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In the NASA conference today regarding Perseverance's successful landing on Mars, they talked about how you can see the cable that held the rover in this picture below (I circled the cable).

I'm wondering: it looks to me like the cable was cut - if so, how did they cut it? And why would they choose to physically cut the cable rather than using some other mechanism that simply disconnects it (I'm sure they have a good reason, I'm just curious)?

enter image description here

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    $\begingroup$ I believe they used explosive bolts. I remember seeing a YouTube video at some point that explains why they use those. I'll try to find it. $\endgroup$ – Phiteros Feb 23 at 8:54
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    $\begingroup$ +1 That certainly looks like a cut cable to me; I don't see how "exploding bolts" explains the image. $\endgroup$ – uhoh Feb 23 at 11:17
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    $\begingroup$ If the cables were simply "unhooked" and dropped, then they may foul on equipment like the mast that has to move. Also they might drag and catch on the ground, or just use more energy to move, or stall a wheel completely. $\endgroup$ – Criggie Feb 23 at 23:46
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    $\begingroup$ Answers are conclusive, but there's also a Wikipedia mention: ...fired several pyros (small explosive devices) activating cable cutters on the bridle and umbilical cords to free itself from the descent stage. $\endgroup$ – uhoh Feb 24 at 2:44
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    $\begingroup$ It costs something like a quarter million dollars a pound to put something on Mars and when you only need to disconnect once explosives are the lightest way to do things and are extremely reliable because additional power is light and easy. $\endgroup$ – Loren Pechtel Feb 25 at 2:38
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All cables, conductive or otherwise, were severed using pyro-cutters. These devices drive a sharp wedge through a cable using force of a small explosion.

But alas! The bundle of cables in question was actually not cut on touchdown, but during rover/descent stage separation. Cutters that were set off to confirm landing are actually still attached to the rover and will be visible in pictures!

Before landing

Top of Perseverance during skycrane descent with bundle and cutters highlighted source: NASA/JPL-Caltech

Here we can see the bundle already severed, but the rover is still in the air. Outlined in green is pair of cutters that are yet to fire to sever supporting cable and final data umbilical.

Descend stage from rover's perspective, suspected cable cutters highlighted source: NASA/JPL-Caltech

Looking upwards at around the same time we can almost make out the cutters that did the job on this one, outlined in yellow to match. Outlined in orange is another pair of probable cutters.

After

Fused picture of rover's top after landing source: NASA/JPL-Caltech and source: NASA/JPL-Caltech

Finally a crudely fused picture with cords in question and final separation cutters outlined in respective colors.

There are also other great closeup pictures of cutters still remaining on Perseverance available through NASA's raw image tool.

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I believe the mechanism is this pyro-cutter, photographed by Emily Lakdawalla here. Specifically, I think they were manufactured by PacSci EMC, since they brag about it in this Facebook post. If you'd like to buy one, they sell them here.

There's a description of the cutters used by MSL (Curiosity) here with some nice pictures of it in action on page 12.

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    $\begingroup$ Pyro cutters were used to separate the Apollo command module from the service module. $\endgroup$ – Uwe Feb 24 at 1:02
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    $\begingroup$ The facebook post does not seem to be available $\endgroup$ – Federico Feb 24 at 7:25
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    $\begingroup$ @Federico it’s available for me in the U.K.. Perhaps a regional issue. $\endgroup$ – Tim Feb 24 at 9:39
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    $\begingroup$ "the cutters used by MSL (Curiosity)" The fact that page 8 of that PDF is an entire collage labeled "EDL Pyro Devices" is all kinds of awesome. (Pyrovalves, thrusters, various separation nuts, two different kinds of "pin puller", three different size cable cutters, and a fourth that's actually labeled "Mega Cable Cutter". 🤩) The MSL clearly did a lot of exploding on its way down. $\endgroup$ – FeRD Feb 24 at 12:37
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    $\begingroup$ if anyone can find a video of the pyro cutters in action that would IMO make this already great post even more interesting. $\endgroup$ – Trevor Boyd Smith Feb 24 at 12:52
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According to the official NASA press release, pyrotechnic guillotines were used:

The footage ends with Perseverance’s aluminum wheels making contact with the surface at 1.61 mph (2.6 kilometers per hour), and then pyrotechnically fired blades sever the cables connecting it to the still-hovering descent stage. The descent stage then climbs and accelerates away in the preplanned flyaway maneuver.

That is not the same thing as an explosive bolt.

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    $\begingroup$ "pyrotechnic guillotine" sounds like a good band name... $\endgroup$ – IronEagle Feb 23 at 20:56
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    $\begingroup$ pyrotechnic guillotines were used for the Apollo mission to separate CM and SM. Electrical cabels and tubes for oxygen and water were cut. Explosive bolts were used for stage separation of the Saturn V. $\endgroup$ – Uwe Feb 24 at 1:07
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    $\begingroup$ @Uwe: Exactly. Guillotines were also used between the LM ascent and descent stages. Explosive bolts held together a large number of things that needed to be separated or deployed (e.g. escape tower, SLA, landing legs, docking adapter). $\endgroup$ – DrSheldon Feb 24 at 2:09
  • $\begingroup$ @IronEagle - you'll see one used on stage in This Is Spinal Tap, Reunion Tour, coming soon. (Don't want to give away what happened, but the stage manager screwed up the props again ...) $\endgroup$ – davidbak Feb 24 at 20:18
  • $\begingroup$ @IronEagle Pyrotechnic Head Clipper. (Because we ran out of hedges?) $\endgroup$ – leftaroundabout Feb 25 at 12:27
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Perseverance's umbilical data cable and bridles were cut by exploding bolts. This is the same method that Curiosity used.

Exploding bolts are very commonly used in space missions. This is for several reasons. First, the fasteners must be strong enough to hold the pieces together throughout all of the vibrations and motion a spacecraft must endure as it launches. There must be absolutely no chance of them separating, otherwise vital components could break. However, they must also be able to release at precisely the right time, and very quickly at that.

An exploding bolt is extremely simple, and has a very low chance of failure. Since NASA has been using these for decades, they know how to use them very well. And NASA likes using technology they already know how to use. Could they have made a fancy mechanism to properly disconnect the cables? Probably. But the more complicated the mechanism, the more likely it is to fail. And it's not like they are going to use those cables later.

This video does a very good job of explaining how explosive bolts (Hold Down Release Mechanisms) work on spacecraft (though it's not the one I remember seeing).

Edit: From the comments and other answers, I see that the communications cable was cut with an explosive guillotine. I'm going to leave this answer up, though, as it provides insight into how the other cables were cut.

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    $\begingroup$ The image in the OP's question certainly looks like a cut cable to me. I don't think that "exploding bolts" explains the image. Is it possible to reconcile the two? $\endgroup$ – uhoh Feb 23 at 11:15
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    $\begingroup$ I would also be surprised at NASA using anything other than tried-and-true explosive bolts... but that picture really doesn't look like them. For one thing, it's clearly a bundle of cables and not a single sturdy, threaded screw. How would a bolt grip tightly around that? For another, NASA themselves use the word "cut" when describing the rover separating from the skycrane $\endgroup$ – BThompson Feb 23 at 13:55
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    $\begingroup$ Surely a pyrotechnic guillotine rather than an "explosive bolt". $\endgroup$ – Organic Marble Feb 23 at 14:26
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    $\begingroup$ psemc.com/products/pyrotechnic-cable-cutter - says "guillotine" on it. $\endgroup$ – Mazura Feb 23 at 15:14
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    $\begingroup$ NASA explained in yesterday's AMA: "we command pyrotechnics to initiate a guillotine like device that cut the cables" $\endgroup$ – Kirk Woll Feb 23 at 16:31

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