Part 1 of this post discusses one aspect of the 2011 letter from Missile Defense Agency (MDA) to then-Senator Kyl about the IDA study of space-based missile defense. The letter raises several additional issues, which I comment on here.
- Vulnerability of missile defense satellites to anti-satellite (ASAT) attack
To be able to reach missiles shortly after launch, space-based interceptors (SBI) must be in low-altitude orbits; typical altitudes discussed are 300 to 500 km. At the low end of this range atmospheric drag is high enough to give very short orbital lifetimes for the SBI unless they carry fuel to actively compensate for the drag. That may not be needed for orbits near 500 km.
Interceptors at these low altitudes can be easily tracked using ground-based radars and optical telescopes. They can also be reached with relatively cheap short-range and medium-range missiles; if these missiles carry homing kill vehicles, such as those used for ground-based midcourse missile defenses, they could be used to destroy the space-based interceptors. Just before a long-range missile attack, an adversary could launch an anti-satellite attack on the space-based interceptors to punch a hole in the defense constellation through which the adversary could then launch a long-range missile.
Alternately, an adversary that did not want to allow the United States to deploy space-based missile defense could shoot space-based interceptors down shortly after they were deployed.
The IDA report says that the satellites could be designed to defend themselves against such attacks. How might that work?
Since the ASAT interceptor would be lighter and more maneuverable than the SBI, the satellite could not rely on maneuvering to avoid being destroyed.
A satellite carrying a single interceptor could not defend itself by attacking the ASAT, for two reasons. First, the boost phase of a short- or medium-range missile is much shorter than that of a long-range missile, and would be too short for an interceptor designed for boost-phase interception to engage. Second, even if the SBI was designed to have sensors to allow intercept in midcourse as well as boost phase, using the SBI to defend against the ASAT weapon would remove the interceptor from orbit and the ASAT weapon would have done its job by removing the working SBI from the constellation. A workable defensive strategy would require at least two interceptors in each position, one to defend against ASAT weapons and one to perform the missile defense mission.
The IDA report assumes the interceptor satellites it describes to defend ships would each carry four interceptors. If the system is meant to have defense against ASAT attacks, some of the four interceptors must be designed for midcourse intercepts. The satellite could carry at most three such interceptors, since at least one interceptor must be designed for the boost-phase mission of the defense. If an adversary wanted to punch a hole in the constellation, it could launch four ASAT weapons at the satellite and overwhelm the defending interceptors (recall that the ASAT weapons are launched on relatively cheap short- or medium-range missiles).
In addition, an ASAT attack could well be successful even if the ASAT was hit by an interceptor. If an interceptor defending the SBI hit an approaching ASAT it would break the ASAT into a debris cloud that would follow the trajectory of the original center of mass of the ASAT. If this intercept happened after the ASAT weapon’s course was set to collide with the satellite, the debris cloud would continue in that direction. If debris from this cloud hit the satellite it would very likely destroy it.
- Multiple interceptors per satellite
It is important to keep in mind that adding multiple interceptors to a defense satellite greatly increases the satellite’s mass, which increases its launch cost and overall cost.
The vast majority of the mass of a space-based interceptor is the fuel needed to accelerate the interceptor out of its orbit and to maneuver to hit the missile (the missile is itself maneuvering since it is during its boost phase, when it is accelerating and steering). For example, the American Physical Society’s study assumes the empty kill vehicle of the interceptor (the sensor, thrusters, valves, etc) is only 60 kg, but the fueled interceptor would have a mass of more than 800 kg.
Adding a second interceptor to the defense satellite would add another 800 kg to the overall mass. A satellite with four interceptors and a “garage” that included the solar panels and communication equipment could have a total mass of three to four tons.
- Space debris creation
Senator Kyl asked the MDA to comment on whether space-based missile defense would create “significant permanent orbital debris.” The MDA answer indicated that at least for one mechanism of debris creation (that of an intercept of a long-range missile), the system could be designed to not generate long-lived debris.
However, there are at least three different potential debris-creating mechanisms to consider:
- Intercepting a missile with an SBI
When two compact objects collide at very high speed, the objects break into two expanding clouds of debris that follow the trajectories of the center of mass of the original objects. In this case the debris cloud from the interceptor will likely have a center of mass speed greater than Earth escape velocity (11.2 km/s) and most of the debris will therefore not go into orbit or fall back to Earth. Debris from the missile will be on a suborbital trajectory; it will fall back to Earth and not create persistent debris.
- Using an SBI as an anti-satellite weapon
If equipped with an appropriate sensor, the space-based interceptor could home on and destroy satellites. Because of the high interceptor speed needed for boost phase defense, the SBI could reach satellites not only in low Earth orbits (LEO), but also those in semi-synchronous orbits (navigation satellites) and in geosynchronous orbits (communication and early warning satellites). Destroying a satellite on orbit could add huge amounts of persistent debris to these orbits.
At altitudes above about 800 km, where most LEO satellites orbit, the debris from a destroyed satellite would remain in orbit for decades or centuries. The lifetime of debris in geosynchronous and semi-synchronous orbits is essentially infinite.
China’s ASAT test in 2007 created more than 3,000 pieces of debris that have been tracked from the ground—these make up more than 20% of the total tracked debris in LEO. The test also created hundreds of thousands of additional pieces of debris that are too small to be tracked (smaller than about 5 cm) but that can still damage or destroy objects they hit because of their high speed.
Yet the satellite destroyed in the 2007 test had a mass of less than a ton. If a ten-ton satellite—for example, a spy satellite—were destroyed, it could create more than half a million pieces of debris larger than 1 cm in size. This one event could more than double the total amount of large debris in LEO, which would greatly increase the risk of damage to satellites.
- Destroying an SBI with a ground-based ASAT weapon
As discussed above, an adversary might attack a space-based interceptor with a ground-based kinetic ASAT weapon. Assuming the non-fuel mass of the SBI (with garage) is 300 kg, the destruction of the satellite could create more than 50,000 orbiting objects larger than 5 mm in size.
If the SBI was orbiting at an altitude of between 400 and 500 km, the lifetime of most of these objects will be short so this debris would not be considered to be persistent. However, the decay from orbit of this debris would result in an increase in the flux of debris passing through the orbit of the International Space Station (ISS), which circles the Earth at an altitude of about 400 km. Because the ISS orbits at a low altitude, it is in a region with little debris since the residual atmospheric density causes debris to decay quickly. As a result, the additional debris from the SBI passing through this region can represent a significant increase.
In particular, if the SBI were in a 500-km orbit, the destruction of a single SBI could increase the flux of debris larger than 5 mm at the altitude of the ISS by more than 10% for three to four months (at low solar activity) or two to three months at high solar activity. An actual attack might, of course, involve destroying more than one SBI, which would increase this flux.