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Message started by RGClark on 08/22/15 at 07:05:03

Title: Simulation of an asteroid retrieval?
Post by RGClark on 08/22/15 at 07:05:03

At some point I'll have to learn how to use the simulator, but I get the feeling it has a steep learning curve. Perhaps someone on the forum would like to try this:

NASA wants to do an near Earth asteroid retrieval. The ideal candidates have low delta-v requirements to move to cislunar space, such as to L2 or lunar capture. For one known asteroid it's particularly low, 2008Hu4, at only 170 m/s. It's orbital parameters are given here:

2008 HU4;orb=1

The closest approach is in April, 2016. How would a chemical propulsion transfer look that provided the needed 170 m/s? For the chemical propulsion transfer you may assume the delta-v is provided in a single short impulse. How long would it take for the transfer at the closest approach?

 Bob Clark

Title: Re: Simulation of an asteroid retrieval?
Post by Tony on 08/22/15 at 11:07:02

The learning curve is steep only if you want to learn the whole program.  To do simple tasks like make the orbit diagram you posted, its easy.
Start here:
Type 2008 HU4 into the Import Object box.  If you think white is boring, click FFFFFF and change the color.  Then press Import.

To play around with the view, adjust the slider bars.  

Slow down the time step to around 16 and press Play (>) to advance into the future.

If you press A on your keyboard, you are given an interface where you can choose to center the screen on the Earth.  Doing so shows you that April 2016 is not 2008 HU4's close approach to Earth.  It is its close approach to Earth's orbit.  Capturing this asteroid in 2016 by applying a Delta-V in 2015 would be difficult.  At closest approach to Earth's orbit, the asteroid is both ahead of Earth and external to Earth's orbit.  Slowing it down to match Earth's longitude would also expand its orbit, increasing its distance to Earth at close approach.  Your best bet would be to try to set up a ballistic capture the next time it makes a close approach to Earth in probably a few decades from now.

Sometimes we get lucky and Earth captures asteroids without our intervention.  2006 RH120 is such an asteroid.  It underwent ballistic capture in 2006.  It was already in Earth orbit when it was discovered.  It has recently been theorized that at any given time, Earth probably has at least one temporary satellite > 1 meter in diameter.  They're just difficult to discover because they're faint.  Perhaps a better strategy would be to develop a technique for discovering them.  Perhaps a space telescope in high earth orbit external to the Moon.  It would be much easier to capture them since they're already here.  We just need to slow them into a permanent orbit.  That would only take a few m/s of delta V on an asteroid that is not very massive.

Title: Re: Simulation of an asteroid retrieval?
Post by Tony on 08/22/15 at 13:31:25

You can try a very low delta-v capture here:

2008 HU4 will make another very close pass of Earth's L2 around the year 2076.  By thrusting at aphelion on Sept 16, 2015, you can change the conditions of the 2076 event.
In the Autopilot Single Event, the lines
delta = 0.1 / v;
delta = 0.075 / v;
apply a retrograde thrust to 2 clones at aphelion of 0.075 m/s and 0.1 m/s respectively, so you can see the effect in 2076.

The challenge is to set up a ballistic capture through Earth's L2 point.

Title: Re: Simulation of an asteroid retrieval?
Post by RGClark on 08/22/15 at 14:06:16

When you click on the "Close-Approach Data" link on that page for 2008HU4 it gives the V-infinity with respect to the Earth as 1.28 km/s. This means their relative speed before it is effected by Earth's gravity and speeded up. This is relevant because remember we don't want to put it in Earth orbit but put it in lunar orbit or at L2:

Asteroid Redirect Mission.

Note then that the Moon's orbital speed around Earth is 1.1 km/s. So it could be a small relative speed between the asteroid and the Moon, but this would depend on the position of the Moon when the asteroid makes its closest approach.

Most discussions of ARM just look at solar electric propulsion(SEP) because it would give a smaller mission size. But the thing is when the delta-v is so small as 170 m/s you could move a 500 metric ton (mT) asteroid with just a single Centaur upper stage, at ca. 20 mT gross mass.

That's the case I want to look at because of the 5 to 10 year transfer time for the SEP case. It should be a shorter transfer time when using chemical propulsion. If you want, you can just calculate what would be the transfer time for the asteroid to get within the Moon's distance of the Earth. We can assume we use ballistic capture or small delta-v burn when it comes close to the Moon to put it in lunar orbit or at L2.

 Bob Clark

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