View Full Version : Throwing a knife in space.

10-31-2012, 11:16 PM
If a knife was thrown in space, would it pick up enough speed to cut through a rope? The scenario is an astronaut on a spacewalk is attached to his craft via a tether/line. Another (bad) astronaut, throws a knife in an attempt to sever the line. It glides towards the line, spinning slowly. The sharp blade connects with the line and slices through it.

Is this plausible? (aside from being an amazing shot and perfectly timed that the blade end hits the line). I'm not sure if a thrown object in space could generate the speed needed to cut the line. But with no resistance I guess it would be faster than on Earth...

Anyone know the official name of the lines astronauts use to do spacewalks and what they are made of?

Thanks in advance.

10-31-2012, 11:29 PM
If a knife was thrown in space, would it pick up enough speed...
Objects in space cannot pick up speed -- there's no force except the initial thrust. A thrown knife cannot cut a rope, there's not enough power. Power is the product of force and velocity and, since velocity doesn't change in space (significantly), it's the force of the throw versus the force of the rope that makes the difference (compounded with the surface size, i.e. blade sharpness).

Drop a piece of rope on a knife blade for an example. The blade would have to be infinitely sharp to sever the rope.

A precise shot from a blaster would do it though. Provided you didn't attend the Storm Trooper Marksmanship Academy... :)


10-31-2012, 11:29 PM
Well it won't gain speed after it is thrown unless you have something going on other than a static vacuum. But nothing stops him from throwing it pretty quickly.

It seems that the line attached to him, he could cut it at that end without throwing the knife? I mean, a tether is probably made of metal or synthetics, not rope. So it is even less possible than it seems.

Drachen Jager
11-01-2012, 12:24 AM
Astronauts are not connected by 'rope'. Whatever they use I highly doubt it can be severed so easily.

Okay, just looked it up, the tether is normally an oxygen line (I presume the suit has a small reserve for emergencies) and an electrical/communication line. No way is a knife going through both of those.

11-01-2012, 12:48 AM
It will continue going at the same speed at which it was thrown unless an outside force accelerates it.

The bigger problem is that it's very unlikely for a knife to slice through any kind of cord or tether in one stroke by falling at it, which is essentially what is happening when you throw something in space.

If you drop a knife at a rope on Earth, it's very unlikely for you to cut through it. And you have the benefit of gravity accelerating the knife.

Can you just have the bad guy cut the tether from the ship?

King Neptune
11-01-2012, 01:51 AM
If the thrower could have cut the line with a thrown knife on Earth, then he could do it in space. The force would be the same. The only difference would be that the knife would not slow from air resistance, nor would it be deflected by gravity (depending on exactly where). I don't know what they make the O2 hoses from, but rubberized nylon or similar would make snes, and that could be cut by a thrown knife, if the knife hit just right.

You might want the knife to just nick the hose, so that the astronaut would stop getting O2.

11-01-2012, 01:59 AM
The others have the general reason the knife throwing won't cut the line, but something else to keep in mind is how hard it is to actually throw something in space. We use leverage to throw on earth - comes from our connection to the ground - and you don't have that in space. Imagine trying to throw a knife while standing on the ice and multiply that by ten or so.

Watch some video of a spacewalk and you'll see that all movements are slow and a little clumsy. Make sure whatever you end up using to cut the tether it takes into account the proper feel of movements without gravity.

Drachen Jager
11-01-2012, 02:19 AM
Leverage wouldn't be much of a problem. 70 kilos or so of Astronaut vs 250-500 grams of knife makes leverage moot.

The hose, from what I can see in pictures, is wrapped together with that reflective metal foil NASA seems to put over everything. I'm sure it's also hardened to ensure it won't get nicked in a simple accident. I really don't think there's any way this scenario is plausible.

If it's science fiction, you may give the astronaut some sort of advanced cutting tool, buckeye tube knife, some sort of burning torch that burns ultra hot and works in space (perhaps with its own oxygen supply), maybe just call it a 'plasma torch' or somesuch and be done with it.

Barring such tech-toys the scenario doesn't work.

11-01-2012, 03:16 AM
Thanks for the replies, I think I will need to change the details of the scene. It's for film so maybe a bit more license to stretch reality. Just thought I would be quite visually entertaining if we see the thrown knife spinning through space, as the astronauts nears the end of the line to safety, only for the knife to cleanly cut through the line, sending him off into the viod of space.

The high-tech blow torch could be an idea - I could have it thrown too! The thrower astronaut is from a secondary craft.

11-01-2012, 05:20 AM
Frankly, I'd laugh and smugly point out how implausible a thrown knife in space was. I'm pretty much a jerk and no one will go to movies with me anymore.

On the other hand, if you take this over the top and have one astronaut hucking a battleaxe at the line of another then I will gladly suspend disbelief. Especially if a huge rocking soundtrack is playing and their space suit has viking horns on the side because if you're going to go - go big.

A blow torch is definitely realistic and super scary. Hitting a tiny target like a safety tether is going to be a hard throw but to make sure and hit it with the burny part so that it cuts through the line - that's going to be an amazing shot.

11-01-2012, 05:32 AM
For ever action there is a reaction, even in space, though it may not be the expected reaction.

You astronaut bad guy puts his arm in motion and throws the knife. Now his body is in motion as well. If he is not tethered to something solid, the throwing motion could have him tumbling end over end...

Also, does the good guy have a space walk maneuver pack on? More than likely he does and even if his tether is severed, he can maneuver back to the ship using the thrusters on the pack...

11-01-2012, 06:33 AM
What makes it difficult to throw accurately if not that you slide backwards, it's that you pivot.

11-01-2012, 10:57 AM
Leverage wouldn't be much of a problem. 70 kilos or so of Astronaut vs 250-500 grams of knife makes leverage moot.

It's not just the mass of the knife. It's also the larger motion of the arm making the throw, and even more important, the impediment of whatever protective armor the thrower is wearing. Sounds completely implausible to me. Maybe you should contact Adam and Jamie.


11-01-2012, 02:02 PM
Where are these two guys in relation to each other and the craft to which presumably both of them are tethered?

11-01-2012, 02:41 PM
The bad guys' ship has attached itself to the good guys' ship via a cord/line. The good astronaut goes outside into space and works his way up the line. (no backpack involved) - and manually disconnects the line. He works his way back down line toward his own ship. The bad astronaut is desperate to stop him, he also leaves his ship (I guess attached to a line as well). Just before good astronaut reaches safety, the object thrown (leaning towards blow torch) by bad astronaut cuts through the line. So good astronaut is no longer attached to anything. The plan was to have him then drift off into the path off the thrusters.

11-01-2012, 04:19 PM
Sorry, but if I saw that, I would laugh my socks off. Then tell everyone I knew how ridiculous it is and why didn't the writer actually check that it was possible.

And then try to guess just how many 'Comic Book Guys' will ripping you apart on the Internet the next day.......

Seriously, don't do it.

11-01-2012, 04:44 PM
mirandashell, the throwing of a knife? Or the blow torch? Or just the general idea of throwing something through space to severe the line?

If a thrown object is too ridiculous, I will have to use another method - perhaps a laser shot, cutting the cord.

It's a shame as I still think an object floating through space, almost in slow motion, slicing through the line would be very cinematic - but not if it takes the viewer out of the scene, thinking 'how the hell'!

It is science fiction, but would like to keep it as real to science as possible.

11-01-2012, 04:48 PM
The general idea of throwing something to sever a line in space. I like the idea of the 'showdown' but you have to stop thinking about what works on the ground and think about what would work in a place with no gravity.

Throwing something is damn tricky, for the many reasons above. Forget that. Think of how people move. How people can die in space. The tiniest of nicks to your suit and you are likely to die. What else? If you want suspense, think about what creates suspense.

TBH, the whole 'floating away into the blackness' has been done so often that it's unlikely to have the effect you want. So think of something else.