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above: GIF from Polular Mechanics which has been kindly modified here to be under 2 MiB.

Harpoons are effective when the target is out of reach, or moving too quickly to catch. Neither is generally true when one spacecraft maneuvers near another in Earth orbit. Getting close enough in relative position and velocity to harpoon something can take a huge amount of Δv, but once there, the additional Δv to approach and contact the target spacecraft is minimal.

The BBC News article Big harpoon is 'solution to space junk' highlights AirBus' research into satellite harpooning. Is this really easier or more reliable than some kind of grappling or connecting or netting o "tying-on-to"? Surely large modern spacecraft have several sturdy things to hold on to already.

Is de-tumbling the presumably dead target spacecraft really easier if it is on the end of a harpoon? It seems much easier to de-tumble something if you are rigidly attached and can apply torque. If you only have it at the end of a line, how could one apply the correct torque to zero out all angular momentum? Wouldn't one just get wrapped around the target in a tangled mess?

The sentence "Care would be needed to avoid hitting any pressurised tanks onboard." in one of the figure captions highlights another issue with ballistic harpooning.

There's more information in the article and accompanying video RemoveDebris: Space junk mission prepares for launch.

Click small images for larger size:

below: "The miniature harpoon to be tested on the upcoming RemoveDebris mission." From BBC.

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below left: "Envisat artwork: Care would be needed to avoid hitting any pressurised tanks onboard." Credit: ESA. From BBC.

below right: "The spacecraft has been assembled in the UK and will soon be packed up for launch." Credit: Max Alexander. From BBC.

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below left: "The spacecraft will establish whether a net could ensnare a small satellite." Credit: RemoveDebris Mission. From BBC.

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    $\begingroup$ A harpoon is much simpler than a robotic arm grabbing the debris. Both methods would fail if pointed to the wrong spot of the debris. $\endgroup$ – Uwe Mar 25 '18 at 11:04
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    $\begingroup$ you may be interested in Necropolis which looks at capturing defunct satellites "The Harpoon capture is u nder development by Airbus Defence and Space with support from ESA Clean Space programme. It was selected against the thrown net solution (also under consideration) because a net may ensnare the Hunter as well as the satellite it is carrying" It also carries a "stinger system that uses the apogee boost motor or the Marman clamp ring as the physical capture point." $\endgroup$ – user20636 Mar 27 '18 at 14:19
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    $\begingroup$ @uhoh, Necropolis was something I was aware of from an article in SpaceFlight, but I don't have sufficient interest to reread and summarise for a full answer. $\endgroup$ – user20636 Mar 27 '18 at 14:44
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    $\begingroup$ Another problem is to move the captured debris. Thrust vector of a satellite should align to the center of mass, before capture and also after capture before thrusting. Pushing the debris would require realigning the thrust vector to the new center of mass for catcher plus debris. Pulling the debris might enable self alignment, but tumbling of the debris will be a problem. $\endgroup$ – Uwe Mar 28 '18 at 12:02
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    $\begingroup$ The best way to get a satellite under control would be a special docking adapter mounted at a position with balanced weight distribution. Every other method without a docking adapter would be more difficult and success is less probable. $\endgroup$ – Uwe Mar 28 '18 at 19:05

This answer responds to the title question "Harpooning satellites? Is this really the best way to get them under control?".

I've studied this problem, the answer is "no", based on this rationale:

  1. all satellites have adequate hard points on them from launch, usually in the form of a launch vehicle adaptor ring
  2. a harpoon will create secondary debris even on a successful engagement in the form of matter released from the surface impact. Any mis-engagement is also likely to involve secondary debris release. Of course any orbital demonstration will show that there is no trackable debris because its all too small, but as we all know that completely defeats the objective as it would have exchanged a single known hazard for a cloud of untrackable hazards.
  3. the dynamic motion of any pair of orbiting vehicles is complex from the time between the first tip off to rigidisation, the harpoon offers no benefits and a worse (longer) time here as it has to be wound-in and brought into a known controlled state.
  4. also in that time the harpoon tether is at risk of becoming entangled in appendages and possibly causing secondary damage as the chasing vehicle attempts to gain full momentum control over the combined stack

I believe the objective has to be to keep satellites intact as much as possible as this is keeps the constituent mass in the best tracked configuration. As a result the emphasis for de-orbit really has to be on preventing secondary debris release.

To address the obvious counter part of the my claim (i.e. if harpoons are not "best" then something else must be better), I believe that there is a large solutions space, available in the timescale, that it is meaningful for debris clearance based on the following

  • large soft bulk capture
  • controlled tracking motion for small capture spacecraft
  • cost effective robotic arms

I don't think adaptor ring motion is a significant in the medium term, no matter the number of appendages. I can see that if one looks through the rigid constraints of "what could be qualified with agency funding next year" then one might be driven into solutions like a harpoon, though I don't think it represents a viable clean-up path given the secondary debris issue. I accept it does help with the removal of mass from orbit, and might seem cost effective in that respect, but by creating hidden debris it is effectively storing up the problem for later.

  • $\begingroup$ to address rationale 1 -- in order to use the adaptor ring the object would need to not be tumbling. Part of the rational for the USIS is the scant lip used by a marmon clamp isn't a suitable target for attaching to. $\endgroup$ – user20636 Mar 27 '18 at 14:12
  • $\begingroup$ Thanks, I didn't address that side of it. answer now updated $\endgroup$ – Puffin Mar 27 '18 at 18:10
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    $\begingroup$ When you say "studied", do you mean you're an aerospace engineer who's spent months/years working on the problem of space debris, or are you a spectator like me who's spent half an hour looking up a few sources? $\endgroup$ – Hobbes Mar 28 '18 at 8:50
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    $\begingroup$ 1 year agency funded study looking at cost effective satellites, robot arms etc. I was the systems lead looking at the mission as a whole including he debris remediation. It made a nice change from supporting geostationary satellite missions. This field has the potential to change quite quickly now given that the price of doing demonstration missions could fall. $\endgroup$ – Puffin Mar 28 '18 at 10:20

The RemoveDebris mission is a low-cost, small mission to demonstrate 4 key technologies needed for removal of large space debris (i.e. defunct satellites).

Surely large modern spacecraft have several sturdy things to hold on to already.

No. Satellites are generally not built to be serviced on orbit, so they don't have fittings that can be reliably grabbed by a claw-type mechanism. Most of the surface is flat aluminium panels. Magnets won't work either. Any protrusions (solar panels, antennas) are not dimensioned to take large loads.

Experience with the Shuttle Canadarm and the ISS robotic arms show that operating a robotic arm is not easy. You can't bump your target, because any bump will lead to the target drifting away from the arm. You pretty much have to have purpose-designed grapple fixture for the arm to hold onto. Most current satellites don't have this.

If you wanted to use an arm to target a large satellite that's tumbling, you'd have to do complex maneuvering to get close enough to the target without hitting its solar panels and other protrusions.

A net has to be very large to capture a large satellite incl. its solar panels.

A harpoon, in contrast, can be fired from outside the swept volume of the target satellite. A small spacecraft can carry multiple harpoons.

To control the satellite, you can always reel in the harpoon until the two spacecraft are in rigid contact.

Here's an example, a Boeing 702-based satellite:

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The satellite body consists of 4 rigid panels on the outside, some of it covered in thermal blankets. Those panels are usually solid aluminium or aluminium honeycomb, made to withstand the satellite's weight during launch. These are the most solid parts of the satellite.

To grab any part of the satellite with a claw you'd need to get close without hitting the many protruding parts (hit one and the sat will float away). Then you need to fit the claw around a small target accurately.

The only place where a claw could be used reliably is the payload adapter.

A harpoon can be fired from a distance at a large target (just about anywhere on one of the sides is a viable target), and then be reeled in at leisure.

  • $\begingroup$ They don't have to be designed with the purpose in order to serve the purpose. And this is not servicing, it is garbage collection and incineration. If you are recovering a dead spacecraft, why couldn't you use the arms holding solar panels or other appendages? I'm not sure harpooning is any more "reliable" than holding on to something large in order to apply a Δv. $\endgroup$ – uhoh Mar 25 '18 at 12:20
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    $\begingroup$ It's not called an experiment for nothing. $\endgroup$ – Hobbes Mar 25 '18 at 12:26
  • $\begingroup$ I'm thinking about Philae's harpooning experiences. $\endgroup$ – uhoh Mar 25 '18 at 12:31
  • $\begingroup$ If the harpoon missed the debris spacecraft it should be possible to reel back the harpoon and reload it into the catapult for the next try. $\endgroup$ – Uwe Mar 25 '18 at 17:02
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    $\begingroup$ @uhoh if you grab antennae or solar panels they will just come off. To make the strong enough to be a useful handle would waste mass. $\endgroup$ – Steve Linton Mar 25 '18 at 17:02

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