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

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.

• @PeterMasiar The real point was how do we avoid getting killed by small, medium and large objects traveling at speeds that are really beyond most human understanding. Even traveling at .1 light is dang fast and dangerous should anything get in front of you. Commented Dec 11, 2014 at 20:06
• actually your only argument why we can't go that fast is because we can't take enough fuel to slow back down. At least according to Wiki, .1 the speed of light is relativistic. Can we make it to .2 or .5 light? Commented Dec 11, 2014 at 20:55
• @PeterMasiar: Remember that the "bigger fuel budget" and increased mass thing is only true from an outside observer. Inside the ship nothing changes. Commented Dec 11, 2014 at 21:15
• I will gladly upvote any answer with better numbers. So far we have none. So far @Jason Patterson is closest, but all answers ignore tyrany of rocket equations - you need a lot of fuel to get small amount of fuel moving fast. Soda can is 94% fuel, 6% can. And none of the answers (even mine) calculates fuel need for return trip. Commented Dec 11, 2014 at 23:46
• Yes you're absolutely right. Most black holes have more gravity than the sun (if not significantly more), so their planetary system (a.k.a. "solar system"), and their Oort Cloud, and their heliosphere will all be larger than that of the Sun. The only reason I wanted to make my comment is because many people don't understand the relationship between a black hole's mass and its gravitational influence; people often think a black hole has infinite gravity, which it doesn't; it has infinite density. Commented Dec 12, 2014 at 17:17

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.

• Good, +1 but now you need additional mass to power magnetic field generator and super-laser. And the question how much safe trip you want to have (chance of deadly collision with a rogue asteroid/comet thrown away from it's solar system) remains open. Commented Dec 11, 2014 at 21:53
• @PeterMasiar The power/fuel comes from the gas itself, so you don't need to bring more mass with you. Using the same magnetic field that deflects it from the crew area (and actually any area of the ship that you don't want completely annihilated before you get very far) you funnel it into a fusion reactor. It's almost entirely monatomic hydrogen and helium out there, so it's basically fusion fuel waiting to happen. The faster you go, the easier it is to collect the fuel. There's definitely the possibility of running into something, and it's not one that I would want to take. Commented Dec 11, 2014 at 21:59
• Project Rho has a long description of relativistic rocket strategies here: projectrho.com/public_html/rocket/slowerlight.php#id--Go_Fast. This mostly consists of more details than you ever wanted to know about the Bussard Ramjet (and various derivatives), but they also describe a concept called Valkyrie, which places an matter/antimatter rocket in front, and uses a droplet cloud for coolant and shielding. Of course, they also use a solid block of supercooled antihydrogen as part of the propellant, so... Commented Dec 12, 2014 at 4:01
• @JasonPatterson I don't think your system is going to be lossless. Are you thinking that destroying an object is going to take less energy than you will gain by scooping it up for fuel? You might be able to recover some of the energy, but most will likely be lost. Not saying it cannot work; just pointing out that you won't simply get energy for free along the way. Commented May 2, 2017 at 19:45
• You can project in a cone in front of you a magnetic field. then you ionize everything within that cone and draw it down to the center. At the tip of the magnetic cone, in front of the ship is a device that produces a powerful magnetic field. This is strong enough to alter the trajectory of atomic particles to direct them to a shaft where it is then ejected out the back at high speeds. larger objects would not be able to change its trajectory as easy and get flung aside. that's how I imagine your device would work. Commented Apr 5, 2019 at 5:05

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.

• I was wondering too! An excellent addition to the answers. It also shows you don't want an actual physical shield for protection. Want to do the energy at .2 speed too? :) Commented Dec 12, 2014 at 20:27
• Just replace all the "0.75"s with "0.2"s and type it into a calculator. I'll add it to the answer though! Commented Dec 12, 2014 at 20:47
• Let's just mention that not only the marble has 113 times more energy than a Hiroshima bomb, it also gets spread over much less mass. Commented Jun 22, 2016 at 5:50

## 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.

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.

• Good answer, but the detection of these objects seems quite difficult to me. If the ship is moving at .75 the speed of light and you are using a detection method restrained to the speed of light, are you really that capable of picking out objects coming at you in order to deflect them? By the time your detection 'beam' has found the object, how long do you really have to deflect it before you hit it? What type of range would this deflector beam need to deflect an object before you hit it, assuming the beam is also constrained by the speed of light? Commented Dec 11, 2014 at 17:13
• I prefer Star Trek solution so far, they did their homework better. Star Wars sounds better but they do not explain very much how their technology work. Dune's spice is too magical. Leaving reality-check aside, for an action storyline/movie/video game nothing can beat the Jedi Knight. Commented Dec 11, 2014 at 17:13
• @HatoruHansou: In Star Wars, they respect that you can't go the speed of light: They go super super super super fast and then phase out realspace and skip light speed. The principle of course is that you avoid the issue. Gravity wells are one cause of concern, as they tend to rip ships apart for some reason. Thereby is the C-Canon Kessel Run explanation, as it is a bunch of black holes. G-Canon just wanted to use "parsecs," I think. Commented Dec 11, 2014 at 17:20
• We could also take the Halo approach. In the Halo universe, ships achieve FTL by opening up a path in slipstream space which avoids this issue all-together. Commented Dec 11, 2014 at 20:19
• This is really comprehensive, but it isn't really science-based; it's more sci-fi-based. Commented Dec 11, 2014 at 21:30

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).

• So far I think this is the best answer, with Jason coming in a close second. Commented Dec 11, 2014 at 19:30
• I've tried expanding on the 'how many light minutes we need to deflect an object in our way'...it doesn't work badly if the object is already moving, but it's not so easy if the momentum of the object is low or near stationary as you have no momentum to work with (IE deflect) and are more in the range of trying to push it out of the way, which just takes more time. Commented Dec 11, 2014 at 19:49
• Can you use that 'deflection' on larger objects to alter your course around it? I think that would help a bit. Commented Dec 11, 2014 at 19:50
• @bowlturner - good point...using the deflector in a different direction would change your trajectory to some degree...even more so if you could bounce it off a moon and back to yourself to push yourself away again (double effect of the push as you generate and the push as it strikes you?)...but that'd be extremely difficult as far as implementation goes, I'd think directional 'thrusters' or something to that extent would make more sense...I'm struggling with deflecting 2 objects simultaneously as well. Commented Dec 11, 2014 at 20:30
• I guess I was assuming if you are sending enough force to 'move' an object in your way, shouldn't that same force be pushing back equally on you? So you'd be 'shoving off' of the large items? Commented Dec 11, 2014 at 20:33

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 :-)

• Avoid the problem altogether, great! :P Commented Dec 11, 2014 at 16:56

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

• "They're scanning us capta--" -Unidentified ball of energized goo floating in space. Commented Dec 11, 2014 at 18:55
• Are you talking about the Heisenberg's Uncertainty Principle? Commented Nov 29, 2019 at 5:50
• @thescribe No. Where did you think uncertainty applied? Commented Nov 29, 2019 at 15:38
• How such weapons work is that they study something with extreme levels of scrutiny. According to the Uncertainty Principle, studying something introduces an unknown level of energy to it. The more closely you are studying something, the more energy is added. If you study something closely enough (such as recording the exact positions of all its atoms) the excess energy will make it explode. As @Crabgor put it, " 'They're scanning us capta--' -Unidentified ball of energized goo floating in space." Commented Nov 29, 2019 at 23:55

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.

• So if a piece of space junk blows up your egg timer, you stay floating forever? Commented Dec 11, 2014 at 23:55
• No, as I remember it, there are 3 devices outside of the stasis field: egg-timer, accelerometer and field-generator. The timer isn't to turn the field off. It provides a positive current to keep the field up until the time runs out. The accelerometer works the same way, providing current until the ship stops moving. Both are needed, along with a fully functional field generator, to keep the stasis field up. If any of the three gets destroyed, the field fades away. So, if the ship is stopped or the time runs out, or anything breaks, the field turns off. It is a deliberately flawed system. Commented Dec 12, 2014 at 0:39
• But if that system fails, you are now at high speed with no shielding. Commented Dec 12, 2014 at 0:42
• yep... SPLAT! That's why pilots always go to sleep with their foot above the break petal! Commented Dec 12, 2014 at 0:48

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.

• to get to Vega (27 ly) and stop there, we need 57kg of fuel for every kg of payload, using 100% effective engine.
• 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.
• Is an answer to a question asking how to detect objects while travelling at .75 the speed of light as "we can't go that fast" really valid? Commented Dec 11, 2014 at 19:29
• Is any attempt to answer question "how I can design a bicycle which will allow fish to colonize moon within limits of physics" valid? Commented Dec 11, 2014 at 19:35
• I'm assuming that type of straw man silliness would be closed. The assumption that we can make it to that speed is stated in the question, if you want to challenge an assumption of the question that invalidates the question, I'd suggest that's what the comments section is for instead of posting two separate answers that really come down to the same 'just don't travel at .75% the speed of light' arguement Commented Dec 11, 2014 at 19:52
• Because of relativity it is exactly as hard to shoot bullets forward at 75% light speed as at 0%. They won't hit the target much sooner than you will, but they'll definitely go forward, and you'll see them moving as fast away from as you would as if you were at rest. You also copied the wrong line for Vega, you didn't use a 0.75c maximum speed, the link you found didn't indicate 99.999% of the energy. The idea of this answer is good, but the quality is very low. Commented Dec 11, 2014 at 23:32
• Not all ships need to carry their fuel with them. Although I'll definitely agree that traveling that fast is definitely outside of current technology, it is not outside of current theory. For example, if one were able to focus a laser tight enough, one could shine the laser from a station in a tight orbit around a star that draws its power from star light converted to electricity. (This is just one of many such possibilities.) Commented Dec 12, 2014 at 0:09

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.

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.

• An interstellar ship would be indistinguishable from a comet. Commented Dec 12, 2014 at 7:20
• ... And you need to accelerate all that ice to 0.7c. Lots of fuel! Commented Dec 12, 2014 at 20:55
• @PeterMasiar: Yep. If remember correctly though that was not a problem since the engines got their energy from another dimension or something like that. Can't remember the exact details, but fuel was infinite.
– Leo
Commented Dec 12, 2014 at 23:08
• Ah, magic, it just melts all engineering problems away! Commented Dec 12, 2014 at 23:14

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."

• That sounds kind of like a snow plow. But the snow doesn't shed off the sides. Commented Dec 11, 2014 at 20:41
• What he's describing is called a Bussard Ramjet. In reality, a Ramjet would the stuff to the center of the vessel, at which point they would be shot out the back. Commented Nov 29, 2019 at 5:53

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.

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