# Interstellar probe designed without FTL?

Thanks to enhanced space telescopy Earth has found some great candidate star systems for colonisation. But they are not yet ready to try sending people.

There's no new physics. However, the cost of getting into space has been substantially reduced from today's costs, let's say 0.5 percent current costs.

The world federation of space exploration has offered a prize to the best solution for the development and deployment of an interstellar probe. It will need to travel around 20 lightyears, but shall then send information back via radio comms of some sort.

There are two prizes available.

• Prize for orbital probe
• Prize for a lander

The lander has the option to be a seeder.

What did the winning entries look like?

• Check this: en.wikipedia.org/wiki/Breakthrough_Initiatives Specifically Starshot seems to fit. – Mormacil Apr 1 '17 at 10:36
• Thanks for the link @Mormacil. They seem to be concentrating on remote powered lightsails, expecting a transit of about 20 years. There isn't mention of how they intend to slow the system down at the other end though. However, I like the idea of StarChip too. – Konchog Apr 1 '17 at 14:43
• I don't think they expect to slow down at all. There's some more information out there not summarized on Wikipedia. – Mormacil Apr 1 '17 at 14:58
• Konchog, @Mormacil You may find this of value "Astronomers Have a Plan to Slow Down a Spacecraft When it Gets to Alpha Centauri" - popularmechanics.com/space/deep-space/a25001/… - This same system could be used to park it at the planet as well as make pass by's of any detectable object on the way in. – Enigma Maitreya Apr 1 '17 at 18:34

Project Orion

Your spaceship is a big disk that carries all the satellites and landers you need (the hard part is getting it into space, but, once there, it works easily). It also has antennas for talking to Earth and with all the probes that it will detach to explore the star system.

As for getting to the star, it tosses out large bombs behind it (anti-matter would be best, if it is attainable, but nuclear bombs would still work) and rides the shockwave, with using a pusher plate to distribute the acceleration over a longer period of time.

Once it arrives at the star, it parks in orbit around the star, and all the probes with science equipment detach and go to wherever.

The orbital ones all use ion engines so they can get very high efficiency, and then have whatever science instruments attached to them, and they transmit back to the original ship. Some will actually hop from one location to another (like Dawn), others just park in orbit around an object and stay there.

The landers are designed for the environment, with ones on air-less bodies using propulsive landing and ones on bodies with an atmosphere using heat shields and parachutes. From there, they use whatever science instruments and transmit the data back, either directly to the mothership or to an orbital craft around its planet. Rovers are out of the picture unless we have good AI because it would take 60 years after landing until we get our first science data out of them.

Furthermore, most of the landers will land 40 or more years after everything else gets there because scientists at home look at the data and decide where to have the landers land. Planets are big, and we don't want to risk them landing in a bad spot when we could have gotten a lot more science out of them.

This was a serious thing being considered back in the earlier parts of the space race. See https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) In 1968, they found that they could get to Alpha Centauri in only 133 years, even with 50,000 tons of payload, but I'm sure people can do better with more advanced technology, so it would take only a couple centuries.

EDIT: In response to comments, the following should be noted:

1. Efficiency of nuclear bombs has not increased, so that part would not be improved without the use of future technologies, such as Antimatter-catalyzed Nuclear Pulse Propulsion or use of Muon-Catalyzed Fusion in fusion bombs instead of Uranium or Plutonium (but neither of those would be considered near-future tech).

2. A bomb using pure antimatter is likely not going to happen because everything annihilates into gamma rays in the end, but it is still possible that (with far-future tech) we could find a way to harness the energy, but there does not seem to be any research into how that might be done.

3. The increase in efficiency from then would be (assuming modern or near-future tech) just advances in materials and manufacturing from then, because the original study used only current or what was then considered to be near-future tech (which we have exceeded).

• Interesting. Orion led to a wikicrack, which introduced me to successor technologies, such as HiPEP, NTR, VASIMR, and MTF/ICF - all of them offering some sort of capability for interstellar transits of around 100 years to AC. – Konchog Apr 1 '17 at 14:41
• "I'm sure people can do better with more advanced technology" When it comes to space travel, change of velocity (delta-v) and rate of change of velocity (accelleration) means everything. To be able to lift off a planet, another important aspect is the thrust to weight ratio. It's not as simple as "more recent technology is better"; the F-1 engines on the Saturn V first stage (mid-1960s design) probably still hold the record as the most powerful rocket engines built to date. Compare e.g. worldbuilding.stackexchange.com/a/64349/29 and space.stackexchange.com/q/13763/415. – a CVn Apr 1 '17 at 14:49
• antimatter explosives wouldn't work, if you're using antimatter it has to be anti protons as they're the only ones that can be contained and are stable, but when you have a proton anti proton collision they emit two gamma rays, which wouldn't be a shockwave, and would have basically zero thermal output – Alex Robinson Apr 1 '17 at 15:23
• @MichaelKjörling I was referring to greater mass efficiency on the explosives (especially if anti-matter bombs are allowed) – Jarred Allen Apr 1 '17 at 15:25
• @JarredAllen I study physics, i know antimatter exists, but you didnt get the gist of what i'm saying: an antimatter bomb would emit high energy gamma radiation, which can pass clean through 12 feet of concrete, so an antimatter BOMB wouldn't help you propel your probe – Alex Robinson Apr 1 '17 at 15:30

Icarus Interstellar, a modern non-profit with the stated aim of interstellar flight by 2100, would likely have an entry. They grew out of the five-year Project Icarus study run by the British Interplanetary Society and Tau Zero Foundation, which was itself a successor to Project Daedalus, which designed an interstellar ship that was a variation on Project Orion.

As of 2014, their design was called the Firefly Starship, and a summary of the design specs can be found on Project Rho. It uses D-D fusion in z-pinch confinement for efficiency and fuel cost, and distance shielding to carry live cargo without crazy amounts of weight. Most of the mass comes from heat radiators. It cruises at 4.5 percent of c, and might make a good choice for your seeding mission.

If you want something different, though, Project Rho is a great resource in general and has three pages of realistic designs, any of which should fit fairly well in hard sci-fi.

For something faster but somewhat less realistic, check out their torchship page for more... interesting stuff. The Zubrin nuclear salt-water rocket is probably my favorite OMGWTF slightly realistic torchship.

• Sorry @Hat, you say that Firefly cruises at 4.5c and is hard science? – Konchog Apr 1 '17 at 16:33
• @Konchog That was their design target. I haven't read the full paper (it costs five pounds, and I lack the patience/skills to fully digest it) but if you look at the cover image, they target a speed greater than 4.3c in the second paragraph under 'The Case for Fusion' section. I am decidedly a layman. However, going by what I know about the ship and the foundation, I'm confident they've targeted something like 4.5c, and this definitely seems like hard-ish science. – Hat Apr 1 '17 at 16:48
• @Konchog Sorry, i stepped away from my computer and instantly realized I'd forgotten the % sign. >.< That's .045c. I'll edit my answer to better reflect reality. :P – Hat Apr 1 '17 at 17:39

theres a realatively easy answer here, and seeing as you have a 1/2 cost multiplier lets get going with costs!

Problem: 20 lightyears

Even if we hit an impressive 1% of c(speed of light in a vacuum) it would take 2,000 years, and being a human we want to claim this prize in our life time, so lets assume people fresh out of uni at age 22, who live to 82... lets assume 10 years building time, leaving us 30 years to travel that distance (and 20 years for the information to get back), so we need to design something to travel at 2/3 of the speed of light

**Solution **

We need powerful lasers, the most powerful laser is MIRACL, or Mid-Infrared Advanced Chemical Laser, which can operate at 1 megawatt for 70 seconds, which is all we need from the laser front, because it's going to accelerate a probe using the momentum of light!

You see, Einstein didn't say E = mc^2, its just a simplified version that works for objects moving at speeds less than about half a percent of the speed of light, you can rearrange that equation to show you the momentum of light:

Why is this relevent? Our probes are going to be the size of pennies, and are going to use tiny solar sails. Oh, and there are going to be millions of them:

Now, current Earth technology estimates that we can get this probe upto about 1/5 of the speed of light, which isn't bad for something not developed yet. BUT remember the 1/2 cost multiplier, and we are going to up the budget. So given that, it's reasonable to assume that we can get this speed up to 2/3 of the speed of light, we just need a lot of lasers and the initial capital, but other than that, you have your 2/3 of the speed of light probe!

This isn't going to win the landing prize, but it really should win the orbital probe prize, as the closer it gets to the star we aim at, the more it will brake because of the solar sail, then you have millions of probes in relatively close proximity to a star that can take lots of measurements to retrieve and send back data.

• This is a great answer @Cursed1701, but I don't think it's complete yet. There's a low cost to orbit (as mentioned in the OP), so is there some way that we can use solar-powered lasers, or are you suggesting we uplift all the deuterium fluoride? Likewise, how are we shielding the micro-probes from the laser? Won't they be incinerated? – Konchog Apr 1 '17 at 15:40
• The lasers are ground based theguardian.com/science/2016/apr/12/… watch that video for an explanation: beyond that, i'm not sure on the specifics as i don't think the technical specification really exists yet, but based on my timeline, we have 10 years to refine technology and still claim the prize – Alex Robinson Apr 1 '17 at 15:42
• we would suffer terribly from thermal blooming if we fired such powerful lasers from the surface. The probes would definitely need to be shielded against such a blast also. Could we not just go for less energy over a longer time? As mentioned below, wouldn't we get far better results boosting the probe(s) first with some sort of disposable HiPep booster? Since we have millions of lightsails, and great telescopy, would we need to radio messages back? Could we message via the telescopes? – Konchog Apr 1 '17 at 15:59
• "could we message via the telescopes" "would we need to radio messages back" YES, your probes are 20 light years away, analogue radio is the most reliable way of transmitting data, ones and zeros, it need to travel 20 light years, i dont know what your telescopes are measuring but they probably wont work – Alex Robinson Apr 1 '17 at 16:38

It's not practical without FTL, it would just take too long... simple as that.

This Link could be wildy inaccurate at 37,200 years to travel one light year, but even 1% of that would make it unfeasible.

If the expanding universe theory is correct then by the time the probe arrived there at whatever speed after however many millenia, the expansion means the stars moved even further. I'm not convinced the probe could even move faster than the expansion with present capabilities so it's not a relatively simple point to point journey, it might never get there, it might just chase the star forever.

All my research (admittedly just googling) has the universe expanding at about 67 km/s per megaparsec. But we have no way of really knowing how accurate that is either. Pretty damn fast anyway.

• That's only 0.4 m/s over 20ly. – ths Apr 1 '17 at 14:30
• ths is correct. There are far better estimates, @Kilisi. Most current engine proposals offer about 100 years for a 4LY jaunt. – Konchog Apr 1 '17 at 14:41
• This makes the assumption that you want to keep the velocity required to stay in orbit around Earth, which is flawed because you don't have that restraint in this problem – Alex Robinson Apr 1 '17 at 15:26
• @Cursed1701 no I'm not positing any restraints at all, it doesn't matter how fast you go with conventional technology, it's still going to take too long. Because even at 20,000 km per hour, the universe is still expanding faster than your craft. So the target star is moving away faster than your craft. It's not a stationary object. – Kilisi Apr 1 '17 at 20:15
• @Konchog so 500 years then? That's too long, plus the star is moving away from you at 67 km/s – Kilisi Apr 1 '17 at 20:22

you want to orbit or even land. This is your biggest hurdle.

Even though you might be able to accelerate up to the speed needed to travel the 20 light years in a reasonable number of millennia, you need then to slow down to orbit a planet, or even more to land on it and not simply fly by or, even worse, crash on it.

Since carrying along all the fuel needed for slowing down is practically impossible, as the rocket equation clearly states,

$\Delta v = v_\text{exhaust} \ln \frac{m_\text{initial}}{m_\text{final}}$

your only hope is to build a solar sail.

1. You use it to accelerate and leave the Sun, using solar wind plus Earth powered laser
2. You fold it during the trip, to prevent damages due to micrometeorites
3. You unfold it when getting close to the star, using the other star wind to slow down your speed.
4. you use the sail as collector of a big paraboloid to communicate one way with homeland (supposing a civilization able to listen still exists)
• @L_Dutch, similar to Cursed1701's analysis? If one can use lightsails to slow down at the other end, why is it necessary not to use an additional accelerator at the home end? Could we not push the probe(s) with a disposable HiPEP booster before deploying the sails to give them additional laser acceleration? – Konchog Apr 1 '17 at 15:52
• @Konchog, I am not going for that high speed (at least I think...) and also for slightly bigger cruisers – L.Dutch Apr 1 '17 at 16:31