Starship Asterisk*

http://www.bbc.co.uk/news/science-environment-20986464 wrote:

[c]Pic du Midi: (99942) APOPHIS at 15 millon km
Images obtenues le 7 janvier 2013 de 2h00 à 3h00 UTC,
poses de 30 sec, champ de 8 arc min vitesse 3 arcsec/min
© François COLAS - IMCCE - CNRS - Observatoire de Paris[/c]

Apophis asteroid: Large space rock 'will not hit in 2036'
BBC news, 11 January 2013

<<A 300m-wide asteroid will not hit the Earth in 2036, US astronomers say. It was thought there was a one-in-200,000 chance that it could strike on 13 April 2036, but revised calculations have now ruled this out.

Instead, Nasa scientists said it would not get closer than 31,000km as it flies past on this date. They were able to study the rocky mass as it made a relatively close approach above our planet, allowing them to better assess its future threat. "Radar data we have collected over the past couple of weeks have completely excluded any chance of impact in 2036. Furthermore, we can now precisely predict its trajectory decades into the future," Marina Brozovic of the Nasa's Jet Propulsion Laboratory told the BBC Stargazing Live programme.

The Apophis asteroid is named after the Egyptian demon of destruction and darkness. It caused alarm after it was discovered in 2004, when scientists thought it could have a one-in-45 chance of smashing into the Earth in 2029. Improved calculations later lifted this threat, but until this week, the very tiny but real chance of a hit in 2036 remained.

If an asteroid of this size did smash into Earth, it would strike with the energy of about 100 of our largest nuclear bombs. But for now, this has been ruled out - at least for Apophis.

Scientists are becoming increasingly interested in potentially hazardous asteroids. So far, they have catalogued more than 9,000 of them, and spot on average another 800 new ones each year. One recent discovery is 2012 DA14. On 15 February, this rock, which measures about 45m in diameter, will pass about 36,000km from the Earth. This is closer to the Earth than some satellites, but again scientists say there is no chance of a collision. 2012 DA14 should be visible with binoculars or small telescopes.>>

neufer wrote:I like the idea of a set of six octahedronally spaced stationary surface ion thrusters gimballed so that at least three always point towards (or away from) the direction of asteroidal motion. (At least three would always point towards the sun for solar power and they would be interconnected to share power.) Ion thrusters which temporarily can't point towards (or away from) the direction of asteroid motion would be turned off.

Chris Peterson wrote:
Also, keep in mind that asteroids are rotating, so in order to provide a force vector in a controlled direction relative to the Sun, you would need to stop its rotation, which is beyond our current technology. Or, you'd have to have your thruster mounted on some sort of track to counter-rotate it, which is a complex engineering problem that is probably also beyond what we're currently capable of doing in space (all the worse if the asteroid is tumbling- rotating about two axes- as many are).

Chris Peterson wrote:

starmanron wrote:
OK ... I'm not a space physicist, but wouldn't it be quicker to just push the asteroid? Sure, one would need to ramp up thrust and have an adequate structure, but ... ???

Most asteroids are loosely clumped balls of rubble.
Where do you push without just breaking it apart
slightly and redistributing material?

• . http://apod.nasa.gov/apod/ap050919.html
  . http://apod.nasa.gov/apod/ap051116.html

starmanron wrote:OK ... I'm not a space physicist, but wouldn't it be quicker to just push the asteroid? Sure, one would need to ramp up thrust and have an adequate structure, but ... ???

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:

neufer wrote:

• 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).

• But the fuel requirements go as the cube of the asteroid's size
  (; e.g., a 1 km asteroid would require a 1,000 tons of fuel).

http://en.wikipedia.org/wiki/Ion_Beam_Shepherd wrote:

[b][color=#0000FF]Ion Beam Shepherd (IBS) deorbiting a space debris[/color][/b]

---

An Ion Beam Shepherd (IBS) is a concept in which the orbit and/or attitude of a spacecraft or a generic orbiting body is modified by having a beam of quasi-neutral plasma impinging against its surface to create a force and/or a torque on the target. Ion and plasma thrusters commonly used to propel spacecraft can be employed to produce a collimated plasma/ion beam and point it towards the body. The fact that the beam can be generated on a "shepherd" spacecraft placed in proximity of the target without physical attachment with the latter provides an interesting solution for space applications such as space debris removal, asteroid deflection and space transportation in general. The Technical University of Madrid (UPM) is exploring this concept by developing analytical and numerical control models in collaboration with the Advanced Concepts Team of the European Space Agency. The concept has also been proposed independently by JAXA and CNES.

The force and torque transmitted to the target originate from the momentum carried out by the plasma ions (typically xenon) which are accelerated to a few tens of kilometer per second by an ion or plasma thruster. The ions that reach the target surface lose their energy following nuclear collision in the substrate of the target material. In order to keep a constant distance between the target and the shepherd spacecraft the latter must carry a secondary propulsion system (e.g. another ion or plasma thruster) compensating for the reaction force created by the targeted ion beam.

The concept has been suggested as a possible solution for active space debris removal, as well as for accurate deflection of earth threatening asteroids. Further in the future the concept could play an important role in areas such as space mobility, transportation, assembly of large orbital infrastructures and small asteroid capturing in Earth orbit. >>

smitty wrote:Realistically (this is supposed to be about reality, right?), how much advance warning would we need to implement such a scheme? Getting the tractor into the vicinity of the asteroid would not happen instantaneously, even assuming we had a supply of such tractors on the shelf and ready to go. Moreover, it sounds as though the tractors would necessarily be sufficiently massive that we probably would need to launch each of them in smaller segments and assemble them in space. And how much would it cost to have this all "in place" and ready to launch at a moment's notice? How much is our world worth?, you might answer. Sounds like an excellent project for a huge international cooperative effort. If we have to finance the whole thing, we're only going to divert asteroids that would have slammed into our part of the world, right? Ooops, but wait; maybe the Chinese and Russians and Brits and French, etc., would all feel the same way? Hmmmm . . . .

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

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.>>

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

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

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...

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...

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...