But space weapons do not
help to reduce vulnerability for most of these threats,. They are
limited to intercepting objects that approach satellites in a noticeably
offensive way, such as hit-to-kill kinetic energy weapons; and that
capability remains to be assessed.
One of the most effective
threats is a microsatellite in the form of a "space mine."
Surrey Satellite Technology Ltd., a Surrey University company, is
a leader in developing microsatellite technology, and has numerous
collaborative programs with other countries and with non-state groups.
Although microsatellites have peaceful and military non-weapon uses--observation,
communication, and the like--they make particularly good antisatellite
weapons. In this role, a microsatellite space mine equipped with maneuver
capability exceeding that of the quarry satellite would sit always
within lethal range (even a few tens of meters) ready to explode at
a moment's notice.
A microsatellite as inspection
device might have been useful in conjunction with Columbia's final
flight, but a long-endurance microsatellite is a more difficult task.
Nevertheless, a cautionary tale is this account of a January 29, 2003,
US microsatellite exercise; the XSS-10 repeatedly maneuvered to within
115 ft of its final-stage rocket, taking pictures. A shotgun shell
could have destroyed a satellite from such a range.
China carried out similar
maneuvers with Surrey technology several years ago.
Since in the vacuum of
space (as was known to Galileo) a feather and lead shot fall at the
same speed without significant drag, a microsatellite with little
payload necessary to devote to other tasks can be equipped to outmaneuver
and outlast a major satellite, the primary job of which is surveillance,
high-bandwidth communication, and the like.
It is difficult to counter
space mines once they are in place. It might be done with defensive
microsatellites, but the asymmetric nature of the threat (i.e., tiny
expenditures for the microsatellite vs. $200 million-plus for a major
US LEO satellite, makes it desirable to prevent the emergence of such
threats.
Two general tools for
resolving the microsatellite dilemma are rules of conduct in peacetime,
and deterrence by holding non-space assets at risk.
In summary, space weapons
are generally not good at protecting satellites. In the case of microsatellites,
one might plagiarize Jonathan Swift and commit to deploy "smaller
still to bite 'em." This is an arms race in which United States
resources far outweigh those of any other state, but this advantage
is outweighed by the vulnerability inherent in the cost of existing
and future high-capability satellites in low Earth orbit.
Power projection and offensive
counter-space
We turn now to the remaining
two uses for space weapons, power projection and offensive counter-space.
Different space weapons have varying degrees of utility in these areas,
so we will now look at the utility of specific weapons.
We have already seen how
useful space mines may be AGAINST those who have valuable satellites
and useless against those who have none.
Another weapon much discussed
is long-rod penetrators. The idea is that these long tungsten or uranium
rods would be orbited, and (according to the RAND Report) de-orbited
by canceling their orbital velocity, so that they would fall essentially
vertically through the atmosphere, striking their target with enormous
energy. Two problems that will not be alleviated by the progress of
technology: the energy is larger the higher the orbit, but the fall
time is greater as well. The energy of high explosive corresponds
to a material speed of 3 km/s, and one does not arrive at a similar
energy per gram from a projectile dropped from altitude until one
reaches 460 km, with a corresponding fall time of 12 minutes; a fall
from GEO takes almost 6 hours and provides about ten times the energy
density of high explosive.
A rod would need to be
guided accurately to strike its target within some meters in order
to destroy a surface target by the explosion.
Long rods might be used
to penetrate through earth to hard or deeply buried targets. However,
the physics of high-velocity impact limits penetration depth as shown
by high-speed photography of a bullet impacting steel at just above
1 kilometer per second. A copper-jacketed lead bullet fragments against
the hardened steel, but in the process produces a pressure sufficient
to leave a small crater. Very strong projectiles impacting earth or
rock at similar speed can penetrate to depths several times their
length.
Tests done by Sandia laboratory
confirm predictions that, even for the hardest rod materials, penetration
is maximum around 1 km/s. Above that speed, the rod tip simply liquefies,
and penetration depth falls off, becoming effectively independent
of impact speed. Therefore, for maximum penetration, such rods would
need to be orbited at very low altitudes, and could only deliver one
ninth the destructive energy per gram as a conventional bomb. The
effort is entirely mismatched to the results.
Dominating the cost is
the need to put the rod into orbit in the first place and later cancel
its orbital velocity so that it drops back to earth. The propellant
required to place the entire weapon in orbit must suffice to lift
both the rod and its attendant deorbiting propellant. For low earth
orbit, the total velocity change of about 15 km/s typically requires
several thousand times the orbiting mass in propellant. Taking the
typical $10,000 per kg launch cost to LEO, and assuming a 0.1 ton
rod with the 3 tons of propellant to stop its orbital motion, the
launch cost to orbit would be some $30 million. And for timely delivery
against a single target at temperate latitude, several rods in each
orbit would be required and a good many orbits-say 10. Clearly, the
more conventional deorbit maneuver would be preferable, with a small
energy change and the use of atmospheric drag (combined with wings
or a lifting-body approach) to preserve much of the orbital velocity
as the rod approaches the vertical.
Whatever the effect actually
achieved against a target, it is far better to propel the rod directly
from launch to target and avoid orbits altogether-- by placing the
rods on ballistic missiles. Specifically, a one-km/s penetrator could
be provided flexibly by a nominal solid rocket motor giving an acceleration
30 times that of gravity-so 300 m/sec2. The desired 1 km/s would be
obtained in 3.3 s, over a distance of 1.65 km. A speed of 3 km/s would
take 10 s and a distance of 15 km. The cost would be some $100,000
or less, plus whatever cost for the terminal guidance system--which
is surely no greater for the ballistic missile than for the orbiting
projectile.
Looking now at the common
aero vehicle (CAV) carrying conventional ordnance or intelligence
payloads, one finds again that this capability is dominated by CAV
delivery by ballistic or cruise missiles-- perhaps guided by observation
from space. Indeed, the role of the CAV itself is largely supplanted
by the familiar "bus" technology for delivering multiple
payloads from a ballistic missile launch.
Space weapons and missile defense
We turn now to space weapons
(and their competition) for missile defense. For boost-phase intercept-BPI--
space-based kinetic-energy (hit-to-kill) interceptors are in competition
with surface-based interceptors (on land or sea, or even on aircraft).
The non-space options excel against a small state such as North Korea,
largely surrounded by water. For BPI, space-based interceptors must
be given acceleration and divert capabilities very similar to those
required for surface-based interceptors, if they are not to pass harmlessly
by the quarry missiles. For missile launches from a small area, space-based
interceptors have their required number multiplied by the number of
simultaneous launches, and also by the "absentee ratio"
because most of the SBI will be on the other side of the Earth and
unable to join the fray for a clustered launch.
However capable the surface-based
interceptors would be against North Korea, Iraq, or even against launches
from Iran, unless based within the target country they are ineffective
against ICBMs launched from China or Russia, because the interior
of those countries is so far from the borders.
Yet China and Russia are
highly capable powers, and it would be much easier for them to destroy
space-based interceptors as the constellation is gradually built than
it would be for the US to use the SBIs to counter ballistic missile
launch. Some observers are skeptical that Russia or China (or France,
for that matter) would destroy SBIs in peacetime, but when the question
is posed what the US would do if another state deployed a vast number
of SBIs, the response of many of my colleagues is that we would destroy
them-"shoot them down".
The airborne laser (ABL)
under development and in early flight test (in contrast to the space
based laser (SBL) for which no US program currently exists) might
serve as a BPI capability against ICBMs launched from North Korea.
In the spirit of a "capabilities based" system, it would
to some extent complicate NK's ICBM program: North Korea would need
to deploy from the beginning countermeasures to mid-course and would
have to consider countermeasures to an ABL BPI defense. Unlike the
mid-course interceptors which, once deployed, would always be ready
for use, the ABL would incur large operating costs to maintain a constant
presence.
Another weapon of considerable
interest is the Space Based Laser. These weapons could attack over
long distances at the speed of light, although space mines and the
ABL could be equally prompt. A SBL could also attack terrestrial targets,
but only with suitable laser wavelengths to penetrate the atmosphere.
The current candidate SBL lasers cannot attack ground or airborne
targets.
A single SBL, costing
billions of dollars, could typically have a range of at most 3000
km, unless the SBL constellation were conceived to have a large number
of redirecting ("fighting") mirrors . Under those circumstances,
a competitive system could use a ground-based laser, redirected by
such mirrors3. Cloud
at the GBL site would cancel the capability of a GBL, so several would
be needed to have high probability that the system would be operable
at any time. In any case, the fighting mirrors might be classed by
the potential victims as weapons in space as well.
An SBL would be a very
expensive means of attacking a satellite, but might be more useful
for missile defense purposes. With relatively few SBL in orbit, one
might need to be used at 3000 km range. At that distance, with no
loss through the atmosphere, a perfect mirror of 3 m diameter, and
laser power output of 3 MW in the 3.8-micron DF band, a target protected
with 3 cm of cork could withstand about 200 MJm-2 before exposing
the target surface to laser heat. (Some Minuteman ICBMs have had a
0.6-centimeter layer of cork to protect the booster from skin friction
heating during launch. Such a layer would be vaporized with about
50 MJm-2 (5 kJcm-2) from a SBL.) The laser consumes fuel at a rate
of some 3kg/MWs, or 9 kg/s, and it would need to fire for 1700 s at
the assumed 3000-km range, thus using 15 tons of fuel, at a launch
cost for fuel of $150 million per target attacked. At a range of 1000
km, the launch cost would be some $16 M per target.
Other countermeasures
are feasible and could be multiplicative-such as the slow rotation
of the booster during launch.
A substantial constellation
of SBLs covering the strategically important region of the Earth could
consist of 20-50 such satellites, which could provide rapid illumination
of most important points, providing that the target can be destroyed
by the laser, and that it is not covered by cloud. Cloud coverage
is typically 30-40%, but can range to 70% or more in parts of Germany
or North Korea.
But, as analyzed in detail
in the RAND publication, many targets are not vulnerable to destruction
by SBL, and many that are can be protected by smoke, by water shields,
or in other ways. Aircraft yes, and combustible targets or thin-skinned
storage tanks. But not bunkers, armored vehicles, or many buildings.
We have already seen that
the use of an SBL can easily cost in the range of $100 million per
target and is contingent on the target being thin-skinned and not
obscured by a cloud. For comparison, a Tomahawk missile costs some
$600,000 and will attack heavily armored and non-flammable targets,
and is not affected by cloud.
Even enthusiasts consider
SBLs a weapon to attack very special targets, while most military
capability against similar targets is to be provided by more conventional
means. In contrast almost all portions of the earth are reachable
by existing cruise missiles (Tomahawk Block 3) launched from outside
the 12 nmi limit. The flight time can be several hours.
For the space-based laser,
"rapid response" is a sometime thing, since it is necessary
to have clear air to allow the laser beam to strike the target-no
cloud in the way.
With these competitive
means of striking the target, observation could still be provided
by non-weapon space assets, so that in addition to attack by navigation
(using GPS) one could use a laser-target designator from space with
observation and designation provided at the time when a destructive
payload arrives in the vicinity of the target-an example of non-weapon
military space capabilities contributing to US military capability.
In summary, the one target
which can surely be held at risk at modest cost is important and costly
satellites, of which the US possesses by far the greatest number and
value.
The US Space Commission
Report is generally considered as support for the proposition that
the US should proceed to develop and deploy space weapons in order
to counter the evolution of space weapons by others, and to effect
the needed reduction in vulnerability of US satellites. In fact the
commission does not specifically advocate the development of offensive
weaponry for deployment in space. In particular, it reads,
