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A way to deviate an exterminator

An exterminator is a big cosmic object, like a comet or an asteroid, that is going to hit the Earth. Such an object is supposed to have contributed to eradicate the dinosaurs, 60 million years ago. The probability an exterminator appears the next coming years is infinitesimal, yet it would be such a disaster officials decided to invest in a space survey program.

Now, what if an exterminator is discovered?

Two Hollywood movies propose to send a team of astronauts towards the exterminator, have them land on it, bore a hole and put a H-bomb inside. Once the exterminator bursts in small rocks, each of those small pieces is inoffensive. They will burn in the atmosphere of the Earth and just be a big firework.

Such a mission has several main disadvantages:

• In order to be able to land on the exterminator the astronauts (or a robot) must follow a path very close to that of the object. The classic method to make a spacecraft get such a trajectory is a cosmic ballet: it bounces from planet to planet, in order to get the same trajectory as the object. Such a method takes years. It could be OK to get towards an asteroid that was discovered to be an exterminator very early. But not for a comet, whose trajectory crosses the orbit of the Earth after less than a year. What's more most spacecraft that use this method rely on the gravity of the target itself: they are catched by the target and just use a little rocket motor to brake and stay in orbit around the target. This cannot be used with a "lightweight" comet or asteroid unless it passes close to another planet before it hits the Earth. Make a spacecraft cross the trajectory of cosmic object is far easier than make a spacecraft follow the same trajectory as that object.
  

• The only method to land quickly on the exterminator is to use a tremendous amount of rocket motors to get directly the right trajectory. Even if this would be possible, it would cost a fortune. Spending the money would be worth the case since mankind depends on it, but anyway another method that would be cheaper allows to get more reliability and accuracy.

• It is quite unsure make the object explode would not leave some big parts. In order to be sure to make a coarse powder out of the object, the H-bomb must be enormous and the hole it is put inside must be very deep.

• Such a method will take a lot of time, to build the spacecraft and to make it follow the right trajectory (even with huge rocket motors and a direct trajectory). So the encounter with the exterminator can only occur when the object is already close to the Earth. This makes no second trial is possible. So several different spacecraft must be launched in order to get a probability at least one of them succeeds. If they succeed and the exterminator explodes, the Earth will be hit by a swarm of radioactive coarse powder (because of the H-bomb).

My proposal is the following:

• Make the spacecraft hit the exterminator at the highest possible velocity. The spacecraft can have almost any trajectory, provided it crosses the path of the exterminator at the right moment. No cosmic ballet is needed, just a strong rocket motor. This makes the mission simpler to conceive and allows to hit the exterminator soon after its discovery.

• Don't try to explode the exterminator, just deviate it. This is made easier by the fact the spacecraft hits the exterminator far from the Earth. Just a little deviation makes the object miss the Earth.

• The spacecraft is a sort of rough A-bomb. For example a big needle of fissile uranium or plutonium. The needle shape makes the critical mass is not attained because too many neutrons are loosed. When the object is hit at a speed of 40 up to 100 km/s, the needle bores itself inside the object and smashes towards a rough sphere shape. So the critical mass is attained and the bomb explodes a few meters inside the object. It will carve a crater inside the object. The matter that is blown away will make the crater be for a short while a big rocket motor that will push the object and deviate its trajectory.

Such a needle can easily be manufactured with today technology. The highly precise navigation in order to hit the exterminator with a few tens of meters precision is not a problem. Such performances are already attained by anti-satellite missiles. A hundred needles can be send towards the exterminator without changing the economical structure of our civilization.

Other rough A-bomb shapes can be imagined. Like a pair of spheres that follow each other closely. The spheres or the needle can be surrounded with a material that reflects neutrons and that is calculated to keep surrounding the fissile material after the target is hit.

This method can also be used to destroy the exterminator. Should it be discovered too late to be deviated, for example. The needles would then be used to carve the object to pieces, the one after the other. They should carry an artificial intelligence system to aim the next needle at the biggest object left by previous needle.

I suppose it would be impossible to build a H-bomb needle. H-bombs are too fragile. I don't know if a big H-bomb exploding just near the surface of the object would be enough to deviate it. A compromise could be to puncture a big needle of U-238 inside the object and then ignite it with a H-bomb exploding just before it hits the object at the place the U-238 needle is situated. This could be a sophisticated last-chance device to destroy the object if it is already close to the Earth.

Another approach I heard of is to land a big ion motor on the exterminator, to slowly push it away from the path towards the Earth. This can only be used if the exterminator is far enough from the impact. For example an asteroid that would hit the Earth in a few years. It also implies the ion motor can be landed on the exterminator. This can be feasible with an asteroid but maybe not with a comet. A solution could be to put the ion motor on a little asteroid and make it cross the road of the comet once it approaches the Earth.

Another way to use nuclear power could be to use it up before the impact to gain a high speed. A nuclear motor can accelerate the impactor and the nuclear fuel core itself be the impactor's mass.

Related data: NASA Deep Impact Home Page

A great idea: http://news.bbc.co.uk/1/hi/sci/tech/4420762.stm