Bruce Conway, a
professor of aeronautical and astronautical engineering. "This would allow
whatever mitigation strategy we use to have the longest time to act."

There are two practical problems that must be solved, however, Conway said.
"The first is simply getting a sizable payload to the object in the shortest
amount of time, and the second is deciding what to do when we get it there."

In a paper published in the September-October (1997) issue of the Journal of
Guidance, Control, and Dynamics, Conway described the optimal low-thrust
interception of a potential collider. The proposed mission scenario would
combine the speed of conventional chemical rockets with the increased
payload capability of continuous-thrust electric propulsion. Having arrived
at the destination, however, what should be done to prevent the impending
collision?

"For years, we assumed that the best mitigation strategy was to blow up the
object with a nuclear warhead," Conway said. "But that may not be such a
good idea. If we blow it up, instead of having just one large mass hurtling
toward the Earth, we could end up with a multitude of smaller -- but equally
lethal -- objects coming at us. A better alternative would be to deflect the
object."

One possible mechanism to accomplish this would involve detonating a nuclear
warhead above the asteroid surface, Conway said. "That would create a
crater, and a large portion of the jet of vaporized material would shoot off
in one direction -- like a rocket -- and push the object in the opposite
direction."

But which direction should the object be pushed to ensure that it will miss
the Earth? And would it make more sense to speed the object up or slow it
down?

Conway's latest research has focused on answering these questions. He
developed an analytical method that, given the orbital parameters of the
object and the interval between interception and close approach, determines
the proper direction in which to push the object to maximize the deflection
in the required time.

Such calculations may never be needed, but they're nice to have just in
case.

"While the probability of a large asteroid or comet colliding with the Earth
is low, the potential for destruction is immense," Conway said. "It's
probably not something we should lose sleep over; but, on the other hand, it
would be really silly not to do anything."

12 responses total.

That "analytical method" has been known for a couple of centuries - its
good old orbital mechanics, and is now solved routinely for space missions.
But the article was written for popular consumption and it wasn't Conway
that claimed anything novel about the calculations. It would seem, if it
hasn't been done, worthwhile to investigate a few scenarios to see what
impulses are necessary to obtimize trajectory alteration (and also not
put the object on an alternate trajectory that would collide at some
future time - putting it into the sun would be the best choice).

Unless an object's current perihelion is well inside Mercury's orbit
(quite unlikely), it will take extremely fine control or a huge delta-v
to drop it into the sun.  So forget it for substantial objects.  If
you're trying to do "get rid of it forever" clever tricks, lunar impact
sounds like the way to go.  (Just watch out for the splash.)

If you're trying to get rid of it forever, a gravity whip onto an
escape trajectory is the easiest.  Only 41% of the delta-V for a
solar impact.

However, there's not much point in doing that.  A rock big enough
to be a threat to Earth is also a rock that might have considerable
value; like fire, a terrible force if wild, a wonderful servant when
tamed.  Using reverse gravity-whips past Luna until it is captured
into Earth orbit would keep it handy, then you can mine it for
raw material for anything you like.  Shielding for geosynchronous
satellites might not be a bad use for raw asteroid-stuff; they
get messed up by solar flares too often.

Why wouldn't a gravity whip into the sun be just as good? I don't
particularly like lunar impact, as that could send a *bigger* lump
into us. Thinking again of the gravity whip...it *would* require
rather fine tuning of the trajectory shift....that would require a
much more refined technology than a "more than sufficient" nudge to
just avoid a collision. 

Russ has a good point, but let me build on it.  How about mining the astroid
by hollowing it out and then converting it into a space habitat?

Of course, we already have a satellite in earth orbit that we are not
mining (yet). Maybe we should practice there first.

If we did that, wouldn't we lose it because its gravity gets less and
therefore obtaining a bigger orbit?  In turn increasing the mass of the
earth and shifting our orbit around the sun?

Changing the mass of the moon would not necessarily change its orbit.
If the moon split in two, the two pieces of half mass each would orbit
at the same distance in the same time. However it would depend on
how the mass of the moon was transferred to the earth - that is, the
reaction forces of departure from the moon and landing on the earth.
This could be done without having any effect upon the orbital periods
of either the moon or the earth. The effects upon the rotational periods
might be more significant, however. 

Re #4:  Sending the Solar Polar mission onto a trajectory over Sol's poles
required a very precise gravity whip past Jupiter.  It would require one
of equal or better precision to hit Sol.  On the other hand, there are
lots and lots of trajectories which go past Jove and never return, or you
could just do a Shoemaker-Levy 9 and hit it.  Or Mars.  Or Venus.
 
Re #5:  Unfortunately, the best projections based on recent measurements
of the density of asteroids doesn't hold much hope for hollowing them
out.  They appear to be heavily fractured and composed of perhaps 50%
void space; any hole carved out might well collapse.  However, this
does seem to make it easier to take them apart, as the job is half done.
 
Re #6:  There are a couple advantages that an asteroid has over Luna as
a source of raw materials.
 
The first:  Escape energy.  Luna's escape velocity is about 2.4 km/sec.
The escape velocity of a 1-mile asteroid is going to be on the order of
*meters* per second, a million-fold difference in energy.  This makes it
really easy to put equipment down and take materials away.  You don't
need rockets; you could literally launch multi-ton loads with bungee cords.
(You won't have any outdoor baseball games, as every hit would be a
home run.  Too hard on the defense.)
 
The second is composition.  The Apollo data show that Luna has almost
no volatile compounds.  There is next to zero carbon, sodium, and many
other things common on earth.  Most of the iron probably fell to the
core as well.  An asteroid may well have been heated less and contain
more of our necessities or in more accessible forms.
 
On the other hand, it appears very likely now that Luna has ice; we
already knew about the iron fines and the helium-3.  We could use
both, I guess.  If all you want is "space dirt" to pile around things
for protection, either will do.
 
Re #7:  Luna's mass is about 7*10^22 kilograms.  If you used a million
tons of it a day (1e9 kg), it would take you 2 billion years to change
its mass by 1%.  You can forget about changing the orbit significantly
unless and until you get *really* serious about it.

My understanding was that "scoop it up with a shovel" on the lunar
surface would usually get you over 8% iron by weight.  (Apollo data.)

Re #10:  Indeed, that is the case.  The average of the Apollo 12
crystalline rocks was 16.6% iron.  Unfortunately, the way the numbers
are presented in my reference doesn't make it obvious how much was
metallic and how much was oxide.  Trying to sum the weight-percentages
of the elements and oxides goes well over 100%.
 
While the surface of Luna has plenty of native iron from impacts of
iron meteroids, it's almost certain that everything else in this
neighborhood of the solar system does too.  This makes it unlikely
that Luna has big advantages in that regard.  Given that an asteroid
mining robot could conduct simple operations with equipment amounting
to a magnet and a big pogo stick, the asteroid would probably be much
cheaper to target for a first attempt.
 
There is one last reason to go for an asteroid.  Most space equipment
used inside the orbit of Mars is solar powered.  Having a "day" which
is much shorter than 300-something hours is another big advantage for
the typical asteroidal body; any equipment sitting still doesn't have
to endure the chill of such a long night.

Mining Luna vs. asteroids sounds mostly like a details decision to me.
Sure, you can drop-kick 10-ton hunks of high-grade off the asteroid -
but where's it headed?  Probably somewhere far away, so you still gotta
pay the delta-v after getting the stuff clear of asteroid #99A52734.
Moving the whole asteroid only works if you've got the time, resources,
right asteriod, and need *lots* of material.  For smaller needs, shorter
lead times, and elements that are there, the moon makes more sense.  A  
bulldozer can quickly pile dirt into cheap, efficient radiation/thermal/
micrometeor barriers.  No air and (relatively) low excape velocity mean
that an economical, efficient mass thrower can deliver a steady stream
of whatever you're mining most of near-earth space.  

Response not possible - You must register and login before posting.

- Backtalk version 1.3.30 - Copyright 1996-2006, Jan Wolter and Steve Weiss