PLEASE NOTE:
*
CCNet-ESSAY, 16 February 2000
-----------------------------
DEFENDING EARTH: FACT VS FICTION
By Michael Paine
Special to SPACE.com posted: 06:23 am EST 11 February
2000
http://www.space.com/space/technology/asteroid_defense_000211.html
First, the good news: Asteroid Eros is not on a collision course
with
the Earth.
At roughly twice the size of Manhattan Island, Eros is huge
compared
with other known near Earth asteroids. A collision by an object
this
size would be more devastating than the impact that is thought to
have
finished off the dinosaurs 65 million years ago.
Eros in the news because, on Monday, after a torturous four year
journey, the NEAR spacecraft will attempt to become an artificial
moon
of Eros. A successful NEAR mission to Eros will show that we have
the
ability to rendezvous with an asteroid and to orbit it.
This ability is crucial if -- some scientists would say
"when" -- an
asteroid is discovered to be on a collision course with the
Earth.
Space missions to asteroids and comets might not seem as exciting
as a
landing on Mars but the social,scientific and commercial benefits
from
these missions could be great. An asteroid or comet impact with
the
Earth is the only type of natural disaster that could instantly
wipe out
human civilization and yet, unlike earthquakes, floods and
volcanoes, it
is within our grasp to prevent the collision.
The know-how needed to protect the Earth from collision could
also be
used for commercial mining in space. Comets and asteroids are
packed
with useful raw materials. Eventually space prospectors might
want to
rendezvous with them and, perhaps, change their orbit.
Catching Comets; Angling Asteroids
To learn more about the physical properties of asteroids we first
have
to reach them with spacecraft.
The NEAR mission is the first attempt to rendezvous with an
asteroid. A
rendezvous involves carefully maneuvering the spacecraft so that
it
follows nearly the same orbital path as the asteroid. The
spacecraft
slowly approaches the object then adjusts its speed so that the
spacecraft and asteroid follow the same path around the Sun. In
the case
of NEAR a further maneuver will put the spacecraft into orbit
around the
asteroid.
Previous spacecraft missions to asteroids and comets have
involved quick
flybys with no attempt to match speed with the object.
These missions
were important steps in our exploration of
these objects but improved
technology was needed to achieve a rendezvous.
One recent space mission was designed to test new technology. In
July
last year the Deep Space 1 spacecraft passed within 10 miles (16
kilometers) of asteroid Braille. This mission successfully tested
two
important technologies - auto-navigation and the ion drive.
Auto-navigation means that the robot spacecraft worked out its
own
location in space and the course to the target object.
The ion drive (pictured below) is an advanced form of propulsion
where
the particles coming out of the exhaust are electrically charged
(ions)
and they are accelerated by electrical means to very high speeds.
Solar
cells or a nuclear generator could provide electrical power.
Deep Space 1 used an advanced solar collector to generate a
stunning
2500 watts of power. By using a steady, reliable power source the
ion
drive can gradually accelerate the spacecraft to interplanetary
speeds.
Within twelve months Deep Space 1 will have consumed all of its
180
pounds (80 kg) of Xenon propellant and reached a speed of 9000
mph (4
kilometers
per second).
Dr Marc Rayman from the Deep Space 1 mission team explained that
the
Braille flyby was a bonus for the primary mission that was mainly
designed to test new technology. The experience gained at Braille
will
help them plan an encounter with Comet Borrelly - the main target
of the
extended mission. He added that the failure, last November, of
the
spacecraft's "star tracker" navigation aid meant that
they had dropped
plans to reach a second comet but otherwise the failure would not
seriously hamper the extended mission.
Sling-shots from planets
Many recent interplanetary space missions have involved a
gravity-assist
flyby of the Earth. This sling-shot technique can produce
substantial
reductions in the size of rocket needed to reach a planet or
asteroid.
For example, in January 1998 the NEAR spacecraft whizzed within
340
miles (550 km) of the surface of the Earth. This planned
encounter
changed the course of the spacecraft so that it reached Eros one
year
later (unfortunately a technical bug prevented the spacecraft
from going
into orbit and the mission scientists had to wait a year for the
next
opportunity).
Of course, an Earth flyby would be very difficult to sell to the
world's
population if the spacecraft was carrying nuclear weapons
intended to
deflect an asteroid. Adding further to the difficulties, the best
time
to nudge an asteroid is when it is closest to the Sun and this
can make
the mission much more challenging.
Alan Harris, Senior Research Scientist with JPL in California
points
out a mission to rendezvous with asteroid 1999 AN10 -- in an
orbit
which is typical of a "potentially hazardous asteroid"
-- would involve
a space mission which is formidable with current rocket
technology.
They run out of fuel well before the necessary speeds are
achieved.
Maybe we should be dusting off the blueprints for the giant
Saturn 5
rockets that were used for the Apollo Moon landings - just in
case we
need to quickly intercept an asteroid or comet on a collision
course
with the Earth. This may not be that easy - in his book
"Mining the
Sky", planetary scientist John Lewis reports that he went
looking for
the Saturn 5 blueprints a few years ago and concluded,
incredibly, they
had been "lost".
Harris cautions that even the mighty Saturn 5 could only deliver
a few
pounds/kilograms of payload to land on, or orbit, an asteroid
such as
1999 AN10. He adds "Ion drive is probably the most feasible
way out of
this quandary."
To Nuke or To Nudge
An asteroid is heading for Earth. With just days to go before the
collision a beefed-up space shuttle is sent to intercept it. A
brave
team of astronauts and oil-rig workers drills deep into the
space
rock, plants a nuclear bomb and blows it in two. The two halves
fly
apart and miss the Earth.
Dream on!
The idea of blowing up an asteroid makes for good movie scripts,
but is
not the way to do it in the real universe. Many of the fragments
would
remain on a collision course and like the blast from a shotgun;
the
fragments can do up to ten times as much damage as the original,
intact
object.
In any case, Erik Asphaug from the University of Southern
California has
modeled "rubble-pile" asteroids and finds that blowing
them up with
bombs may be much more difficult than with asteroids made of
solid
rock. It is a bit like the difference between hitting a sandbag
and a
solid sandstone block with a sledgehammer -- the sandbag absorbs
the
impact with little disruption but the sandstone block shatters.
"Stand-off" nuclear explosions are favored by some
scientists (see
animation) and might work with both solid and rubble-pile
objects.
A nuclear bomb is detonated several hundred yards away from the
object.
Surprisingly, it is the intense radiation generated by the
explosion
that does the job. In one scenario, the radiation grills one half
of the
asteroid and causes a very thin surface layer to vaporize and fly
off
into space.
Tens of tons of material blasting off the asteroid at high speed
would
be sufficient to jolt the asteroid in the opposite direction. The
effect
is like the recoil of a rifle -- a small bullet moving at high
speed
causes the heavier rifle to recoil at low speed.
One thing most scientists agree on is there is no need to
maintain an
arsenal of nuclear weapons in space ready to intercept rogue
asteroids.
They also point out that there are ways to deflect asteroids that
don't
require nuclear explosions and we should be looking at these
methods
more closely.
Deflection
In theory, an asteroid that is found to be on a collision course
with
our planet can be deflected to avoid an impact.
The deflection involves changing the asteroid's course with a
sideways
push or, preferably, changing its orbital speed so that it
arrives
before or after, rather than when Earth crosses its path. In
either
case the deflection is far more effective if it can be carried
out
years or decades ahead of the predicted collision.
For example, after twenty years, a nudge of just 1 m.p.h. (1.6
kilometers per hour) would change an asteroid's location in space
by
about 170,000 miles (273,500 kilometers). That is more than
halfway to
the moon.
Recent discoveries suggest that deflection of some
Earth-threatening
asteroids may be easier than first thought. Most schemes for
nudging
asteroids into a safer orbit assumed a single catastrophic
encounter
with Earth. This meant changing the course of the object by at
least
4,000 miles (6,300 kilometers) -- the radius of Earth.
Alan Harris, from NASA's Jet Propulsion Laboratory, explains that
scientists now realize an asteroid will usually make several
close
passes by the Earth before a collision occurs.
The recently discovered 1000-yard (1-kilometer) wide asteroid
designated
1999 AN 10 provides an instructive example. It will make a close
pass of
Earth every few decades. During each pass the asteroid is
deflected
slightly by the Earth's gravity.
Astronomers in Italy have calculated that a critical deflection
could
occur in 2027. This would involve the asteroid passing through an
imaginary hoop in space they call a "keyhole". If the
asteroid were to
pass through this keyhole, which is only about 60 miles (100
kilometers)
across, then it would collide with the Earth on its return in
2039.
When the initial calculations were made, astronomers didn't know
the
orbit well enough to determine if it might pass through the
keyhole.
After important follow-up observations were made they have now
pinned
down the orbit enough to be sure that it will not pass through
any
keyhole in 2027 and there is no chance that it will collide with
Earth
in the next century or so.
If, however, they had determined instead that there was a chance
it
would pass through a keyhole in 2027, then a mission to place a
transponder, like a radio homing device, on the asteroid would
have been
wise so that its orbit could be determined precisely.
Harris explains that such a high level of precision would likely
be
required to determine for sure if the asteroid were on a course
through
a keyhole and, if it came to be, to measure the success of any
deflection efforts. In this case a deflection of just a few
hundred
miles prior to the 2027 keyhole event would be all that was
needed to
avoid the 2039 collision.
Deflection of dangerous asteroids that are not in a
"keyhole" orbit is
more difficult because a larger change in course is required. The
task
is still feasible provided that sufficient warning time is given.
If a serious global effort is made to discover most large
near-Earth
asteroids within the next decade, then we should have decades, or
even
centuries of warning before a devastating impact. With such lead
times
only a relatively small nudge is required to change an asteroid's
course
so that, decades later, it will miss Earth.
Sailing with Sunlight: Non-nuclear Asteroid Deflection
Asteroid expert Jay Melosh from the University of Arizona has
looked at
a range of ideas for deflecting asteroids without resorting to
nuclear
weapons. They include:
Deploying a giant parabolic mirror to concentrate the Sun's rays
and
vaporize rock on the surface of the asteroid. The vaporized
material
flies off at high speed and generates a recoil action that pushes
the
asteroid, slowly but surely, in the opposite direction.
Landing cannon-like devices on the surface to fire asteroid
material
into space. This also depends on recoil action. An ion drive, as
used on
the Deep Space 1 spacecraft, might do the trick.
Attaching a giant solar sail to the asteroid
The solar sail uses the small pressure of sunlight acting over a
large
area to steadily move the asteroid.
Melosh points out that the sail needs to be steerable, like the
sails of
a modern yacht, to tug the asteroid in the right direction:
"An
along-orbit push (at right angles to the Sun) is by far the most
effective in changing a collision into a miss," Melosh says.
There are two other ideas related to the solar sail concept: a
giant
silvery balloon(which in theory would be easier to deploy than a
sail)
and wrapping the asteroid in foil (or painting it) to increase
its
reflectivity. Melosh explains "with such a reflector it is
hard to steer
-- it can only apply a force directly away from the Sun, which is
the
least helpful direction".
Melosh is cautious about techniques that depend on being attached
to the
asteroid. "The asteroid is rotating and perhaps tumbling --
a hard
object to tie anything up to," he says. "It would
probably have to be
enclosed by a system of gimbals anchored to the asteroid surface:
a
mechanical nightmare begging for a catastrophe."
The solar mirror scheme, preferred by Melosh, has the advantage
that it
could avoid the need for physical attachment to the asteroid.
During the
1960s NASA did some work on solar mirrors for use in space, but
little
has been done since then.
Space Missions: Chasing Comets and Asteroids
Several researchers are using super-computers to predict the
effects of
asteroid deflection techniques. One day these simulations may be
needed
to plan a mission to save the Earth from a collision.
But the physical properties of asteroids and comets are poorly
understood, and so the information gathered from space missions
to these
objects is crucial for these simulations.
Several challenging missions to asteroids and comets are underway
or are
planned.
(List of Space Missions to Asteroids and Comets)
Although knowledge about asteroids is important for protecting
the Earth
from collisions it is more likely to be used, ultimately, for
commercial
purposes.
Over the next few decades an impact by a large asteroid is highly
unlikely (but cannot be ruled out). During that time commercial
mining
of asteroids may be commonplace.
Many asteroids are rich in the raw materials needed for
manufacturing in
space, and some are easier to reach than the Moon. Of course, one
way to
deal with an earth-threatening object is to mine it away to
nothing.
c2000 Space.com
-----------------
CCNet-ESSAY is part of the Cambridge Conference Network. It
includes
interesting and thought-provoking essays about our place in space
and
the prospects of a planetary civilisation that is in control of
our
terrestrial and extraterrestrial environment. Contributions to
this
ongoing debate are welcome. To subscribe or unsubscribe from
CCNet,
please contact Benny J Peiser at <b.j.peiser@livjm.ac.uk>.
The fully
indexed archive of the CCNet, from February 1997 on, can be found
at
http://abob.libs.uga.edu/bobk/cccmenu.html