APOD: Gravitational Tractor (2012 Jul 07)

[neufer]

A ten year gravitational tractor pull which ultimately changes an asteroid's velocity by 1 cm/s is actually (paradoxically) reducing its velocity by 1 cm/s by effectively placing the asteroid in a higher, longer and slower orbit. Hence, rather than being just 1578 km ahead in its trajectory after ten years of tugging (as one might expect) the asteroid actually ends up 4734 km behind in its trajectory; and it gets 947 km further behind for every year thereafter. Given a few decades, it is not difficult to avoid hitting the earth using such a ten year gravitational tractor.

[henrystar]

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 . . . .

Better yet, maybe we'd get 3 or 4 expeditions from different countries all nudging in different directions......what fun!

[Flase]

I still say blow it out of the sky with nukes

[Goddard_69_Alum]

A quick calculation shows that the maximum acceleration that can be provided by a 20 (metric) ton spaceship at a distance of 100 m (the asteroid radius) is 1.335E-10 m/s^2. If the distance to be nudged is one earth radius, the time required is nearly ten years. (This assumes moving it perpendicular to its' orbit. Speeding it up or slowing it down might be a better option, depending on the orbit.) If it makes a close pass to earth much earlier and can be nudged a smaller amount, fine and good. If not, this is not very workable. For one thing, it takes a whole lot of energy (and reaction mass) to get the craft where it would need to be. Even with an earlier pass, it would still take a lot of energy and reaction mass to match the orbit of the asteroid.

Trying to break up the asteroid with either a nuclear weapon or a direct kinetic energy kill would be a better option. Even hitting it and relying on a total change in velocity of an intact asteroid from hitting a stationary (relative to earth) spacecraft would change the path by one earth radius in about 10 days, assuming a relative speed of 1 km/s and an asteroid density of 5 g/cm^3. If we managed to break it up, so much the better. Only those fragments above a certain critical size will actually reach earth, and the rest will burn up in the atmosphere and results in a lot of dust and not much more. The 2002 Eastern Mediterranean event was estimated to be a meteorite of about 10 m in diameter and exploded in the air. This can be used as a lower limit on the size of a remnant chunk that could impact earth.

[neufer]

• 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).

Contrarily, for asteroids ~100 meters and smaller the weight of the fuel
becomes negligible (< 10%) such that an Ion Beam Shepherd (IBS) PUSHER makes more sense.

[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. >>

[neufer]

<<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:

Note that a ballast mass of just 2 metric tonnes could be dangled on a tether just 13 meters above the asteroid's surface.

The spacecraft itself could then be a full kilometer away with its ion jets
pointing almost directly backwards (and fuel mass loss would be irrelevant).

[starmanron]

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 ... ???

[Chris Peterson]

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? The advantage of using gravity (which happens to be an attractive force, not a repulsive one) is that you apply force to all parts of the body (although tidal effects mean the force isn't quite the same everywhere), and thus move the whole structure. 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).

[neufer]

[b][color=#0000FF][size=115]And some asteroids you might prefer just not to touch altogether[/size][/color][/b] :!:

---

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

[neufer]

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).

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]

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.

That's an elegant solution... although still quite an engineering challenge, especially for something that is expected to operate for a decade or longer without service. Moving parts in a vacuum have always presented reliability problems.

[neufer]

[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.>>

[c]. http://astrobob.areavoices.com/2013/01/ ... h-in-2036/[/c].

NASA Rules Out Earth Impact in 2036 for Asteroid Apophis
NASA JPL-Caltech | 2013 Jan 10