# Additional EGT Musings: Gravity Tractor and Enhanced Gravity Tractor Overview

[Editor’s Note: When I posted the Enhanced Gravity Tractor article earlier in the week, I didn’t have time to really dig into the concept the way I had originally intended. Josh Hopkins suggested on Twitter that I could cheat and revisit the topic in a follow-on blog post since I’m trying to do this blog-a-day for this last month leading up to the 10th anniversary of Selenian Boondocks. This is probably the first of two or three follow-on posts]

The previously linked-to EGT paper had a great introduction to the concept of using a gravity tractor for deflecting potentially hazardous asteroids. In all gravity tractor concepts you’re using the mutual gravitational attraction between the spacecraft and the asteroid itself as a way of transferring thrust from the spacecraft’s thrusters into the asteroid itself. You can do this using an in-line tractor orientation, where you cant the spacecraft engines outwards at an angle sufficient to avoid plume impingement on the asteroid, and you eat the cosine losses on the thrusters.

Or you can place the spacecraft into a halo orbit around the asteroid, and fire the thrusters due backwards.This shifts the orbit from orbiting around the equator of the asteroid to an offset halo (that looks like a spiral from a sun-centered perspective).

I prefer the halo approach, both because you can probably make it passively safe (where a thruster failure doesn’t involve the spacecraft colliding with the asteroid), because you can probably have your spacecraft a lot closer to the asteroid thus increasing the gravitational acceleration (and thus peak thrust you can impart), because you avoid the cosine losses from canted thrusters, and because it’s a lot easier to add multiple gravity tractors flying in formation with the halo/spiral approach.The equation governing the thrust you can impart into an asteroid in such a halo orbit is:Where rho is the radius of the halo orbit, and z is the axial offset distance of the halo orbit, G is the universal gravitational constant, Mast is the mass of the asteroid, and Msc is the mass of the spacecraft. As you can see, the closer you are in, and the heavier your spacecraft, the more force you can transmit into the asteroid, hence the idea of augmenting the spacecraft mass with locally harvested regolith, rock, and boulder materials. It is pretty easy to increase the effective towing mass by >10x using locally harvested materials. While traditional gravity tractor methods required more than a decade of advanced notice, enhanced gravity tractoring might only take a year or two of advanced notice if you already have the infrastructure in place to deal with a threat.

As an interesting aside, I noticed the other night that it looks like my coblogger John Hare might have actually beat the NASA guys to coming to this conclusion by at least a few months, based on this Selenian Boondocks blog post from February 2013, which describes a concept almost identical to the one shown in Figure 20 from the paper.

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#### Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.

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Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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### 13 Responses to Additional EGT Musings: Gravity Tractor and Enhanced Gravity Tractor Overview

1. Simon says:

Cool!

2. john hare says:

Ego boost, thanks.

3. MBMelcon says:

Isn’t there still a cosine loss since since the force vector F is not parallel to the velocity vector? -MBM

4. MBM,
Actually, no there isn’t a cosine loss for the halo case due to the gravitational vector being at an angle if you pick the halo orbit velocity such that the off-axis gravitational component is balanced by the centripetal acceleration of the halo orbit. This is described at the bottom of page 6. The only cosine losses you would get are if the spacecraft can’t align parallel with the velocity vector you’re trying to impart (due to gravitational effects or other considerations).

~Jon

5. MBMelcon says:

Got it, the plume is anti-parallel to the velocity vector. Does that make plume thrust (z/d)Fn? -MBM

6. MBM,

I think the plume thrust is also just Fn, though I could be wrong.

~Jon

7. Stan Witherspoon says:

For improved traction, would it help to have a “Bag of Rocks(tm)” on a tether being held just off the surface of the asteroid? If all of the extra mass is in the BoR, you don’t have to spend fuel accelerating it into the halo orbit,and making sure your thrust vector always goes through the center of mass of a loose collection of material. You can drop the tether if there is a problem and the BoR is well away from your spacecraft and will settle on to the asteroid for reuse.

8. Jonathan Goff says:

Stan,
That’s basically what John Hare proposed in his post, and what Dan, Josh, et al showed in Figure 20 of the linked report. It has some merits, but also its share of GN&C challenges. But not a bad idea.
Jon

9. Stefan says:

Apologies if this is a stupid question, but if you’re going to land anyway, why not anchor and use thrust directly? Wouldn’t the total impulse imparted on the target be the same, whether you’re attached mechanically or by gravity? You might be able to bring a drill and chuck pieces of the asteroid away as well.

10. Stan Witherspoon says:

Two problems mentioned in the paper are that asteroids typically are not monolithic objects and that they are rotating or tumbling.

11. Jonathan Goff says:

Stefan,
In addition to what Stan said, I can see several reasons:

1- If the asteroid is tumbling you’re going to end up in the shade more often than you would like. Also getting the velocity vector where you want it will be a lot harder. You could try despinning the asteroid, but that will take a lot of time and patience.
2- The surface of an asteroid is likely a pretty hostile environment for a spacecraft, especially one with large solar arrays and electric thrusters. You have a lot of dust, and the potential for shorting is high.
3- Thermal cycling going between light and shade is going to wear out your spacecraft faster.

Using a gravity tractor, and keeping the time in contact with the asteroid to the minimum is probably your safest bet. I do like the idea of expelling asteroidal material to augment the propellant you bring from home. But that’s a topic for another blog post (the next one I had planned, in fact).

~Jon

12. Stefan says:

Jon,

I completely buy those as reasons to avoid landing in the first place. But doesn’t the “enhanced” part of Enhanced Gravity Tractor require landing and collecting material anyway? Although, I suppose limiting the time you spend on the surface is still helpful in that case.

13. Jonathan Goff says:

Stefan,
Exactly what you said in your last sentence. Spending 30min-1hr on the surface is worlds different from spending 1-2yrs on the surface. The ARM mission intends to do the landing, extraction, and launch from the asteroid all within a single sunlit period to minimize thermal cycling, power issues, etc. There are still challenges compared to a non-enhanced gravity tractor, but it seems like EGT is simpler than landing and staying.

~Jon