Effect of collisions while travelling at relativistic speeds?

[Fanghur]

I recently watched the new movie 'Passengers', and in the beginning of the movie, the starship has the misfortune of travelling through a cloud of space debris while travelling at approximately 50% of the speed of light. In the movie, the ship had some manner of force field to shield it, but I was wondering what would have happened in the real world if a ship travelling at that speed or greater were to collide with, say, a piece of debris the size of a grain of sand or a small pebble?

 

I know that at those speeds, the impact would have a kinetic energy dwarfing any (possibly even all) weapons we have in our arsenal by several orders of magnitude, but what would actually happen at the moment of impact? Would it simply obliterate (or at the very least severely damage) the ship by creating an explosion comparable to an asteroid impacting a planet? Or would the effect be more akin to a bullet going through wet paper? That is to say, it would simply punch a hole clear through the ship like it wasn't even there and continue on?

 

I'd imagine the answer would depend on how thick the ship's hull is, but I can't say for sure.

[Pugdaddy]

http://www.biblehelp.org/ufo4.htm

[Janus]

A typical grain of sand at 50% of c has a kinetic energy equivalent of ~2 kg of TNT.

[Airbrush]

Very good question that I've been wondering about for a long time also.

 

50% light speed seems a bit unrealistic, maybe 10%. In the movie the speed scale was WAY off. Traveling at 50%C means going so fast that there would be nothing to see. What would really happen, does anyone know?

 

This leads me to believe that a star ship that is on a mission to another star, traveling at a relativistic speed, should have a SHARP shape, made of very tough material (depleted uranium?) so it will simply penetrate and pass through any obstacle. If you are traveling at 50%c and the ship detects a field of asteroids in its' path, or a rogue planet, could the ship fire some retro rockets to push the ship to the side, perpendicular to direction of travel to dodge obstacles?

 

For years my signature was "How do you dodge a bullet when you are traveling at 12%C?"

Edited May 1, 2017 by Airbrush

[Janus]

Very good question that I've been wondering about for a long time also.

 

50% light speed seems a bit unrealistic, maybe 10%. In the movie the speed scale was WAY off. Traveling at 50%C means going so fast that there would be nothing to see. What would really happen, does anyone know?

 

This leads me to believe that a star ship that is on a mission to another star, traveling at a relativistic speed, should have a SHARP shape, made of very tough material (depleted uranium?) so it will simply penetrate and pass through any obstacle. If you are traveling at 50%c and the ship detects a field of asteroids in its' path, or a rogue planet, could the ship fire some retro rockets to push the ship to the side, perpendicular to direction of travel to dodge obstacles?

 

For years my signature was "How do you dodge a bullet when you are traveling at 12%C?"

It depends greatly on how far away you could detect the obstacle and how much thrust you can generate with the maneuvering rockets. In interstellar space there is going to be little light to see them by, and, if you are heading directly towards an object, little to no side to side motion to detect.

For example, if 1 light hour ahead is your best detection range ( ~7.5 AU), then at 0.5c, in order to miss an Earth-sized planet in your direct path, you would need to apply a sideways thrust of ~1/10th of g from the moment of detection up until passing the body, or for 1 hr (If you can detect it at a distance of 1 light hr away, it will be 1/2 light hr away when you receive the detection information.)

A 2 light hr detection range drops the needed thrust to 0.028 g ( applied for 2 hrs)

[MigL]

Unrealistic, maybe.

But Jennifer Lawrence sure looked good.

[Airbrush]

It depends greatly on how far away you could detect the obstacle and how much thrust you can generate with the maneuvering rockets. In interstellar space there is going to be little light to see them by, and, if you are heading directly towards an object, little to no side to side motion to detect.

 

Then for example let's use the speed of 10%C. In theory, if the spaceship can detect obstacles (using radar?), retro rockets can automatically steer the ship around one object, but probably not find a path through a field of asteroids?

[Janus]

 

Then for example let's use the speed of 10%C. In theory, if the spaceship can detect obstacles (using radar?), retro rockets can automatically steer the ship around one object, but probably not find a path through a field of asteroids?

I really don't think you are going have to worry about fields of asteroids in intergalactic space. Mutual gravitational attraction between the bodies would tend to cause them to clump together as a single object. If they are orbiting a common center of gravity, the field cannot be very densely populated. (even if it started out so, collisions and gravitational interactions will thin it out.) Forget about the thickly strewn collection of objects like that depicted in "The Empire Strikes Back".(For example, you can pass right through the heart of our own Solar system's asteroid belt and would be lucky to even see an asteroid with the naked eye.)

[Airbrush]

In that case, the 10%C spaceship could probably steer around a single object in its' path? Fields of asteroids is unlikely as you pointed out, and impacts coming from the sides would not be an issue either. So travel between stars could be rather safe, as far as collisions while traveling at relativistic speeds.

[mistermack]

I've seen it said that two galaxies could and probably would "collide", without one single collision between stars. Such is the distance between them.

If that's the case inside galaxies, there's probably very close to zero chance of a collision in intergalactic space.

[swansont]

I've seen it said that two galaxies could and probably would "collide", without one single collision between stars. Such is the distance between them.

If that's the case inside galaxies, there's probably very close to zero chance of a collision in intergalactic space.

Non-sequitur.

 

Stars not colliding with stars does not mean there would not be collisions with smaller objects. At the very least you have to worry about the hard x-rays from colliding with protons with many MeV of KE, relative to your ship.

[mistermack]

Non-sequitur.

 

Stars not colliding with stars does not mean there would not be collisions with smaller objects. At the very least you have to worry about the hard x-rays from colliding with protons with many MeV of KE, relative to your ship.

Well, intergalactic space is far more empty than galaxies. If galaxies with billions of stars can pass right through each other without any stars colliding, then your chances of hitting something like a pebble or even a grain of sand in intergalactic space are an order of magnitude smaller.

[swansont]

Well, intergalactic space is far more empty than galaxies. If galaxies with billions of stars can pass right through each other without any stars colliding, then your chances of hitting something like a pebble or even a grain of sand in intergalactic space are an order of magnitude smaller.

Saying it doesn't make it so. Do you think there are fewer small objects than there are stars?

 

Edit: how many meteors hit the earth every day?

[Airbrush]

Too bad that intergalactic space is so much further away. Let's be realistic, beyond a radius of 100 light years is out of reach for a long, long time.

[mistermack]

Saying it doesn't make it so. Do you think there are fewer small objects than there are stars?

 

Edit: how many meteors hit the earth every day?

Meteors hitting the Earth are from the Solar System, which is much more likely to have comet trails etc than the space between galaxies. And the Earth is a big target with strong gravity.

 

If you think of the most distant stars that Hubble can get an image of, then that light hitting Hubble has travelled for 13.8 billion years, obviously at the speed of light, without all of it being deflected, and still clear enough to form an image.

And a constant image that persists, not just a flash in the pan.

 

So those photons have travelled all that time at c and never hit a particle of dust.

[swansont]

Meteors hitting the Earth are from the Solar System, which is much more likely to have comet trails etc than the space between galaxies. And the Earth is a big target with strong gravity.

 

If you think of the most distant stars that Hubble can get an image of, then that light hitting Hubble has travelled for 13.8 billion years, obviously at the speed of light, without all of it being deflected, and still clear enough to form an image.

And a constant image that persists, not just a flash in the pan.

 

So those photons have travelled all that time at c and never hit a particle of dust.

 

 

I would be inclined to believe you if you could back this up with any kind of analysis. Like finding the star density of a galaxy, and looking at the numbers. The Milky Way is about 100,000 LY across, so it has an area of over 10^10 LY. If there are 100 billion stars, then that's a star per tenth of a square light year. That's why a stellar collision is unlikely. And that has squat to do with the OP.

 

Another analysis would be to calculate the number of photons emitted by stars. The fact that some of them make it to us says nothing about obstacles some might encounter along the way. There are areas where the dust is dense enough to be opaque.

https://www.sciencedaily.com/releases/2006/04/060404091933.htm

 

Think about this : since you point out it's easy for stars to pass by each other in a galactic collision, it should be equally easy to see stars if you look through the galaxy. And yet we have difficulty doing that

"we can out see only roughly 6000 light years in the plane of the Galaxy because of the dust."

http://www.astronomynotes.com/ismnotes/s2.htm

[mistermack]

It's interesting that the dust in the galaxy blocks out light from 6,000 light years away.

Of course, that's side-on to the Milky Way, in the plane of it's disc, so it's not too surprising.

If you compare that to the light I mentioned coming to Hubble from the farthest galaxy yet detected, then that light has travelled about two million times farther, without being blocked out. Which illustrates the point that intergalactic space is much less likely to have a particle of dust in the way.

It's actually confusing how the various sites refer to the space between galaxies, and the space inside galaxies.

The term "intergalactic space" generally refers to the space between galaxies, but the term "intergalactic medium" seems to be used for the space INSIDE galaxies rather than between them.

I would have thought that intergalactic should always refer to the space between galaxies, and just galactic medium for inside.

The figures that I've seen, for particle density in and outside of galaxies, are that galaxies are about a million times denser.

The space between galaxies contains about one hydrogyen atom, per cubic metre, and inside galaxies, it's about one hydrogen atom per cc.

If dust particles follow the same pattern, then that explains why the Milky Way is so much more opaque, looking towards the centre.

Of course, looking side on through the disc gives maximum opacity.

I think it give a rough guide, that you are a million times more likely to hit dust inside a galaxy, than between galaxies, which isn't really surprising, since dust is the result of exploding stars, and isn't very likely to leave the gravity of the galaxy.

[imatfaal]

1. dust in the intersteller medium (just over 1% of mass) --> rho = 2e-26 g/cm^3

 

2. an hour at .5c is 5.4e11m S = 5.4e13cm

 

3. estimated frontal area of space craft --> 10m x 10m --> 1000cm x 1000 cm --> A =10e6 cm^2

 

4. volume cleared of intersteller space by ship cruising at .5c for an hour --> V = A x S = 5e19 cm^3

 

5. mass of dust impacting space ship per hour --> V x rho = 10e-6 g/h = 1 microgram per hour

[swansont]

It's interesting that the dust in the galaxy blocks out light from 6,000 light years away.

Of course, that's side-on to the Milky Way, in the plane of it's disc, so it's not too surprising.

If you compare that to the light I mentioned coming to Hubble from the farthest galaxy yet detected, then that light has travelled about two million times farther, without being blocked out. Which illustrates the point that intergalactic space is much less likely to have a particle of dust in the way.

Again claimed without evidence. The fact that light makes it to us says nothing about how much might have been attenuated along the way.

 

 

It's actually confusing how the various sites refer to the space between galaxies, and the space inside galaxies.

The term "intergalactic space" generally refers to the space between galaxies, but the term "intergalactic medium" seems to be used for the space INSIDE galaxies rather than between them.

I would have thought that intergalactic should always refer to the space between galaxies, and just galactic medium for inside.

 

The figures that I've seen, for particle density in and outside of galaxies, are that galaxies are about a million times denser.

The space between galaxies contains about one hydrogyen atom, per cubic metre, and inside galaxies, it's about one hydrogen atom per cc.

 

If dust particles follow the same pattern, then that explains why the Milky Way is so much more opaque, looking towards the centre.

Of course, looking side on through the disc gives maximum opacity.

 

I think it give a rough guide, that you are a million times more likely to hit dust inside a galaxy, than between galaxies, which isn't really surprising, since dust is the result of exploding stars, and isn't very likely to leave the gravity of the galaxy.

A million times less dense is not zero, and it only takes one to ruin your day. And as I said earlier, even hitting protons means dealing with the Bremsstrahlung radiation. One proton per cubic meter is one proton per square meter of area per second, for each m/s of speed. Really hard x-rays, too.

[Airbrush]

When the next generation of space telescopes, TESS and Webb which are to be launched in 2018, start searching, they should be able to find the nearest Earth-like planets. Then over time we can image these possible destinations for space probes. Would we be able to tell from the Webb telescope if significant dust or obstacles are in the path of a space probe, sent at 10%c, towards an Earth-like planet? If such a probe passed through a dust cloud could it be damaged?

Edited May 9, 2017 by Airbrush

[imatfaal]

5% of interstellar dust is in the higher mass region of >10e-11 grams. But even these dust grains at the larger end at .1c have little energy (about a joule). However when they impact they will give off radiation - which could be very hi-energy gamma rays. You can see how much mass will be cleared by a spaceship in my calc in above post - multiply by 86000 to give 10 light years worth of interstellar dust. Still not much

 

However going through a dust cloud raises the density of the particulate matter by over ten thousand fold. I cannot see a breakdown by size of particles in a dust cloud but I assume that the number of high mass particles will be greater (they aggregate). A grain of about a microgram has the same kinetic energy as a family hatchback at the motorway speed limit. This is the danger. You only need one - in your journey of a hundred years...

[mistermack]

It's interesting to think about how you can shield against collisions.

 

If you have a shield at the front, clearing the way, you can site it quite far ahead of the rest of the craft.

Of course, if the particle has lateral movement relative to your direction of travel, it can miss the shield, but still hit the craft.

But the faster you are travelling, the less likely this is to happen. So if you are travelling at a significant proportion of c, your shield should be clearing virtually all dust particles.

I don't know about the motion of the hydrogen atoms that are floating in intergalactic space, but I'm guessing that they haven't got much energy, so they are not very likely to miss the shield either.

 

You would have to work out the best compromise, for how far in front of the main craft you would put the shield.

But the faster you are going, the further in front you can put it.

[Airbrush]

Why should the shield be quite far ahead of the craft?

 

Just design the spacecraft aerodynamically, like a jet, and the nose area would be very thick depleted uranium, so the craft will pierce dust clouds or small obstacles. When a significantly large obstacle is detected in its' path, then retrorockets fire to push the craft to steer around it. Right?

[mistermack]

Why should the shield be quite far ahead of the craft?

 

Just design the spacecraft aerodynamically, like a jet, and the nose area would be very thick depleted uranium, so the craft will pierce dust clouds or small obstacles. When a significantly large obstacle is detected in its' path, then retrorockets fire to push the craft to steer around it. Right?

Well, without doing any research, I believe that collisions at very high speeds generate Xrays which can be harmful to health and equipment. So if the shield is a significant distance ahead, this could be minimised.

You would have to arrive at a best compromise, between distance ahead, and efficiency of shielding.

I don't think that detecting obstacles would be doable in practice. At very high speeds, you would have to detect them a very long way ahead, and that would be pretty much impossible for dust-sized particles.

[KipIngram]

Unrealistic, maybe.

But Jennifer Lawrence sure looked good.

 

Seconded - she really looked fantastic. Saw the movie last night.

 

PS: To give equal time, my wife thought Chris Pratt looked just fine as well.

Edited May 25, 2017 by KipIngram