Tau Ceti's planets nearest around single, Sun-like star | Hacker News Reader

Tau Ceti's planets nearest around single, Sun-like star

bbc.co.uk

43 pointsmorphics32 comments

Just looked on Wikipedia, it is 12 light years away. So how long to get there with conventional technology? I suspect speed isn't even the worst problem, harder to deal will radiation?

Also I suppose any such mission would have to be fully automated, remote debugging would be too frustrating with a 24 year round trip for information.

Have there been any spacecraft besides Voyager even launched beyond the boundaries of our solar system?

> Have there been any spacecraft besides Voyager even launched beyond the boundaries of our solar system?

No.

> So how long to get there with conventional technology?

I don't have any precise numbers but I guess it would be in the neighborhood of 100000 years one-way with today's technology (if even possible)

I just looked it up on Wolfram Alpha, apparently the speed of Voyager is 17km/s, or ~1/18000th the speed of light? So it would take Voyager 216000 years to make the journey? Bummer :-( We probably couldn't even build a probe that would last long enough?

50 km/s is a speed I think we could reach (by clever gravitational maneuvers for example). Even then the trip would take 72000 years. Better wait another few hundred years until we hopefully can launch a probe at a significant fraction of the speed of light.

Ion engines have been tested at effective exhaust velocities of 90 km/s [0], which allows delta-v's up to 500 km/s or so.

[0] https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion

We'll need either three or four quantum leaps in technology, or a sudden discovery that invalidates much of what we know about physics and/or cosmology, before even the Alpha Centauri planets become reachable. "Like it or not, for the moment, the Earth is where we make our stand."

A quantum leap is (on the order of, if you skip a level) the smallest possible increment. We're going to need much bigger leaps than that to do it in four or five leaps. :)

Relevant SMBC: http://www.smbc-comics.com/index.php?db=comics&id=2628

Some writers of dictionaries disagree: http://www.merriam-webster.com/dictionary/quantum%20leap.

You're using the technical (or jargon) meaning, which is not the same as the mainstream ("popular") meaning.

I think that's generally how jokes work.

I was using the term in neither the mainstream nor the technical sense, but in the Dr Sam Beckett sense. Oh, boy!

Using 1960s engineering (and many, many nuclear bombs), an Orion-powered starship could make it there in less than 150 years, one-way. And no slowing down; it would be a flyby.

http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi...

I don't see why it can't use atompsheric braking, or reverse slingshot effect amount the planets to slow down?

1000km/s on the energy-limited Orion would be hard to slow down using aerobraking without potentially disrupting the planet below, and reverse slingshots would probably occur too fast to do anything more than staying in-system for a few weeks...

How about a solar sail for braking? (We've got to be creative, I can't imagine there's no solution for slowing down.)

I imagine you would need a ridiculously large solar sail to slow down the mass of an Orion.

It's not that slowing down is impossible...you just start throwing bombs the other direction. But by slowing down you increase your travel time, so the 150 year estimate is no longer valid.

Can't you just turn around midway and start throwing bombs out in front of your path of travel to slow down?

One of the interesting things in that Wikipedia link is that you only need to run your Orion for 36 days to get it up to speed. Beyond that, detonating more bombs won't accelerate you any further.

So yes, you could turn it around. But surprisingly it seems you can wait until a month before arrival to do so. The downside, however, is that your max velocity gets cut in half if you bring enough bombs to slow down.

With better explosives, you could get a higher max velocity and therefore closer to the "accelerate halfway, decelerate halfway" ideal case, but at the moment such explosives don't exist.

I'm not sure how that works out.

Do 36 days of acceleration at the beginning (or spread it over a year to reduce peak acceleration) -- go from 0 to 0.1c or whatever the cruise speed is.

Coast for ~100 years.

Spend the last year going from 0.1c to 0.

It does mean you have to carry twice as many bombs, and carry half of the bombs for 149 years, but bombs are really lightweight relative to the ship. It also means you can't just jettison the pusher plate, which is probably a bigger deal.

In this case, the weight of the bombs is larger than the rest of the ship: 50K tons for the ship structure, 300K for the weight of the bombs [1]. Twice as many bombs essentially doubles launch mass.

Orion is a great concept for tooling around the solar system easily, but what it buys us over interstellar distances is that it makes macroscopic payloads and reasonable travel times possible; they're still not easy.

[1] Table 2 in http://galileo.phys.virginia.edu/classes/109.jvn.spring00/nu...

Just to reinforce: Orion is still a rocket. And not a particularly efficient rocket, either. It uses an incredibly energy-dense fuel, but not so dense to allow you to ignore the rocket equation.

What if instead of braking Orion itself, it just drops off a lighter satellite that is able brake without expending so much energy?

I'd think it would still have to brake using Orion technology, so it is doubtful if it could be built smaller than Orian itself.

It could use solar wind, or gravity-slingshot (which can slow as well as accelerate a satellite). So yes it could be much smaller.

I wonder if Elon Musk has ever spoken about Orion.

I'd love to live in a world in 2030 or so where a private shuttle runs Earth-Mars every few weeks using Orion technology, since it wouldn't then need to operate on energy-conserving orbits.

It's not twice as many bombs, it is (using your numbers) six time as much bomb mass as payload mass, so 50Kt payload, 300Kt bombs for slowing down, which gives you 350Kt payload for the acceleration phase, which would require another 2100Kt of bombs to get up to speed, for effectivley seven times the launch mass

Sure, but it raises the number of nukes needed at launch considerably, holding the travel time constant. More than double, even leaving aside considerations like the additional storage size.

Also, designing nukes that last for 150 years before being used is non-trivial (can't use tritium in the initiator, for example; it decays too rapidly for that).

A pretty interesting read that addresses these questions is

Deep Space Propulsion: A Roadmap to Interstellar Flight

http://www.amazon.com/Deep-Space-Propulsion-Interstellar-Ast...

Radiation can be handled, just put shielding around the astronauts. That adds weight of course, which makes the spacecraft harder to accelerate.

Btw, even Voyager 1 is only 0.002 light years from earth.

Perhaps Tau Ceti is even close enough to not be trapped to the paradox of interstellar travel: "No matter when you launch a voyage somewhere, some one else will build a better spaceship and get there before you."

Is there a sci-fi novel about this? (an interstellar space voyage for a remote planet discovers humans launching after them already have colonized it when they get there)

"The forever war" uses the time dilation of relativistic travel, which is a related concept.

I remember playing Outpost as a kid. Tau Ceti was definitely the best bet. I loved/hated the fact that you could send your colonists to a solar system that wasn't habitable and then the game would just be over in 5mins; your creepy AI giving you the bad news.

- http://en.wikipedia.org/wiki/Outpost_(video_game)

Reply on news.ycombinator.com