The most effective way to deflect an asteroid is a hydrogen bomb and all other methods are far weaker. Hydrogen bombs are the best because they have the most energy per mass. A hydrogen bomb can deflect a 1 km asteroid if given a year's warning. To defend against asteroids, we should have a rocket with a hydrogen bomb in Earth orbit, ready to go if an asteroid is found. We also need wide-angle telescopes for detecting asteroids, such as the Pan-STARRS and LSST telescopes.
The smallest asteroid that we need to worry about is 50 meters, the minimum for getting through the atmosphere. Such an asteroid has the energy of a 10 Megaton fusion bomb. The minimum for creating a megatsunami is 200 meters. The LSST telescope will find all 200 meter and larger asteroids that are in near-Earth orbits, and then hydrogen bombs can redirect any that will impact the Earth. For asteroids from more distant regions of the solar system it won't find them soon enough to deflect them. For this we need more powerful telescopes.
The following table shows impact damage as a function of asteroid size.
Asteroid Energy Tsunami Crater Impact Equivalent energy diameter height diameter interval meters EJoules meters km years 8 .0001 0 0 5 Fission bomb, 25 kton TNT equivalent 80 .100 0 1 3000 Fusion bomb, 25 Mton TNT equivalent 200 1 10 3 20000 Krakatoa Volcano, 1883 400 10 20 5 100000 Mag 9.5 quake. Chile, 1960. 2000 1000 200 40 1000000 Hurricane 10000 100000 4000 200 100000000 Asteroid that killed the dinosaurs1 EJoule = 1018 Joules.
In the film "Armageddon", an asteroid is on course to hit the Earth and the astronauts deflected it with a hydrogen bomb. The scientists bickered over if it was better to detonate the bomb on the surface or underground. The answer is that for fixed explosion energy, the asteroid recoil momentum increases with the mass ejected by the explosion, and so one should detonate the bomb underground.
If you have more than a year of warning before impact then there is enough time to send a manned spaceship to match speeds with the asteroid, land on it, and bury a hydrogen bomb underneath the surface.
If there isn't enough time to match speeds with the asteroid then one has to settle for detonating the bomb at the surface. A rocket is sent to intercept the asteroid and the bomb is detonated just before it hits. The detonation is delivered to the side of the asteroid to give it a sideways deflection. The deflection can be amplified by crashing an object into the asteroid just before the bomb detonates, which kicks up a cloud of material and increases the mass ejected by the bomb. In the appendix below we calculate the momentum delivered to the asteroid as a function of bomb energy.
The Pan-STARRS telescope specializes in finding asteroids. It has a wide field of view and takes short exposures, allowing it to cover the entire sky in 8 days. The upcoming Large Synaptic Survey Telescope (LSST) will cover the sky every 2 days.
Telescope Diameter Field of Exposure Sky survey Year (meters) view (deg) (seconds) time (days) Pan-STARRS 3 3.0 60 8 2010 Hawaii LSST 8.4 3.5 15 2 2021 El Penon, Chile Flux limit Magnitude (Watts/m2) limit Human eye 3e-11 7 Pan-STARRS 5e-18 24 LSST 2e-18 25 Keck 10 meter 1e-19 28 Hubble 1e-20 31 Webb 5e-22 34
We calculate the asteroid recoil momentum as a function of hydrogen bomb energy.
Mass ejected by the explosion = m Energy of the explosion = E Momentum imparted to the asteroid = Q = (2 m E)½You want the bomb to eject as much mass from the asteroid as possible. If you detonate the bomb from above the surface, most of the explosion energy goes into space without ejecting any mass. You can increase the mass ejected by arranging for an impactor to hit the asteroid just before the bomb arrives. The impactor blasts material into space and the nuclear explosion heats that material, giving an impulse on the asteroid.
Fi = Fraction of the mass of the spacecraft that impacts the asteroid Fb = Fraction of the mass of the spacecraft that is a hydrogen bomb Z = Mass of material ejected by the impactor / Mass of the impactor e = Energy per mass of the hydrogen bomb W = Total mass of spacecraftZ is a dimensionless number that parameterizes how effective the impactor is at ejecting material. We can expect that Z > 100 and it can conceivably be much larger.
The detonation should maximize Q/W
Explosion energy = W Fb e Mass ejected from the asteroid = W Fi Z Q/W = (2 W Fb e W Fi Z)^(1/2) / W = (2 e Z Fb Fi)^(1/2) Maximizing (Fb Fi)1/2 subject to the constraint Fb + Fi = 1 gives Fb = Fi = 1/2Hence,
Q/W ~ (e Z)^(1/2)Setting e = 1e13 and Z = 100,
Q/W ~ 3e7 meters/second Strategy Q/W (m/s) Hydrogen bomb with impactor 3e7 Hydrogen bomb without impactor 3e6 Impactor without hydrogen bomb 3e5
A cannon provides an example calculation.
Energy = E Mass = M Velocity = V Momentum = Q = M VMomentum conservation:
Mcannon Vcannon = Mball Vball Eball / Ecannon = MballVball2 / (McannonVcannon2) = Mcannon / MballIf we assume that the cannon is vastly heavier than the ball then
Mball << Mcannon Eball >> EcannonThe cannonball gets all the energy.
E = .5 Mball Vball2The momentum of the recoiling cannon is
Qcannon = Mball Vball = (2 Mball E)1/2For fixed gunpowder energy, cannon recoil increases with cannonball mass. This suggests that if you want to deflect an asteroid by detonating a hydrogen bomb that you should arrange for as much material to be ejected from the asteroid as possible. If you can land on the asteroid then you want to bury the bomb before detonating it. If you can't land on the asteroid then you can arrange for an impactor to eject material before detonating the bomb.
Suppose an asteroid is on a collision course with the Earth and we deflect it by giving it sideways speed. To estimate the required speed,
Distance from Earth when the asteroid is spotted = X Velocity of the asteroid = V Time for the asteroid to reach Earth = T = X / V Radius of the Earth = R Sideways speed required to deflect the asteroid = Vside = V R / XThe earlier the asteroid is spotted, the larger the value of X and the less sideways speed is required to deflect it.
Near Earth asteroids (NEA) approach the Earth at a characteristic speed of ~ 20 km/s. Retrograde comets can approach the Earth as fast as 75 km/s.
NEA: near Earth asteroids SPC: short period comets HTC: Halley-type comets LPC: long period comets"Near-Earth object velocity distributions and consequences for the Chicxulub impactor" S. V. Jeffers, S. P. Manley, M. E. Bailey, D. J. Asher, Mon. Not. R. Astron. Soc. 327, 126–132 (2001)
If you want to land a spaceship on the asteroid you need at least a year of maneuvering to match speeds with it. If you don't have this much time, then the next best strategy is to send out a hydrogen bomb from the Earth and arrange for the bomb to detonate just before it hits the asteroid. The detonation is arranged to give the asteroid sideways momentum.
The theoretical maximum energy density of a fusion bomb is 2.4⋅1013 Joules/kg, and in practice the energy density is half this.
Suppose you send a spacecraft to intercept the asteroid.
Speed of asteroid = U = 20 km/s Speed of the spacecraft = u Mass of the spacecraft = m Energy density of a hydrogen bomb = e = 1013 Joules/kg Energy of the nuclear explosion = E Momentum imparted to the asteroid = QThe spacecraft hits the asteroid at a speed of u+U. Typically, u << U, and so we can approximate the collision velocity as U.
If the spacecraft is a hydrogen bomb, then
Energy of the hydrogen bomb / Kinetic energy of the spacecraft = e m / (.5 m U2) = 50000There is vastly more energy in the nuclear explosion than in the impact.
Suppose an asteroid is on course for a direct hit on the Earth and we're going to deflect it with a hydrogen explosion.
V = Speed of the asteroid on its way to the Earth = 20 km/s (a typical value) v = Sideways speed delivered to the asteroid by the hydrogen bomb M = Mass of asteroid m = Mass of material ejected by the hydrogen bomb R = Radius of the Earth = 6.371e6 meters E = Energy provided by the hydrogen bomb e = Energy/mass of the hydrogen bomb = 1e13 Joules/kg Z = Mass of material ejected / Mass of spacecraft ~ 100 D = Distance of the asteroid from the Earth when the hydrogen bomb detonates T = Time between the hydrogen bomb detonation and when the asteroid reaches the Earth detones = D / V From the cannonball calculation, M v = W (e Z)^(1/2) Deflecting the asteroid requires v T > R W (e Z)^(1/2) T / (M R) > 1 (eZ)^(1/2) / R ~ 5 To deflect the asteroid the spacecraft must have a mass of at least W > .2 M/T If an asteroid has Size = 1 km Mass = 1e12 kg T = 1 month then W = 77 tons
Exhaust (km/s) Hydrogen+oxygen rocket 5 Dawn ion drive 31 VASIMR ion drive 50 Nuclear thermal rocket, H2 exhaust 9 NERVA design Nuclear thermal rocket, H2O exhaust 1.9 NERVA design Solar thermal rocket, H2 exhaust 9 Solar thermal rocket, H2O exhaust 1.9 Orion fusion rocket 10000 Antimatter rocket ~ 1/2 cAll of these rockets are possible with current technology except for the antimatter rocket.
Ion drives cannot move heavy objects because of their low thrust.
If a thermal rocket can operate at a temperature high enough to dissociate H2 into elemental hydrogen, larger exhaust speeds are possible.
The performance of a solar thermal rocket depends on its proximity to the sun. Nuclear thermal rockets work everywhere.
Heat capacity of air ~ 1.0 Joules/kg/Kelvin Mass of atmosphere ~ 5.1 kg Let F = the fraction of the asteroid's kinetic energy that goes into heating the atmosphere. The atmospheric heating is Mass of asteroid Speed of asteroid Heating ~ 40 kelvin * F * ---------------- * ( ----------------- )^2 10^15 kg 20 km/sA 10 km asteroid has a mass of ~ 10^15 kg. If the asteroid is less massive than this then you don't have to worry about cooking the atmosphere. The dinosaur-extinction asteroid was ~ 10 km in size.
The collision betwen the Milky Way and Andromeda galaxies, 4 billion years from now.
A maurading star disrupts the orbit of the Earth.
In 5 billion years, the sun will explode in a nova and consume the Earth.
The moon is spiraling outward and in 1 billion years will be stolen by the sun. After this, the Earth will have no defense against angular momentum and its spin orientation will start to drift.
The ice caps melt and the Earth overheats.
Protons probably decay and the half life has been theorized to be in the range of 1040 years. After this, the universe will consist of electrons, positrons, and neutrinos.
If the masses of the Higgs boson and the top quark have unfavorable values, then the universe is unstable to vacuum decay. This would destroy the entire universe without warning.
Black holes emit radiation by the Hawking mechanism. In 1070 years they will have radiated all their mass and will end their lives in an explosion of gamma rays.
In 10 billion years the dark energy will expand the universe, leaving behind only the galaxies of the local group.
If dark energy has an unfavrable equation of state then the universe will end in a "big rip", where all matter is shredded into its fundamental particles.
Q = Radius of closest approach / Radius of Earth Q Diameter Date Energy (meters) (Mtons TNT) Chelyabinsk 1.0 19 2013 .44 Tunguska 1.0 50 1908 12 Flattened a forest Arizona asteroid 1.0 50 -50000 10 1 km crater 1972 Fireball 1.0089 ~ 6 1972 Skimmed the upper atmosphere 2011-CQ1 1.87 1 2011 2008-TS26 1.96 1 2008 2011-MD 2.94 10 2011 2012-KT42 3.26 ~ 7 2004 Apophis 4.9 325 2029 510 2013-DA14 5.35 30 2013 2012-KP24 8.99 25 2004 2012-BX34 10.3 8 2012 2012-TC4 14.9 17 2012 2005-YU55 60.00 400 2005
If you detonate the bomb at the center and if the asteroid is too large, gravity will bring the asteroid back together. For a uniform-density sphere,
Gravitational energy = .6 G Mass^2 / R
Suppose the hydrogen bomb has the energy of 10 megatons of TNT, which is 4*10^16 Joules. What would you estimate is the largest value for the radius of an asteroid that this bomb can shatter?
In "Star Wars", a Death Star shatters a planet. If the planet is identical to the Earth, how much energy does this take? If the energy were provided by a sphere of antimatter with the density of iron, what is the radius of this sphere?
Mr Miyagi: Best block... not be there