How to avoid objects when traveling at greater than .75 light speed. or How Not to Go SPLAT?

Question

After asking my question about seeing black holes in your path between the stars, I also wondered abut other more mundane objects. Black holes affect large areas of space so even if you didn't get 'close' it could still affect your trajectory. The answers there are good for black holes.

Now, say we get to traveling at very fast speeds. How do we keep from going splat? Running into almost anything at say .75 light speed I think would be catastrophic. Even something the size of a marble would have devastating consequences. (someone want to do the math?)

I know space is huge and it's actually fairly unlikely that you'd ever hit anything between planets much less between stars, but...

So I'm guessing we'd have to have some kind of deflector shields for small object, maybe even a way to turn the impacts into energy for the ship. But at some point the objects will be dangerously too big and avoidance would be the better idea. How do we detect and avoid object the size of a pickup (or larger maybe a small mountain) when traveling at these speeds? Or does our reaction time (light bouncing off object and returning to be observed + the time it takes for the ship to adjust course) dictate how fast it is safe to travel?

I'm assuming the ship would be doing the monitoring and course adjustments. This question is for conventional linear travel, I want to ask a question about 'Alcubierre drives' later.

Answer 1

In real life, the issue would not be rocks or even dust particles, but single atoms. The presence of a small but not insignificant number of atoms/ions/molecules in any volume of space would create enormous amounts of both friction and radiation for any vessel traveling at that speed.

The best solution I've seen that is physically realistic is a large (1000's of km in extent) magnetic field and a laser/EM beam sufficiently strong to ionize any matter in front of the vessel. Once ionized, the gas/dust will be affected by the magnetic field and can be funneled either A) into the ship's engines for fuel, as in the Bussard ramjet or B) around the vessel.

Larger objects are extremely sparse in the universe and if you happen to come across one, realistically speaking, you're dead. It's unlikely that you will, but not impossible. Very large objects could be seen using a telescope and navigated around.

Answer 2

Just to answer how catastrophic the marble would be: $$\text{marble size} = 1 \text{ cm}$$ $$\text{glass density} = 2.65 \text{ g/cm}^3$$ $$\text{marble volume} = 4/3 \times \pi \times 1 \text{ cm}^3 \approx 4.2 \text{ cm}^3$$ $$\text{marble mass} = 11.13 \text{ g} = 0.01 \text{ kg}$$ $$\beta = 0.75$$ $$\text{lorentz factor} = \frac{1}{\sqrt{1- \beta ^2}}$$ $$\text{lorentz factor} \approx 1.5$$ $$\text{traditional energy} = 0.01 \text{ kg}\times (224844343 \frac{m}{s})^2 \approx 5.05549788 \text{ PJ}$$ $$\text{actual energy} = 0.01 \text{ kg}\times 1.5 \times (224844343 \frac{m}{s})^2 \approx 7.58324682 \text{ PJ}$$ $$\text{hiroshima bomb} = 67 \text{ TJ}$$ $$\text{marble energy} ~ 7583 \text{ TJ}$$ $$\text{ratio} \approx 113$$

or alternatively, for $0.2c$:

$$\beta=0.2$$ $$\text{lorentz factor} = \frac{1}{\sqrt{1-\beta ^2}}$$ $$\text{lorentz factor} = 1.0206$$ $$\text{trad. energy} = 0.01 \text{ kg}\times (59958491\frac{m}{s})^2 = 35.950206429 \text{ TJ}$$ $$\text{actual energy} = \text{trad. energy} \times 1.0206 = 36.690780682 \text{ TJ}$$ $$\text{ratio} = \text{just about a half} :($$

That means that marble hitting your spaceship/whatever at 75% the speed of light would deliver the equivalent energy of 113 time the Hiroshima bomb, but at 20% the speed of light, it is reduced significantly, to just over a half of the Hiroshima bomb. It may be a little less or more, as these are quite rough, and it doesn't answer your actual question, but there you go. I was just wondering.

Answer 3

In true Sci-Fi fashion, someone has thought of that!

In Star Trek, the Navigational Deflector is used for everything imaginable as a shield for just this purpose, deflecting small debris that could otherwise damage the vessel. It works alongside the Deflector Shield.

It's operation is like this:

In order to prevent crashing into debris, I can imagine something like this would be highly desirable. Any sort of shield should do, however.

What about bigger stuff?

Star Wars uses the Navicomputer and Astromech droids to perform the large calculations required for plotting a complex series of jumps through space. Initial plotting would have been done with meticulously short jumps to slowly plot out the rough locations of large stellar bodies. To help with large bodies that may move (or be moved by pirates!) into common hyperspace lanes, the Hyperdrive has built-in fail-safes to cut power if a gravity well is detected. This isn't just about collision avoidance, of course, but that's out of scope here I feel.

Star Wars also has its own Deflector Shield, which is designed to prevent collisions or damage of any kind, it seems. There are a few references to meteor protection being a benefit, specifically the Particle Shield variety.

Answer 4

I think this might be one of those times where 'shipping lanes in space' actually makes sense as we'd be able to clear a line of space and ensure there aren't large objects in the way. Best theory I can come up with is the deflector beam theory...detect the object infront of you and use a beam of sorts to push it out of the way.

going to assume:

• nothing can go faster than the speed of light for this answer as it's the best we know right now.

• We can deflect. If an object was moving directly at the ship, could we really deflect it, or would we just push back on it with a deflector beam?

Lets say an object is 1 'light minute' in front of us. That would mean we could hit it with a detection beam in 1 minute. At 75% the speed of light, we would be 45 seconds of the way there with 15 seconds remaining by the time the detection beam strikes the object. When the detection beam turns around and bounces back, we are approximately 51.5 seconds towards it (8.5 seconds away). If a beam travelling light speed could be instantaneously generated (assume zero cal time), we then have 2.125 seconds for the force of this to deflect it...which I'm going to assume is improbable to impossible. Of course we can extend this so we are detecting further than 'one light minute' infront of us, and I'd have to go down the path how long it would take such a body to deflect out of our path to get an idea of how many 'light minutes' we'd need to detect these in advance.

Should also note that generating this beam and projecting it forward will slow down the ship generating it, requiring more propulsion to keep the craft at 75% the speed of light.

I would think this ultimately comes to the conclusion that there would be two very separate travel styles...one that goes into the unknown at a significantly slower speed, and one that travels known routes that are intentionally kept clear (space highways?) where speed could be significantly faster (then you get into the issue of a slower space craft getting in your way)

As an expansion, I'd be curious what type of force would be needed to change direction as some objects would need to be avoided entirely (say it's a comet that is coming into the pathway).

Answer 5

Much safer and faster is to make hyperspace jumps, where you skip (move discontinuously) over all the areas with debris. Of course to calculate hyperspace jumps you need to see into the future (because you arrive there sooner than light can), so you need spice from Dune :-)

Answer 6

How to detect things coming at you at 75% lightspeed? Radar.

REALLY POWERFUL RADAR

One scifi series I read had ships with radar, lidar and other sensors that were so powerful they would actually act as weapons at closer ranges. This is necessary, because you need a really long range to detect objects so you can dodge. And this was only at 20% lightspeed.

The math is bad. You need to spray a cone ahead of your ship with enough RF to guarantee a return signal from anything big enough to hurt you and far enough out to react. Things big enough to hurt you are quite small and you're approaching them very quickly.

I'm not going to do that math now but I suspect the ship won't need any additional laser weapons. Anyone foolish enough to approach it can have their individual atoms blown into plasma just by being scanned.

Answer 7

Just because no one else has offered it…

How about using Larry Niven's Slaver Stasis Field. He describes it as a bubble of space/time in which time is effectively stopped. Objects enclosed in the bubble cannot be affected in any way by objects outside.

So, for a long space journey, you just get up to your desired cruising speed and then turn on the field. Your now invulnerable ship barrels forward, along your desired course, crashing through the small stuff and boring through or bouncing off of the bigger stuff. Then, at a pre-designated time (controlled by a glorified egg timer and an accelerometer, both stored outside of the stasis field, in a heavily shielded box), the field turns off, allowing you to "wake up" and see if you are anywhere near where you wanted to go. If not, turn the ship, ramp up the speed and do it all over again.

So my answer to your question, with all credit going to Mr. Niven, is that you don't navigate around the stuff that is in your way. You just plow on through it.

Answer 8

It is a non-problem.

If we want to stay within limits of science: edit: and engineering

• It will be extremely hard to accelerate to 0.75SL and then decelerate to 0. Fuel would need to be like 99.999% of the weight of the rocket, even with stages. So expect to travel at 5-10% of SL.
• It will be extremely hard to shoot any bullets forward, to destroy any objects, again because of relativistic physics, because they would be moving extremely fast relative to our ship.
• And of course shooting anything forward to destroy obstacles slows you down. Newton's laws apply for relativistic travel too.

The only way to travel within the limits of physics as we know it now is to travel slowly, in multi-generation ship, of some hibernation. At such speed, detecting obstacles is simpler, and you have more time to eliminate them by laser (which has less impact on your forward momentum than shooting a bullet to destroy such object).

Of course it is less fun to travel that slowly.

Another option is to discover new physical approach. Like discontinuous jumps in hyperspace, but those are not within limits of laws of physics.

If you want to stick within laws of physics to solve this - you will not be able to get to such speed, and if you ignore physics, you can hyperjump or tractor-beam obstacles or whatever your imagination wants.

Edit: numbers provided by relativistic rocket

• to get to Vega (27 ly) and stop there, we need 57kg of fuel for every kg of payload, using 100% effective engine.

See also http://en.wikipedia.org/wiki/Space_travel_using_constant_acceleration

• Soda can is 94% fuel, 6% of construction. Build structures capable to withstand such forces and cary huge amounts of fual is very hard - read The Tyranny of the Rocket Equation

That's why I argue it is not feasible to travel at such speeds (within limits of physics and engineering as we know it today) so you don't need protection for such speeds. And I agree that it is less fun than zoom around like in Hollywood movies. Tough beans.

In other words: when we will have (now unknown) technology and engineering capable of building such ships, protecting them would be relatively simple task. That's why I say it is a non-problem.

Edit2:

• if rocket will not carry fuel/reaction mass with it, where it will come from? From empty space? Or it will be powered by magic? Wishful thinking cannot power a space ship.
• Of course shooting at rest and at 0.7c follow the same laws - that what EXACTLY theory of relativity says. Problem is the lead time. At 0.7c, universe looks "length contracted" so distant object are closer. And they are coming at you at 0.7c speed, so even if you hit target, you have good chance to get hit by debris, because it is not much time to disperse.
• rifle recoil does not "seems" to slow locomotive because of difference between mass (and inertia) of rifle and train. But Newton's laws still apply, action = reaction force. Anyone who believes that there is no effect on train by shooting rifle from train, cannot expect his opinion about physics be taken seriously.

Physics continues to work, even if some people prefer downvote my answer when I remind them inconvenient facts.

Edit: length contraction - that's cool part about relativity:

• in spaceship: ship length remains same, universe contracts.
• for external observer: distances in universe remain same, ship contracts.

Answer 9

Neither fictional example is "hard SF" which I assume is what the question means by science based.

A realistic answer is that objects need to be detected far enough away to do something about it. A real-life magnetic shield works for gas and space dust when facing forward; when decelerating the exhaust will clear your way. Larger items, found by lidar, can be blasted apart or hit by an advance countermeasure bullet. I suppose those should be hit as to throw the debis out of your path. Finally, a static shield will take impacts. This can be your reaction mass store in the form of ice, or a tower of plates with gaps between them to isolate events from the body of the ship.

In short, you have three options: get out of the way, get it out of your way, or take the hit.

Answer 10

Alastair Reynolds had a pretty nice way of dealing with it in his books. If I remember correctly, the ships in his books solved the problem by wrapping the hull in a very thick (I think it was something like hundreds of meters) layer of ice. All small particles would impact the ice without causing any direct damage to the ship itself. It seemed plausible when I read it, but can't really recall all the details.

Answer 11

This very issue has been adressed in Orson Scott Card's series called Ender's Game.

More precisely, we travel into the book called Ender in Exile.

Just to throw a little bit of background: Earth was attacked by an alien race, called the Formics. They had much better technology, etc.

Warning: some might consider it as a spoiler.

"All those old vids showing spaceships dodging through asteroid clusters — they weren't actually far off. Because when you hit a molecule of hydrogen when you're near lightspeed, it releases a huge amount of energy. Like hitting a huge rock at a much slower speed. Tears you up. Any shielding scheme our ancestors came up with involved so much additional mass, or cost so much energy and therefore fuel, that it simply wasn't practical. You had so much mass that you couldn't carry enough fuel to get anywhere."

[...]

"So how did we finally solve it?" asked Ender.

"Well of course we didn't," said the captain.

"The formics did it for us," said the captain with delight. "When they got here, yes, they devastated parts of China and damn near whupped us in the first two wars. But they also taught us. The very fact that they got here told us that it could be done. And then they thoughtfully left behind dozens of working starships for us to study."

[...]

"Please don't tell me it's an egg," said Ender.

The captain chuckled. "Don't tell anybody, but the engines of this ship, and all that fuel — they're just for maneuvering near planets and moons and such. And getting the ship going. Once we get up to one percent of lightspeed, we switch on this baby, and from then on, it's just a matter of controlling the intensity and direction."

"Of what?"

"Of the drive field," said the captain. "It was such an elegant solution, but we hadn't even discovered the area of science that would have gotten us to this."

"And what area is that?"

"Strong force field dynamics," said the captain. "When people speak of it, they almost always say that the strong force field breaks apart molecules, but that's not the real story. What it really does is change the direction of the strong force. Molecules simply can't hold together when the nuclei of all the constituent atoms start to prefer a particular direction of movement at lightspeed."

Ender knew he was pouring on technical terms, but he was tired of the game. "What you're saying is that the field generated by this device takes all the molecules and objects it runs into in the direction of movement and uses the nuclear strong force to make them move in a uniform direction at lightspeed."

Answer 12

I just had a novel idea.

Consider an "erasure channel" in broadcasting data. If you are sending some resource to another star system, break it up into small loads (say, n loads) and add redundancy, launching (n+k) loads.

If this is a seed infrastructure to set up a self-reproducing manufacturing capability, then the parent can continue sending "packets" as it manufactures them.

Some loss in transit is expected. Only n (any n of the set) need arrive safely.

Obvious approach if framed this way. What about people? Well, why aren't we sending information by laser (or whatever) and reconstructing on the other side? Or living as pure software all along.

If it's easier to "sneakernet" than beam a powerful laser, then the information making up the passenger would be treated in the same way, and included in the shipments.

Answer 13

Just make an ice shield in front of the spaceship, in order to absorb impacts.