I was reading Politico's Biden’s space policy: One giant leap for climate change and some algorithm embedded a "Learn more about MEV; Northrop Grumman" advertisement in the story. (screenshot of advertisement; I have no affiliation whatsoever)

The video below shows a cool activity! A communications satellite in GEO moves itself to GEO Graveyard and MEV rendezvous with it, inspects it at a distance, matches the orbit, moves 1 meter away, and then docks using the original satellite's engine and bell.

But what bothers me is the placement of MEV's ion engines on long articulated "sticks" extending far away from the spacecraft's center of mass seeming to invite unwanted torque and vibration issues, and yet an articulated electric thruster might also provide some really handy advantages!

Question: How does MEV use its "ion thruster on a stick" technology? What are the advantages of and technical challenges to putting them out there rather than mounting them on the body of the spacecraft?


Screenshot from Northrop Grumman's Mission Extension Pods (MEPs) showing two versions of "thrusters on sticks" technology being deployed

Northrop Grumman's Mission Extension Pods (MEPs) "thrusters on sticks" technology being deployed

deployment after 01:05 in video:

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    $\begingroup$ Ah. I've reverted the edit. $\endgroup$ Oct 29, 2020 at 14:24

2 Answers 2


I suspect that what you suspect is the problem is actually the answer. MEV is intended to dock with spacecraft that were not intended to dock with, and control the orientation of those spacecraft. Therefore, there are a number of situations that it has to deal with, where the center of mass will be different. It has to control that center of mass when it is flying on its own, as well as when docked with arbitrary spacecraft. Having the point of thrust so far out will allow for one to adjust how it is pointed, allowing it to point through the center of mass depending on the spacecraft it is docked with.

Ion thrust is low enough that vibrations and the like aren't likely to cause a significant issue, but long term pointing from off the center of gravity can in fact have a major issue.

It will involve running plumbing over a longer distance, but as ion engines don't have the temperature concerns, it shouldn't be a huge problem. Overall it seems like a fairly simple solution to a potentially complex problem.

  • $\begingroup$ -1 because I have always found the "I suspect that..." format troubling when used in Stack Exchange answer posts. We always encourage users to source their answers. If everyone started posting guesses in answer posts following the lead of the site moderator, we'd be in big trouble. $\endgroup$
    – uhoh
    Oct 29, 2020 at 22:15
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    $\begingroup$ That's a better topic for chat or meta, but I can say I looked pretty carefully for an answer to that question and didn't see anything. I can say that if I were going to design a spacecraft for that purpose, I would want to move the position of thrust, and putting it on a stick is the best way I could think of to do that. Not proof, but it does make sense. $\endgroup$
    – PearsonArtPhoto
    Oct 29, 2020 at 22:47
  • $\begingroup$ Are "I suspect..." answers okay? What if we all started writing them? $\endgroup$
    – uhoh
    Oct 29, 2020 at 22:52
  • $\begingroup$ "I looked pretty carefully for an answer to that question and didn't see anything." means you don't know the answer. In that case one should not reach for the answer post button, but rather simply leave a comment. Isn't that exactly what we tell new users to do? Isn't that what "This was posted as an answer, but it does not attempt to answer the question. It should possibly be an edit, a comment, another question, or deleted altogether" flag is for? In this site "attempt to answer" should include the use of sources, no? Again, what if everybody started writing "I suspect..." answers? $\endgroup$
    – uhoh
    Oct 29, 2020 at 22:56
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    $\begingroup$ While this answer could be improved by some sources, it seems otherwise useful. $\endgroup$
    – DrSheldon
    Oct 30, 2020 at 2:13

If the MEV could adjust orientation traditionally, with reaction wheels, then no advantage is gained by applying thrust in a line that doesn't pass near the craft's center of mass. But as far as I can tell, the MEV eschews reaction wheels because its mission is to repair satellites whose wheels have failed. Even if the failed ones are decades older, management may be leery of the embarrassment of getting the tow truck itself stranded for the same reason. So reduce the parts count.

So use the primary thrusters for docking maneuvers too. That isn't efficient, but ion thrusters have good $I_{sp}$, and their low thrust makes them less of a concern "on a stick."

I'd guessed incorrectly. A description of the MEV-1 includes reaction wheels. Maybe that description, more than a mere press release, can guide someone to a better explanation.

Merely as a second unattributed guess: the thrusters are that far outboard to clear the connected satellite. At least when the MEV undocks, they'd have to point at the satellite.


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