"Defense is Easier From the Ground," Op-Ed piece by R.L. Garwin
published in Space News, March 11-17, 1991.
Richard L. Garwin
P.O. Box 218
Yorktown Heights, NY 10598
(914) 945-2555
FAX: (914) 945-4419
March 7, 1991
DEFENSE IS EASIER FROM THE GROUND
At a Pentagon press conference February 12, the Strategic
Defense Initiative Organization's Director, Henry Cooper,
argued that a system of 1000 child-sized space-based
interceptors (Brilliant Pebbles or "BP") could defend
against the stretched Scud missiles then being launched by
Iraq against Israel and Saudi Arabia.
But it is clear that the more urgent and important the task
of countering tactical or theater ballistic missiles like
the stretched Scud, the less one wants to rely on Brilliant
Pebbles, and the more one wants to rely on ground-based
interceptors.
Whether you need a new refrigerator or new software for your
personal computer, the beginning of wisdom is comparison
shopping. What would a system of space-based interceptors
need to do to destroy a stretched Scud (call it Scud-S), and
how else might this be done? As modified by Iraq, Scud-S
has a range of some 800 kilometers (500 miles) and a flight
time of 480 seconds, with apogee (maximum height) of about
200 kilometers (124 miles).
The 1000 orbiting BPs would be pre-authorized to destroy
missiles rising from a certain region, Iraq in the present
instance.
So the BP defense against a single Scud-S would need to
observe the launch of the missile, assign a BP to the
intercept, orient the BP to project itself to near-collision
with the predicted apogee of the Scud-S path, and maneuver
during the last seconds of final approach, at a speed of
some 10 kilometers per second (22,000 miles per hour)
through the rarefied atmosphere to collide with the missile
and to destroy it by impact. During the 200 seconds or so
from Scud-S launch to apogee intercept, the orbital speed of
the BP would have carried it some 1600 kilometers (1000
miles).
Cooper says the United States will not deploy the BP unless
some invention makes it possible to solve the problem posed
by (Soviet) ICBM warheads equipped with countermeasures.
These countermeasures might include enclosing the re-entry
vehicle or missile in an inflatable multilayer balloon,
accompanied by similar balloons tethered at a distance of
10 meters (33 feet) or so. In reality, it is highly
doubtful that a BP could identify and reject tethered decoy
balloons.
The interesting point is that the country should not want to
deploy Brilliant Pebbles against Scud-S even if that
long-sought invention is forthcoming. Because Scud-S could
be flown with 100 km (62 mile) apogee with minor loss of
maximum range, even against simple missiles without
countermeasures the BP's homing sensor must be able to
operate in the residual atmosphere at 100 kilometers
altitude, which at the BP's high velocity would rapidly heat
the frontal area of the sensor.
If we imagined that the BPs could somehow do the job, we
could do it better, sooner, and more reliably with BPs of
lesser capability based on the ground in the target country:
ground-based interceptors, "GBI".
A BP homing head on the ground anywhere within 100-200
kilometers (62-125 miles) of the expected target could be
launched at least as soon after the Scud-S launch as could a
BP cued by the same system with which the U.S. observed
every Iraqii Scud launch. Instead of the enormous
additional reachout speed provided the space-based BPs to
compensate for their being in the wrong place when needed,
the ground-based interceptor would require maneuver and
launch speeds each less than 3 kilometers per second (7700
miles per hour) to meet the Scud-S at apogee. Needing less
fuel than the BP, a GBI could carry a larger warhead with a
lethality-enhancement device such as an umbrella or mesh.
Furthermore, launching the interceptor from the ground could
be done with a rocket in the 200-kilogram (440-pound) weight
class; one would need a rocket five or 10 times as large to
launch each BP to orbit (even with a BP homing head
one-fifth the mass of the GBI's).
The ground-based interceptor would need to detect its target
only in a very limited uncertainty area at a distance not
exceeding 200 kilometers, and the closing speed for the
intercept would be less than half the closing speed for a BP
intercept. The infrared sensors of the ground-based
interceptor would operate on battery power without the
necessity of an on-orbit solar power system, refrigerator,
station-keeping, etc. Rugged semi-active radar homing, with
apogee illumination from a ground-based radar is an
alternative for a GBI provided good cueing information.
Furthermore, the ground-based interceptor would need a
sensor operating life not exceeding five minutes, versus
five years in the space environment for the space-based
interceptor. Its infrared sensor has an easier task than
the space-based BP; it does not need to see its target
against the background of the Earth or the dense atmosphere,
and the atmospheric heating for the slower ground-based
interceptor is about 1% that suffered by the space-based
interceptor at the same altitude. In contrast to the few
BPs that could respond to any multiple launch, ground-based
interceptors could be launched from the target region as
many at a time as desired.
The conclusion is clear. The more urgent the need for
protection against Scud-S and its more accurate successors,
the less one wants to deploy a constellation of space-based
interceptors for that task. Against theater-range missiles
without countermeasures, dedicated, ground-based
interceptors with existing radar-homing sensors could be
smaller and cheaper than the existing Patriot air-defense
interceptor that can destroy only those Scuds that would
impact close to the Patriot launcher. If Brilliant Pebbles
homing technology works brilliantly, the ground-based
interceptor would no longer need the Patriot radar. If
discrimination outside the atmosphere cannot be achieved
(the long-sought invention does not emerge), one will have
to fall back on intercept as the missile descends through
the atmosphere, and for that purpose space-based
interceptors are even less suitable than they are for apogee
intercept.
Richard L. Garwin
Richard Garwin is science adviser to the director of research
at the Thomas J. Watson Research Center, Yorktown Heights, N.Y.
These are his personal views.
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