Starship Asterisk*

It is the year 2464

You are commander Bob, in command of the Terran Empire's greatest warship, The Space Conqueror.

A meteoroid has been discovered on a collision course with Earth and it is now your mission to use your vessel's arsenal of weaponry to save the planet.

You are mankind's last and only hope, Commander Bob, so get your act together and measure up!

APOD Robot wrote:The steady thrust would gradually and predictably alter the course of the tug and asteroid, coupled by their mutual gravitational attraction.

Flase wrote:I always imagined that this would be the only reasonable purpose of nuclear warheads.
Have them smack into the side of the asteroid at maximum velocity and if you catch it early enough, it wouldn't take much to push it away from a collision course.

• The Burning

Flase wrote:

APOD Robot wrote:
The steady thrust would gradually and predictably alter the course of the tug and asteroid, coupled by their mutual gravitational attraction.

What sort of time period would this be?

• ~ 10 years.

http://en.wikipedia.org/wiki/Gravity_tractor wrote:
<<Let us suppose that a NEO of size around 100 m, and mass of one million metric tons, threatened to impact Earth. Suppose also that a velocity correction of 1 cm/s would be adequate to place it in a safe and stable orbit, missing Earth that the correction needed to be applied within a period of 10 years.

With these parameters, the required impulse would be: V × M = 0.01 [m/s]×10⁹ [kg] = 10⁷ [N-s], so that the average tractor force on the asteroid for 10 years, = 3.156×10⁸ s, would need to be about 0.032 newtons. An ion-electric spacecraft with a specific impulse of 10,000 N-s per kg, corresponding to an ion beam velocity of 10 km/s (about twenty times that obtained with the best chemical rockets), would require 1,000 kg of reaction mass (Xenon is currently favored) to provide the impulse. The kinetic power of the ion beam would then be approximately 317 W; the input electric power to the power converter and ion drive would of course be substantially higher. The spacecraft would need to have enough mass and remain sufficiently close to the asteroid that the component of the average gravitational force on the asteroid in the desired direction would equal or exceed the required 0.032 N. Assuming the spacecraft is hovering over the asteroid at a distance of 200 m to its centre of mass, that would require it to have a mass of about 20 metric tonnes, because due to the gravitational force we have:



Considering possible hovering positions or orbits of the tractor around the asteroid, note that if two objects are gravitationally bound in a mutual orbit, then if one receives an arbitrary impulse which is less than that needed to free it from orbit around the other, because of the gravitational forces between them, the impulse will alter the momentum of both, together regarded as a composite system. That is, so long as the tractor remains in a bound orbit, any propulsive force applied to it will be effectively transferred to the asteroid it orbits. This permits a wide variety of orbits or hovering strategies for the tractor. One obvious possibility is for the spacecraft to orbit the NEO with the normal to the orbit in the direction of the desired force. The ion beam would then be directed in the opposite direction, also perpendicular to the orbit plane. This would result in the plane of the orbit being shifted somewhat away from the center of the asteroid, "towing" it, while the orbital velocity, normal to the thrust, remains constant. The orbital period would be a few hours, essentially independent of size, but weakly dependent on the density of the target body.>>

• Note that the mass of the tractor goes with the square of the asteroid's size
  (; e.g., a 1 km asteroid would require a 2,000 ton tractor).

Flase wrote:If you use the continuous acceleration of a thruster, could you not attach a more heavy-duty rocket to the rock's surface ... ?

rstevenson wrote:

Flase wrote:If you use the continuous acceleration of a thruster, could you not attach a more heavy-duty rocket to the rock's surface ... ?

What we've learned recently is that many, perhaps all, asteroids are just bunches of rocks and gravel and maybe ice. If you attach anything to pull them, they would come apart. If you go around back and try to push them, they would come apart. I think even pushing them with light pressure would cause them to come apart...

Flase wrote:

rstevenson wrote:

Flase wrote:
If you use the continuous acceleration of a thruster, could you not attach a more heavy-duty rocket to the rock's surface ... ?

What we've learned recently is that many, perhaps all, asteroids are just bunches of rocks and gravel and maybe ice. If you attach anything to pull them, they would come apart. If you go around back and try to push them, they would come apart. I think even pushing them with light pressure would cause them to come apart. Gravity is the only thing that will pull all of the asteroid evenly in one direction. (I suppose you could "bag" them, but we're talking a really, really big bag, and a pretty strong one too!)

The real problem will be getting the large, heavy gravity tug into position in time to do any good. This will only work if we have many years of advance warning.

I suppose you could ascertain the structure of the asteroid before making the decision. Some of them seem to be largely solid iron metal and you could attach a mining operation to build a ship if you have ten years...

Flase wrote:I suppose you could ascertain the structure of the asteroid before making the decision. Some of them seem to be largely solid iron metal and you could attach a mining operation to build a ship if you have ten years...

Chris Peterson wrote:
comets are a different problem... but there's currently no practical technology to deflect those.

neufer wrote:Land a surplus Japanese nuclear reactor on the surface and set it for meltdown.

Rusty Schweickart wrote:
Sorry APOD... you are out of date. Many years ago we (B612 Foundation) worked with NASA when their Prometheus mission was in planning. It was a HUGE ion engine propelled spacecraft... which we realized could be easily modified to have gravity tractor (GT) capability. This led detractors to claim that the GT had to be a HUGE spacecraft. NOT! Any spacecraft using electric propulsion can serve as a GT and e.g. deflecting Apophis from the 2029 keyhole would require a 1 metric ton spacecraft (modest size) only a month or so of towing to avoid the keyhole/impact.

http://en.wikipedia.org/wiki/Rusty_Schweickart wrote:
<<In May 2005 Schweickart told the U.S. Congress that a mission to attach a device such as a radio transponder to asteroid 99942 Apophis (formerly known as 2004 MN4) should be a high priority; it is estimated that this asteroid has a 1 in 6000 probability of striking the earth in the 21st century. The latest data indicates that the chance of Apophis impacting the earth is 1 in 45,000 in 2036.>>

http://en.wikipedia.org/wiki/Gravitational_keyhole wrote:
<<The asteroid Apophis was once estimated to have a 2.7% (1 in 37) chance of striking the Earth in 2029. Further observations and revisions of the estimated path of the asteroid have ruled out impact in 2029, but identified a 660 m wide keyhole that the asteroid may pass through on its approach in 2029, thereby causing a deflection that may result in impact in 2036; currently, the estimated probability of impact in 2036 is 1 in 250,000 (0.0004%). Apophis is estimated to be as large as 400 m across, and could cause millions of casualties if it were to hit Earth.

Scientists from the B612 Foundation, a private foundation dedicated to protecting the Earth from asteroid strikes, have proposed that Apophis be nudged out of its present orbit into an orbit that takes it further from the keyhole. NASA scientist David Morrison says, "After 2029, the deflection would have to be vigorous enough to miss not just a tiny keyhole but the much larger target of the Earth itself. And such a deflection is far beyond present technology for an asteroid this large."

Additional observations of the trajectory of Apophis revealed the keyhole would probably be missed. As of October 7, 2009, the probability of an April 13, 2036 impact is considered to be 1 in 250,000. An additional impact date in 2037 was also identified; the impact probability for that encounter was calculated as 1 in 12.3 million.

The gravitational keyhole for Apophis is only 660 metres in diameter. Calculations showed that if Apophis' velocity could be changed by only 0.00016 km/h, then in three years its orbit would be deflected by more than a mile, enough to miss the keyhole. The problem is that the keyhole is so small that it becomes extremely difficult to predict precisely if Apophis will pass through it or not. Orbit projections made in 2006 for Apophis in 2029 have a margin of error of about 3,200 km, which as of early 2010 is well within the estimated distance between Apophis (18,600 miles from Earth) and the location of the keyhole (18,893 miles from Earth). As time passes the error ellipse will be reduced but NASA may have to wait until enough data accumulates to reduce the error ellipse to one mile (1.6 km) before it knows if Apophis will hit the keyhole or not.>>

Rusty Schweickart wrote:Sorry APOD... you are out of date.