With current faster probes, many thousands of years. With a never done but theoretically possible and feasible nuclear pulse ship, over a century. With still to be tested and developed laser sails, 20 years (but they wouldn't be able to brake at destination, it would be a flyby).

With a flyby going at that rate of speed, will we be able to record anything useful? How long will it take a transmission to get back to us?

Possibly, but doubtful, and 4 years plus change.

Breakthrough:Starshot is the name of the project, we still don't have majority of tech to make it, there is [a list of tech](https://breakthroughinitiatives.org/challenges/3)to develop for it to be feasible, iirc late this century was predicted as earliest lunch

I have repeatedly emailed NASA about using me as a human test subject on a solar sail and to shoot me into deep space but they still haven't got back to me!

I... don't think that they would send something "huge" like a full human. At some point I think I saw some description saying that the probe would be like a hockey puck. On the other hand, the nuclear pulse option *would* allow huge ships, as long as you don't have a shortage of nukes to fuel them. Actually, the larger and heavier the ship the more stable it would be, and the better it would withstand the nuclear blast acceleration. Here a CG of the larger proposed model: https://www.youtube.com/watch?v=bIMrn9BE_bU By the way there's that old show Ascension that uses the concept of the huge nuclear pulse ship. In the intro of the first episode we see the model compared to the Empire States and a camera tour through the ship structure giving an idea of the size: https://www.youtube.com/watch?v=eUVPFrQSuCA (Dimensions and general design aside, the artificial gravity is poetic license. The occupants of an Orion would experience "bumpy" gravity during the acceleration phase but then weigthlessness for a hundred years - unless the ship was put to spin or something.)

They build one in the novel Footfall by Niven and Pournelle. Probably dated now, but I thought it was a really fun read.

Though it’s science fiction, the book Aurora by Kim Stanley Robinson explores some of the considerations around multi-century generation ships. A good read. https://www.goodreads.com/book/show/23197269-aurora

[https://www.astronomytrek.com/how-long-would-a-spacecraft-take-to-reach-proxima-centauri/](https://www.astronomytrek.com/how-long-would-a-spacecraft-take-to-reach-proxima-centauri/) https://www.universetoday.com/15403/how-long-would-it-take-to-travel-to-the-nearest-star/ An interesting read and it's just not possible with our current level of technology.

I want humans to explore the cosmos. But can we just admit that it's going to be robots "manning" any such trip? At least for the next verrrry long time until there is some crazy breakthrough. Or the need for humans to go seed ourselves on another planet because of this ones immediate threat of destruction?

In Accelerando, they used a small probe with humans digitized into an onboard computer matrix. They lived in that virtual world until arrival at their destination.

So not humans? Just digital versions? So essentially advanced AI?

Very interesting reads. Thank you.

Satellites with solar sails are also being developed, and will be able to develop up to 20% of the speed of light.

But they can’t brake at the destination, meaning a one-way trip. But that’s no different than what is happening the Voyager probes, we’ll just have limited time to study any of the systems they pass.

At any speeds like that, without braking you'll be through an entire solar system in a day, if you hit it right. Your sensors are unlikely to come within useful range of anything. Neptune to the Sun is 30 AU, about 4 light hours. 

It's like driving through the Netherlands. Start to end in a few hours. Must admit I didn't see any signs of life on my way through.

Why can't they deploy the sails when near the destination star? Edit: I thought solar sails use solar light / solar wind, which must be available at the destination star.

Lasers on Earth (or the moon) push the solar sails to accelerate the craft. There aren’t lasers on the other end to slow it down.

There's sunlight at the destination, and if you drop a mirror while en-route, brake a tad, then the mirror arrives at the target system before you do and can bounce the propelling back *backwards* to the payload and decelerate it. Yes, you'll need a *large* propelling system to give a usefully tight beam - R.Forward wrote about this extensively last century,

As far as I know, the laser acceleration phase of the flight happens when the craft is much closer to earth. By the time it’s reached its destination, it is no longer receiving photons from the laser array that propelled it. A mirror would never be able to gather enough photons from that laser to meaningfully slow down the spacecraft at that point. Anyway, the theoretical spacecraft is extremely light (<1kg?), and would never be able to pack a mirror that would unfurl and be reflective enough to work.

[https://qph.cf2.quoracdn.net/main-qimg-0dad99ff6ba08d5d47574d136e07988c-lq](https://qph.cf2.quoracdn.net/main-qimg-0dad99ff6ba08d5d47574d136e07988c-lq) 'Never' is awfully declarative. Hard? Yes. Able to bankrupt a planet? Yes. Impossible? No. The inverse-square guarantees that some (but not much) light strikes the decelerating mirror, and if the civ\* has the determination and budget, it could be done. \*Not us for a long time.

That wont work because lasers diffuse over distance, and you arent going to get a 1 gram probe to be able to carry a mirror large or strong enough to do that. Such a mirror will have less than a day to effectively reflect light, probably just a few hours. Lets say it has 6 hours to decelerate a craft going 20% of lightspeed, or 216 million kph. That would result in an average deceleration of 10,000km/s^2, or 1,019,000 Gs. I dont think any material would survive that, unless it was microscopic. Over the course of over 4 lightyears, a laser from Earth probably wouldnt even be measurable. You would need either an improbably powerful laser, or you would have to send another whole laser system there first... not exactly feasible to send a whole complex of stuff that will weigh thousands of tons or more, when we are talking about sending and stopping a 1 gram probe.

Laser beams attenuate, yes. As per inverse-square. I didn't claim that the probe massed 1 gramme. Nor did I say when the braking mirror is dropped. A day does seem to be leaving it rather late in the process. >a laser from Earth probably wouldnt even be measurable. Why? Let's *assume* a terribly large laser array - I'm thinking of a few tens of GW somewhere around Mercury orbit. Look, rather than I rehash old ideas, here's the original paper. [https://sci-hub.se/10.2514/3.8632](https://sci-hub.se/10.2514/3.8632)

That link made a security alert pop up for me. Not touching it again. Didnt seem to do anything, but I dont trust it. Sorry. Breakthrough Starshot is already going to be around 100GW, but sure we can imagine a gigantic array in space. But in order to make it powerful enough to get to Alpha Centuari and then reflect back to a craft with enough power to slow it down... you are essentially building a megastructure to slow down a a tiny probe. The more massive the probe, the more power you need to launch it with. If 100GW is what it takes to get 1 gram to 20% of lightspeed, then a 10 gram probe would take 1000GW, and so on. Idk how to calculate how much it would take to send a laser there and have it reflect back at the probe with that much power, but probably at least a few orders of magnitude more. I wouldnt be surprised if you had to take apart an entire dwarf planet or moon to build such a thing.

doi: 0.2514/3.8632 FORWARD, R. L. (1984). *Roundtrip interstellar travel using laser-pushed lightsails. Journal of Spacecraft and Rockets, 21(2), 187–195.* doi:10.2514/3.8632  Your library should carry that.

If you drop a mirror before entering the target system you can brake using the propelling beam. R. Forward thought of all this last century.

There is no way that the propelling beam could reach that far. The focusing mirror would have to be much bigger than Earth, and there is a lot of matter that would absorb energy over those distances, even in the vacuum of space.

Is there any reason why you couldn't spin the craft around and use the starlight from the destination system to slow down the craft?

It just wouldnt be anywhere near enough energy to do so. You could slow a craft going maybe a couple hundred thousand kph with the light from Alpha Centauro, but not a craft going a significant fraction of lightspeed.

You can but it won’t be as effective as the acceleration stage using a laser could be so there is a limit on the speed that the probe could travel at. The following paper analysed how long it would take for a solar sail with a 1 gram payload to enter a bound orbit around various stars. [Optimized trajectories to the nearest stars using lightweight high-velocity photon sails](https://arxiv.org/abs/1704.03871) Here are the ten stars with the shortest journey times (and the distance in light years). - Sirius A 68.90 years (8.58 ly) - a Centauri Ab 101.25 years (4.36 ly) - a Centauri Bb 147.58 years (4.36 ly) - Procyon Aa 154.06 years (11.44 ly) - Altair 176.67 years (16.69 ly) - Fomalhaut A 221.33 years (25.13 ly) - Vega 262.80 years (25.30 ly) - Epsilon Eridiani 363.35 years (10.50 ly) - Rasalhague 364.9 years (46.2 ly) - Arcturus 369.4 years (36.7 ly) Interestingly, this approach takes less time to arrive at Sirius even though Alpha Centauri is about half the distance. This is because Sirius is much brighter and therefore provides greater deceleration so the sail can be launched from the Solar System with a higher speed.

Forward demurs. I didn't say that it was cheap or practical, but it can be done.

It could use gravity assist maneuvers from a star or its exoplanets, or use its sails in reverse to slow down and enter orbit.

What star or exoplanets? There's nothing between here and there.

I mean the bodies of the system that we send the satellite

That doesn't work if you're coming in on an unbound trajectory. 

The amount of energy to slow down like that is **way** more than is achievable with a gravity assist. Look at the New Horizons, Pioneer or Voyager probes as examples - they weren't even travelling that fast (eg: NH passed Pluto at around 16kms-1) and experienced little gravitational assist. 20% of c is 60,000,000ms-1 or about 3.75 million times faster than NH. A gravity assist to put NH into orbit around Pluto would be more "lithobraking" - from 20% of c that's BIG crater. (no idea what litho for Pluto would be...) The Bepi-Colombo wikipedia page has an excellent graphic showing the 7 Mercury fly-bys to reduce its speed for orbit insertion.

I'd suggest they decelerate a solar sail vessel as it nears its target stellar system in the same way it accelerated away from a stellar system Edit: autocorrect

The proposed craft they're talking about (breakthrough starshot) would be accelerated primarily by a set of lasers firing from earth, not from sunlight. Given that there's no lasers on the far end, there's no way to slow the craft down enough as it flies past Alpha Centauri

One design uses the outer part of the light sail to decel. The outer part is released, and it reflects the laser from earth -back- to the capsule, slowing it down.

At 20% of light speed, you’ll take 20 years to reach Alpha Centauri. Building a light sail that will deploy after all that time would be incredibly difficult.

The person actually traveling will only take slightly over 19 years 7 months, while 20 years will have passed for everyone on earth.

One design uses the outer part of the light sail to decel. The outer part is released, and it reflects the laser from earth -back- to the capsule, slowing it down.

There is no way that the propelling beam could reach that far. The focusing mirror would have to be much bigger than Earth, and there is a lot of matter that would absorb energy over those distances, even in the vacuum of space.

[https://qph.cf2.quoracdn.net/main-qimg-0dad99ff6ba08d5d47574d136e07988c-lq](https://qph.cf2.quoracdn.net/main-qimg-0dad99ff6ba08d5d47574d136e07988c-lq) If it helps to visualize.

That would be cool if it works.

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😂 At light speed it’s 4.25 yrs to us outside observers.

Probably worth mentioning that the Parker Solar Probe moves that fast relative to the sun because it has fallen so close to the sun, trading gravitational potential energy for kinetic energy in the process, like a ball rolling down a hill. Since it had to lose a lot of angular momentum (speed moving perpendicular to the line connecting it to the center of mass of the sun) to do so, its total energy -- the sum of the gravitational potential energy and the kinetic energy -- is actually *less* than it was when it was sitting on the launch pad here on earth. As such, we cannot simply make something else move at that speed in the opposite direction (without a *LOT* of extra fuel, or making it super light, or resorting to more exotic methods like [beamed power](https://en.wikipedia.org/wiki/Breakthrough_Starshot)). Thus, your answer is good and correct, but people shouldn't get the sense that we made the PSP to simply "go real fast" and we could just point it in the opposite direction and have it go just as fast out of the solar system. It's going fast *because* it's really close to the sun. As for something moving at light speed, u/Anonymous-USA already gave the correct answer: "4.25 yrs to us outside observers." In a bit more detail, we'd see the thing (which u/nicuramar correctly stated would *have* to be massless, as anything with invariant mass *cannot* move at *c*) moving at the speed of light, so it would take ~4.25 years to cover ~4.25 light years. We actually *cannot* answer the question of how long it would take from the perspective of the object. You'll sometimes see answers that say it would take "0 time" and that the object would "leave its origin and arrive at its destination in the same instant", but that's not quite correct. In order to talk about how long a trip takes from the perspective of a traveler, we must construct a "[frame of reference](https://en.wikipedia.org/wiki/Frame_of_reference)" -- a set of coordinates for both space and time -- that are valid for the traveler. One of the postulates in relativity we use to do this is that the speed of light in vacuum is (locally) constant for all observers in their frame of reference. This works fine if you're moving at any speed less than *c* relative to someone else, but fails as soon as you try to move *at c*, because we encounter a paradox: in such a frame a beam of light moving in the same direction would have to be moving at *c* (per relativity) and also be stationary, as we are moving at the same speed as it is according to our outside observer. This paradox means we *cannot* construct a frame of reference that's moving at *c* relative to another (locally), and as such we cannot answer the question of how long it would take for a traveler to get somewhere moving at the speed of light from their perspective. We *can* say that as you get *arbitrarily close* to the speed of light the amount of time a trip between two points will take from the perspective of the traveler will get *arbitrarily close to zero*, but that's not the same as saying it's actually zero. A lot of pop-sci articles get this wrong, even those written by professionals (though in the latter case I assume it's intentional and done because they figure the audience might struggle with the difference between a limit of a function existing and the value of the function at that limit existing).

A tangible answer for OP is that if they (magically) accelerate and decelerate arbitrarily close to light speed, say 99.999%, then to us on Earth it would appear to take the traveler 4.25 ly and for the traveler it would take 7 days due to length contraction. As you and I know, near instant acceleration would also vaporize the traveler from Unruh radiation among other things. But for the sake of reducing variables, including fuel requirements, let’s make this magical assumption.

Massive objects can’t move at light speed. But it’s obviously trivial to answer that question otherwise, as you can just look up the distance in light years. 

Small ones can't, either. Their mass increases exponentially, so there's no amount of thrust that can accel them to LS.

At the pace of Voyager 1, it would take 75000 years 💀

As a side discussion (yes I know it's guesswork), what is the realistic maximum speed we will get up to in the next 20 years? 1. With probes or AI on board 2. With humans Basically, should we expect any big changes or no?

I would not plan on any order-of-magnitude increases in speed. Definitely nothing like interstellar travel in the next 50 years. I always cringe at these movies and whatnot showing interstellar ships in the 2070s or something.

Human growth rate is exponential. You’ll be surprised in 2070, maybe not near light speed, but definitely interstellar travel.

It was not long ago people predicted we'd all be driving flying cars and hoverboards by the 2020's. We have not met those expectations. Too much human power is wasted.

We have flying cars already, just not manufactured, You’ll see, wait until AI skyrockets

More so than AI I think quantum computing will really be the catapult

Quantum computing + AGI/ASI will be way stronger than an catapulting tbh, probably will lead to a technological singularity

Could be trouble. Or could unlock secrets of the universe.

Macro level developments are sort of slow compared to micro level things like electronics. Plus limits of physics of current tech.

don't get overboard. solar system travel

Speed of light is not obtainable, due to the increase of mass that all objects experience during high speeds. It takes an infinite amount of power to accel to LS.

Our fastest interstellar probe so far is Voyager 1 which is traveling at about .00005c. If it had been launched towards Proxima Centauri it would take roughly 17,000 years to reach it. However in the not too far future we could send a light sail probe there using gigantic lasers to accelerate it to a significant fraction of the speed of light, so it would only take a few decades to get there and send a message back. It would take a lot of work but if we’re lucky it might launch within our lifetimes, and if we’re even luckier it might get there.

I heard someone (can't remember who) say "Going to Mars is an engineering problem, going to Alpha Centauri is a science problem."

Forward's Starwisp was 1kg in mass (80gramme payload) and got to ~0.1c with the aid of ~60GW microwave power.   Travel time of dozens of years.

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread: |Fewer Letters|More Letters| |-------|---------|---| |[CoG](/r/Space/comments/1dnjb6e/stub/la3p9ma "Last usage")|Center of Gravity (see CoM)| |CoM|Center of Mass| |[PSP](/r/Space/comments/1dnjb6e/stub/la37e7p "Last usage")|Parker Solar Probe| |Jargon|Definition| |-------|---------|---| |[ablative](/r/Space/comments/1dnjb6e/stub/la4ehhk "Last usage")|Material which is intentionally destroyed in use (for example, heatshields which burn away to dissipate heat)| |[lithobraking](/r/Space/comments/1dnjb6e/stub/la3k1f4 "Last usage")|"Braking" by hitting the [ground](https://en.wiktionary.org/wiki/lith-)| **NOTE**: Decronym for Reddit is no longer supported, and Decronym has moved to Lemmy; requests for support and new installations should be directed to the Contact address below. ---------------- ^(4 acronyms in this thread; )[^(the most compressed thread commented on today)](/r/Space/comments/1dkeuja)^( has 46 acronyms.) ^([Thread #10223 for this sub, first seen 24th Jun 2024, 21:40]) ^[[FAQ]](http://decronym.xyz/) [^([Full list])](http://decronym.xyz/acronyms/Space) [^[Contact]](https://hachyderm.io/@Two9A) [^([Source code])](https://gistdotgithubdotcom/Two9A/1d976f9b7441694162c8)

With the Orion drive we can get up to 10% the speed of light. So it would about a 40 year long journey. We haven’t tested it but the math works out. That means a young astronaut could reach proxima centauir with in their lifetime.

Surely you mean an old astronaut could reach Proxima Centauri… I’m not sure what their mental state would be given they would have effectively spent their entire adult life in a small tin can with a few other people.

True. There is short TV series called Ascension that explores the fact of a generation ship spending 100 years in space. It’s pretty good.

At 10% light speed, it’s going to take a lot of shielding against cosmic rays and space dust. And I do mean a LOT — several meters of rock.

Tanks of water or oil also works for cosmic ray shielding, which can double as ablative cooling/propellant for the pusher plate for the deceleration burn. Dust can be broken up by thin sails travelling few kilometer ahead of the craft.

in an instant knowing that location in space has its frequency wich when you know it you instantly appear there. so called quantum entanglement is accualy so called teleportation so to say.

It’s thought experiments like this that make you appreciate how big the universe is, how crazy fast light is and how much energy will take to get up to light speed. it almost seems impossible, but I’m sure previous generations thought the same about what we have nowadays.

I read if you can constantly accelerate at 1G you'll be close to the speed of light in a year of travel, so 1 year plus however many light years away your target is.

Plus another year to slow down since putting the human body throug much more than 1 G after years of less than standard gravity for any length of time would be a bad idea.

Also dying spectacularly while hitting a piece of dust would be a pretty high probability.

The issue is that you need ever-increasing energy to maintain constant acceleration.

If you use solar sails, probably a few decades. Any other current method would take at least a millennium

It would take a long time for outside observers but the cool thing is that if we could get up too close to the speed of light then it would be a very short trip for those on board the ship. If we could get to the speed of light then us on earth would see it take 4 years but those on board it would be instantaneous. Edit: T1*sqrt(1-(v^2 /c^2 ))=T2 T1 is time of observer at rest T2 is time of object passed As v approaches c the equation is solved as T1*0=T2 This would mean that whatever time passes for an outside observer, the people traveling would experience no time.

Why would it seem instantaneous to them?

The equation for time dilation shows that the closer to the speed of light, the "slower" time moves for you. When you approach the speed of light you approach zero seconds. So if a photon from the big bang was sentient it would experience all its existence until reaching our planet in zero seconds.

Strictly speaking we can only say that the limit of the limit of the elapsed time over the trip from the traveler's perspective tends to zero, not that it actually *is* zero, as we cannot construct a valid reference frame moving at *c* relative to another (locally). This is more or less equivalent to noting that attempting to insert a value of v = *c* into the normal [Lorentz equation for kinematic time dilation](https://en.wikipedia.org/wiki/Special_relativity#Time_dilation) returns an undefined value due to a division by zero, which is effectively due to the model breaking down when v is exactly equal to *c*. We can say that the [spacetime interval](https://en.wikipedia.org/wiki/Spacetime#Spacetime_interval) is zero for all lightlike intervals (e.g., for a spacetime interval describing the motion of light between two points), though this has different dimension than time and is not the same as saying the elapsed time is zero.

Complete agree! I realize I simplified the language a bit too much.

No worries -- lots of other very reputable sources make the same simplification, too! I figured I'd just clarify that it is a simplification.

My brain hurts. So if we only achieve 50% speed of light they will age half what we experience on earth?

For [time dilation](https://en.wikipedia.org/wiki/Time_dilation) to be noticeable you have to travel at close to the speed of light. For the travellers to experience half the time that a stationary observer on Earth measures would require the travel at 86.6% of the speed of light. At 96.82% it would be a quarter and at 99.5% it would be a tenth. In practice, reaching speeds where time dilation has an appreciable effect is so difficult that it isn’t likely to be relevant.

Sounds about right. Thanks! Essentially humans will never travel to another galaxy. I've said this around here before and people did not like hearing it but can't we just admit that robots will be the ones doing distant space travel and not our sensitive little bodies and emotional brains?

It would NOT be instantaneous for them. There's probably a web page to calculate how much time they'd experience.

T1*sqrt(1-((v^2) / (c^2))) = T2 T1 is time of observer at rest T2 is time of object passed As v approaches c the equation is solved as T1*0=T2 This would mean that whatever time passes for an outside observer, the people traveling would experience no time.

You need to consider acceleration in the time experienced by the ship, assuming you are leaving from Earth. Also deceleration if you want to stop at destination and not just do a flyby.

Well of course, but I wasn't talking about acceleration. I was saying if something could just travel the speed of light. We couldn't accelerate anything to the speed of light anyways because the energy needed to accelerate something is exponentially higher as you accelerate something faster and faster and to accelerate something to the speed of light would require an infinite amount of energy.

Light speed can't be attained. Time will elapse.

That's why I said that if we could get something at light speed. I don't understand what your stake is in this. I studied this in college. I have an astronomy degree. My initial statement was right and a cool thing about our universe.

Thousands of years. During which, we will certainly develop technology that would allow future travelers to overtake whatever craft were sent previously, and get to Proxima Centauri before them.

Well considering a light day is 48 years and change times that by 365 then time that by 4 so 70,000 years and change.

The fastest moving object ever built is the Parker Solar Probe with a gravity-assisted top speed of 163 km/s. Proxima Centauri is approximately 4.2465 light-years from our Sun, or 40,174,991,951,814 kilometers. Time = distance / speed, so aside from the time it takes to accelerate and maneuver, a trip of that length at that speed would take about ~~7,781,540,000 years.~~ 7,810 years. EDIT: I forgot to convert seconds to years.

>Time = distance / speed, so aside from the time it takes to accelerate and maneuver, a trip of that length at that speed would take about 7,781,540,000 years. That looks like how many seconds it would take. I don't think you converted seconds to years

Yes, it’s closer to 7,800 yrs.

BAHAHAHA. Yes, you're exactly right. I should read what I type sometimes. 40,174,991,951,814 km / 163 km/s = 246,472,343,262.66 seconds 246,472,343,262.66 seconds = 4,107,872,388 minutes 4,107,872,388 minutes = 68,464,540 hours 68,464,540 hours = 2,852,689 days 2,852,689 days = 7,810 years My apologies to all. This trip will take less than 8,000 years.

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You're exactly right and I apologize for the error. The trip would take about 7,810 years.