Dr. Thompson is a professor at the University of Maryland, Baltimore and is a frequent contributor to THE SUBMARINE REVIEW.
Periodically, one hears in the news of the impending collision of an asteroid or comet with the Earth, with the consequent end of life as we know it, downfall of civilization, etc. Typically, the news is followed within a few days by the announcement that the object in fact will miss Earth by millions of miles and there is no cause for alarm. While these “false alarms” are cause for some merriment, some responsible opinion holds that the threat of a significant catastrophe from such a collision is small, but not zero (Morrison, et al., 1994). Moreover, the colossal destruction wrought by even a modest size object (like the estimated 50 meter Tunguska meteorite whose kinetic energy of roughly 20 megatons flattened 1200 square kilometers of Siberia in 1908) argues that steps should be considered to avoid it if possible. It turns out that for a subset of these objects the Submarine Force, and the SSBN in particular, offers unique advantages in deflecting or destroying objects that might threaten the Earth.
Near Earth Objects
Near Earth Objects is the term that has been coined to describe any of a variety of space home matter likely to pass in the vicinity of the Earth. Some of these are familiar, including comets (kilometer-sized dirty snow batls whose out gassing as they are warmed in proximity to the Sun results in the characteristic tail), meteorites (sand grain and larger bits of rock whose fiery entry into the upper atmosphere gives rise to shooting stars) and asteroids (kilometer-size and larger aggregates of rock which are mainly found between the orbits of Mars and Jupiter). While most meteorites are small and fall harmlessly, in the Earth’s history it has collided several times that we know of with mutikilometer-sized objects, which caused global scale devastation.
The best known of these is the comet or asteroid that hit the Earth near the present-day Yucatan, 65 million years ago, creating a dramatic climate change which resulted in the annihilation of the dinosaurs (Alvarez, et al., 1980). The impact of even a smaller object (some hundreds of meters across) is likely to be a substantial catastrophe, with epochal earthquakes and tsunamis devastating entire ocean basins and killing millions. The high impact velocity (estimated at 20 km/ sec) of a 7 5 m iron meteoroid caused the milewide Meteor Crater in Arizona. The Christmas 2004 tsunami that killed more than 200,000 people in the Indian Ocean basin underscores the devastation that tsunamis can cause; the fact that 70% of the Earth is covered by oceans makes a tsunami a likely consequence of any substantial impact. The energy release of the Tunguska object (which burst 8 km in the air) was comparable to that of the earthquake off Sumatra which caused the 2004 tsunami.
Threat to Mankind from Near Earth Objects
Yet, how likely is such a collision in the foreseeable future? Recent estimates of the likelihood vary. One estimate is that the odds of a one kilometer-sized meteorite striking in the next century are one in five thousand, whereas an encounter with a meteorite like the one that devastated Siberia in 1908 should occur roughly once a century. A more recent estimate based on military satellite observations of300 meteorite explosions in the atmosphere suggests the frequency is ten-fold less. Nevertheless, there have been some recent close encounters. On March l 81h, 2004, a boulder 30 meters across (named 2004 FH) passed within about 30,000 miles of the earth; it had been discovered just 3 days previously. On September 29″‘ the largest asteroid known to pass close to Earth (named Toutatis, 4.6 km across) came within about a million miles of Earth. On the 19th of December a relatively small object (5 meters) named 2004 YD5 passed within 22,000 miles of earth (closer than geosynchronous satellites). Having approached the Earth from the direction of the Sun (and towards the Southern Hemisphere, where there are fewer telescopes), it was not detected until two days after it had passed over Antarctica. A five meter object would most likely have broken up upon entering the atmosphere and caused little damage. By comparison, objects sizable enough to cause global catastrophe (kilometers in diameter) are estimated to impact the Earth only once every 300,000 years or so. Thus while the threat is small, it is to some extent quantifiable, and the potential devastation of even a modest size object argues that steps to avoid this should be considered.
Detection and Interception
The first issue is whether the object can be detected soon enough to take any action. A multikilometer asteroid impact might be devastating, but is also likely to be detected years in advance because of its size: Toutatis’ encounter last year was predicted years in advance. Most NEO’s may be found roughly in the plane of the Earth’s orbit around the Sun, and with modem telescopes even advanced amateurs can observe them; for instance, asteroids such as Pallas (300 miles across) can be observed millions of miles away. Several thousand objects of kilometer size and larger have been discovered and their orbits around the Sun determined with high accuracy. NASA has a Congressional mandate to find and determine the orbital parameters of all NEO’s 1 km or larger; it is believed that there are roughly 500 remaining uncataloged in the Earth’s vicinity. There are other ongoing watches maintained, perhaps the best known is the Spacewatch Project of the University of Arizona. While the kilometer-sized objects are trackable at long ranges, smaller objects (-100 meters or so) are less detectable: under favorable circumstances they can only be detected a few days prior to impact. Certainly these objects are more abundant than kilometer-sized objects, and although they are perforce less destructive, their abundance and difficulty of detection might represent a greater threat. A related issue is the detectability of even large objects having low reflectivity.
Also germane is the question of what, if anything, may be done about it if an NEO is likely to collide with the Earth. For large objects whose encounters can be predicted decades in advance, one can imagine launching a vehicle to rendezvous with the asteroid, as the Deep Impact spacecraft rendezvoused with comet Borrelly in 2001, and undertaking some intervention to prevent the impact. This intervention might take the form of demolition of the asteroid (perhaps using a nuclear device), or deflection of its course by attachment of some source of thrust to its surface (Canavan, et al., 1994). Given sufficient time (years), changing the orbital velocity of an asteroid by only I cm/sec should be adequate to avoid a collision. For smaller NEO’s detected only days in advance of impact, rendezvous is clearly infeasible. However, nearby detonation of a missile nuclear warhead should be quite capable of deflecting 100 meter-sized NEO’s, if not breaking them up altogether.
The really salient question is can an intercept mission be mounted sufficiently long enough before impact (e.g., sufficiently far away from the Earth) to adequately deflect or break up the NEO, given that a smaller (hundreds of meters and below) NEO is likely to have been detected only days before, and only tracked with adequate precision for the last several hours. Ideally one would wish to intercept as soon as possible, to maximize the time for any deflecting impulse to steer the target wide of the Earth. Thus a typical mission might only have a few hours to intercept, putting a premium on a quick response and a high speed vehicle. For a launch on short notice the preferred vehicle is of course a solid-fueled missile, which can be stored essentially indefinitely and launched within minutes of order receipt. Ideal candidates are ICBMs and SLBMs, designed to be launched within minutes of receipt of the order. By comparison, current boosters used for interplanetary launches are at least partly liquid-fueled, and thus take days or weeks to prepare for launch.
The typical flight profiles for vehicles leaving Earth’s gravitational field comprise launch into a low parking orbit, followed by an injection burn to achieve escape velocity. This is done to maximize the payload for a given amount of launch thrust, and to utilize the I 000 mph additional velocity enjoyed by rockets launched towards the east from sites near the Equator such as Cape Canaveral or Kourou. Such profiles are only feasible for rocket stages which can be restarted in space, which do not include current US ICBM’s or SLBM’s. For an NEO interception mission (where payload may be less of an issue, and time is of the essence) such flight profiles are probably sub optimal. By comparison, a more direct ascent to the target is faster. Clearly a direct ascent of this sort could be made by a suitably modified ICBM with its MIRV multiple warhead bus replaced by a lightweight single warhead to maximize speed.
SLBM’s for NEO Interception
The unique and crucial advantage the SLBM enjoys over land-based ICBM’s is that it can be based anywhere in the world’s oceans, and thus have a more direct, higher speed flight path to targets arriving from different azimuths. For an NEO approaching the Earth, launch sites in North America only face the target part of each day, and given a firing solution at any time, may have to wait up to twelve hours to launch-a delay that may prove unacceptable. By comparison SSBNs in the Atlantic, Pacific (and potentially Indian) Oceans give much more frequent opportunities to launch. For a direct ascent to a target approaching (for instance) from a high southern latitude (like 2004 YD5), a missile launched from North America would have to take a less direct path than one launched from the Southern Hemisphere, like that in the Figure. This would result in a delayed intercept. Obviously several launch sites exist in the Northern Hemisphere in Europe and Asia, but many fewer south of the Equator. Possible launch sites might include Diego Garcia or Kwajalein Atoll, but the political issues in basing nuclear-tipped missiles there (even for a manifestly good cause) are obviously substantial.
As a potential asteroid interceptor the Trident SLBM has an advantage over Minuteman ICBM’s due to its greater throw weight, which translates into greater terminal velocity for the same size payload carried by the Trident. Exact figures are classified, but the relative size of the missiles and their maximum payload (3 RV’s for the Minuteman vs. 14 for the Trident) gives an idea of their relative capabilities. The MIRV bus on the missile will be replaced by a lightweight warhead carrier, capable of modest maneuver for terminal guidance. The nuclear warhead itself need not be encapsulated within a heavy reentry vehicle and current “physics packages” for cruise missiles weigh less than 200 pounds. SLBM’s already possess high precision inertial guidance systems, but they obviously are programmed for targets on the earth’s surface. However, the interception point is likely to be refined by further observations of the target while the interceptor is en route so command guidance for the terminal phase is likely to be necessary. In intercepting an object not trying to evade interception, the NEO interceptor in some ways has an easier task than our kinetic kill ABM’s which must actually hit the target. However, the relative speeds of the interceptor and target NEO will be much larger than that of an ABM intercepting a reentry vehicle, and the interception must take place thousands of miles up in space. The warhead will require a radar-directed proximity fuse to detonate the device at closest approach.
A third advantage of the SLBM is that being launched in mid-ocean, it can be launched at any azimuth without passing over inhabited land early in its flight path. By comparison, ICBM’s launched from the American Midwest in any direction but north are likely to pass near population centers on the American coasts, and be dropping spent first and second stages near populated areas. The same might be said of missiles launched from many other sites in the Northern Hemisphere. By comparison, the SLBM drops its stages at sea, and the launch is unlikely to even be observed, except by satellite.
An SSBN can carry out this mission with little impact on its primary mission of deterrence. The SSBN would go to sea on deterrent patrol as usual, except that two of its missiles would have asteroid interceptor payloads instead of MIRV buses. Inasmuch as the interceptor missile differs from the Trident SLBM only in its payload, it can be stored and launched almost identically. The small interceptor warhead would appear overtly different from the standard payload from the standpoint of arms control verification. From the standpoint of the SSBN the launch procedures also need differ little. Probably a salvo of two missiles would be launched a few minutes apart to provide a backup in case the first fails. If further refinement of the track of the NEO reveals it will in fact miss the Earth, the warhead need not be detonated and it will proceed harmlessly into interplanetary space.