Correcting the TV show Salvation’s response to a 7000 meter asteroid hitting in 6 months
The show chooses to try to use an EMdrive, a propellantless propulsion system to deliver a gravity deflection mission to divert the asteroid.
EMdrive is under some experimental study and there is some experiments which seem to indicate millinewtons of thrust per kilowatt. There are rumors that an EMdrive was flown in space by China and by the US Air force but there is no confirmation that the systems really work or that there have been any space based tests and definitely no indication that a useful and superior space drive has been developed.
Gravity deflectors generally need ten years or more to deflect an asteroid. 6 months would not be enough. The TV show is also incorrect about an asteroid detecting around Jupiter hitting the earth in 6 months. Most asteroids are going at a speed that would take 9 months to reach the Earth from that distance. It would have to an object with a weird and faster orbit to travel that fast.
There a 149 page study of asteroid deflection written in 2010.
* Since 1998 NASA has discovered over 95% of the objects larger than 1 kilometer in diameter that would cause global devastation if they were to hit. These comprise the largest 1,000 or so of the over 12,000 NEOs that have been discovered to date.
* Congress has mandated that NASA discover 90 percent of all near-Earth objects 140 meters in diameter or greater by 2020. This effort has not been adequately funded but there are telescopes and surveys being performed.
* If an asteroid is bigger than 500 meters and it will hit in less than five years, then the best option is to try to hit it with a nuclear device.
* Over a 1000 meters in size and even with 100 years of notice it is best to nuke the asteroid
For a 6 month response time, we would have to take over existing rocket missions and convert them into nuclear deflection missions. It would be best to convert all of them into nuclear deflection missions to have multiple attempts to pulverize or deflect the asteroid.
If an asteroid is pulverized early enough then only a small fraction will hit the earth. It is unlikely that a 7000 meter in diameter asteroid would be pulverized because it would literally be a mount Everest sized object.
Perhaps ten to 20 launchers could be converted.
We can make statistical statements about the probability of an impact
* Objects larger than about 30 meters in diameter probably strike Earth only about once every few centuries
* objects greater than about 300 meters in diameter only once per hundred millennia.
* objects greater than 7000 meters in diameter are once every 100 million years.
Even objects only 30 meters in diameter can cause immense damage. The cosmic intruder that exploded over Siberia in 1908 may have only been a few tens of meters in size; yet this explosion severely damaged a forest of more than 2,000 square kilometers. Had an airburst of such magnitude occurred over New York City, hundreds of thousands of deaths might have resulted.
Nuclear explosives constitute a mature technology, with well-characterized outputs. They represent by far the most mass efficient method of energy transport and should be considered as an option for NEO mitigation. Nuclear explosives provide the only option for large NEOs (> 500 meters) when the time to impact is short (years to months), or when other methods have failed and time is running out. The extensive test history of nuclear explosives demonstrates a proven ability to provide a tailored output (the desired mixture of x rays, neutrons, or gamma rays) and dependable yields from about 100 tons to many megatons of TNT-equivalent energy. Coupled with this test history is an abundance of data on the effects of the surface and subsurface blasts, including shock generation and cratering.
Various methods have been proposed for using nuclear explosions to reduce or eliminate an NEO threat; for a given mass of the NEO the warning time is a primary criterion for choosing among them. With decades of warning, the required change in velocity (I"V) from the explosion is millimeters to a centimeter per second and can be met for NEOs many kilometers in diameter. This range of values is much less than the 25 to 50 cm/s escape velocity from moderate to large (500 to 1000 meter) bodies, so it is reasonable to assume that such a small I"Vwould not lead to the target's fragmentation or to excessive ejecta (i.e., debris thrown off the object). This expectation is met in hydrodynamic simulations presented here that show that nuclear explosions can provide I"Vfrom 0.7 to 2.4 cm/s, for payload masses less than a ton (including the nuclear device's fuse and environmental cocoon).
In models of NEOs with surface densities as in terrestrial environments, nearly 98 percent of a body remains bound as a single object through only its own weak gravity. The small amountof ejecta expands over the decades to form a large cloud of low-density debris, reducing its posed threat by another factor of 104to 105. The amount of the ejecta depends on the surface porosity. As in the caseof kinetic impacts, a dissipative, low-density surface will reduce the amount of ejecta, thus reducing the I"V.
Alternatively, when the time to projected impact is short, it may be impossible to apply a sufficient I"Vwithout fragmentation, but the limiting factor is assembly and launch. A nuclear package with a new fuse (i.e., a fuse that is not designed for terrestrial use) and a new container requires a cylinder about a meter in length and 35 cm in diameter, with a mass under 220 kg. The longest lead-time item for incorporating such a device in a rocket system is the development of a container to deliver the device and a fusing system capable of operating with the timing constraints required by the spacecraft velocities near impact with the NEO. Specifications for a nuclear bus could be the same as those for a kinetic-impactor mission, but would be very challenging to construct and integrate with the booster rocket and the nuclear package in under a year. This "latency time" between the decision to act and the launch can be reduced dramatically (perhaps 100 fold) by designing and testing these critical components in advance of discovering a hazardous NEO.
Except for NEOs 10 kilometers in diameter or larger, it is generally likely that nuclear explosives can provide a more than large enough I"V, with little material loss and with essentially no danger of fragmentation.
Nuclear explosives can provide considerable protection against a potential NEO impact. This may be the only current means to prevent an impact by a large (over 500 meters in diameter) hazardous object with a warning time under a decade or by a larger (over 1 km in diameter) object with a warning time of several decades. With decades of warning for such large objects, the preferred approach uses a standoff detonation. Neutron output has certain advantages (Dearborn, 2004), as the energy coupling is relatively insensitive to the surface composition and density of the NEO. The simulations show that speed changes (I"V) of order 2 cm/s are achievable with gravitational binding mostly maintaining the NEO as a single body. About 2 percent of the body mass is ejected, evolving to such a low density that it would likely pose no threat to Earth. Very low yield surface explosives also showed great promise for speed changes of order 1 cm/s. As the NEO size decreases, and the required yield of the nuclear explosive drops below the tested regime (100 kilotons), the kinetic impact approach will have to be used. While the nuclear option provides considerable mitigation potential, above some size NEO tested limits will become inadequate.
Although no detailed simulations have been done, NEO diameters greater than 10 kilometers are likely to be problematic for the devices in the nuclear stockpile, which go up to megatons of equivalent energy. Modeling the shock dissipation of highly porous materials appears to be the primary uncertainty for both impactors and standoff bursts. This uncertainty holds particularly true for NEOs with very low-density aggregates that can exist only in low gravity environments. At present, the simulations have not examined the affects of the range of structures, shapes, and rotational states, but with Defense Threat Reduction Agency support to extend the present studies, these simulations could be done. Currently the United States and several other nations maintain nuclear stockpiles and the infrastructure to build them for purposes of national defense. While efforts to reduce those stockpiles continue, it seems likely that they will exist for some decades. Whendefense concerns no longer apply, the governments involved may either accept the longer response time for a Manhattan-Project-like effort, or decide if adequate safeguards can be developed for some entity to maintain a small number of nuclear explosive packages to allow humanity to counter an NEO that could, for example, cause mass extinctions.
