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posted by martyb on Wednesday November 14 2018, @10:33PM   Printer-friendly
from the with-an-oink-oink-here-and-a...oh,-wait dept.

Exoplanet discovered around neighbouring star

Astronomers have discovered a planet around one of the closest stars to our Sun.

Nearby planets like this are likely to be prime targets in the search for signatures of life, using the next generation of telescopes.

The planet's mass is thought to be more than three times that of our own, placing it in a category of world know as "super-Earths".

It orbits Barnard's star, which sits "just" six light-years away.

Writing in the journal Nature [DOI: 10.1038/s41586-018-0677-y] [DX], Guillem Anglada Escudé and colleagues say this newly discovered world has a mass 3.2 times bigger than the Earth's.

Barnard's Star.

Also at phys.org.


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  • (Score: 3, Informative) by Immerman on Thursday November 15 2018, @04:54PM (6 children)

    by Immerman (3985) on Thursday November 15 2018, @04:54PM (#762237)

    Not really - if the fuel tank explodes, everybody's dead anyway - doesn't really make much difference if it's instantly or after they run out of supplies. Earth (or anywhere a rescue ship could reach in time) is a special case - anywhere else your long-term survival depends on that fuel, so all other design decisions can be based on the assumption that the fuel tanks won't explode.

    As for your "towing engines" - there's a few big problems:
    #1: 100k feet of carbon fiber is heavy, radically increasing the mass of the ship, and thus reducing acceleration.
    #2: Carbon fiber is flammable - fire up your engines and it will be rapidly incinerated by the exhaust and leave you behind.
    #3: Shorter, heavier, flame-proof cables mean your ship is blocking a lot of the exhaust, neutralizing its thrust.
    #4: Tilting multiple engines outward so that your rope and ship aren't in the exhaust means a percentage of the thrust is working against itself, so you're wasting acceleration potential.

    Of course if your nuclear/antimatter/whatever drives have enough specific impulse to waste a fair percentage some, then #1 and #4 can be solved with more money, and it could work.

    As for a space elevator - that's actually a great idea for visiting super Earths. *IF* we could develop a material with enough tensile strength to support it's own weight. Sadly, multiwalled carbon nanotubes, the strongest material we know of, are only barely strong enough to work on Earth, with safety margins that would make any responsible engineer run screaming for the hills. Even more sadly, there's reason to believe they're approaching the maximum strength-to-weight ratio of any material that could exist - C-C molecular bonds are (among?) the strongest known, and bonds generally get weaker as you move up the periodic table, while density increases considerably more quickly, so there's not even much hope of eventually finding some sort of exotic element that could do the job better.

    Also, you'd probably need a LOT more than 100k feet, though that depends on the rotational speed of the planet. On Earth, you need 22,236 mi (~12 million feet) to reach geostationary orbit.

    Of course, that only applies if we're talking about a traditional "beanstalk" elevator. If instead we used a "tumbling cable" skyhook in low orbit, you could use a much shorter and weaker cable tumbling fast enough that the lower end neutralized most of the low-orbit velocity while just skimming the atmopshere, while your landing craft could be a high-speed rocket-plane that could make up the difference and rendezvous with the cable at negligible relative speed.

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  • (Score: 2) by edIII on Thursday November 15 2018, @10:00PM (5 children)

    by edIII (791) on Thursday November 15 2018, @10:00PM (#762367)

    Too much risk that a fuel tank will explode, and like you say, you are very very far away from supplies. I was thinking multiple engines and fuel tanks though, all kept far away from the main ship. One engine tows you from the front, and the trailing engines are towed behind you. If those trailing engines are oriented to fire backwards, you could use it as a way to slow down too. Although, calculating orbits is much less fuel intensive for sure.

    As for #2, I was imagining a wing like structure in the towing engine such that the tips have the cables, the center has the engine, and the ship itself is a torus. The thrust could be channeled through the center of the ship, with nothing blocking its path. With the torus, you could spin the inside for gravity, but also have the ability to twist to compensate for the thrust.

    #4 doesn't apply, because the engines are not tilted. Straight backwards towards a center point.

    #1 is a very good point. I thought we would have breakthroughs to make it stronger still, but your observation that we may be at the theoretical limits is sobering. My idea only works with cables that have the kind of strength and durability required, and hopefully, the ability to be manufactured out in space.

    Your last idea about the skyhook sounds very interesting, but isn't the problem of a super earth that any landing craft lack the ability to reach escape velocity? I would assume that would make it difficult as well to reach the upper atmospheres, unless you factor in much denser air. If the latter, a high speed rocket plane might best be replaced with a hybrid dirigible.

    Of course the whole idea centers around light and strong cabling that you kinda shot down, so not sure it is workable as I thought.

    --
    Technically, lunchtime is at any moment. It's just a wave function.
    • (Score: 2) by Immerman on Friday November 16 2018, @02:21AM (4 children)

      by Immerman (3985) on Friday November 16 2018, @02:21AM (#762472)

      So where do you put the second fuel tank so that it's not destroyed when the first tank explodes? Not to mention, even if you can pull that off you almost certainly need ALL that fuel to make it home, so destroying one tank is just as deadly as destroying all of them.

      That would have to be a phenomenally large torus, as the exhaust plume expands rapidly thanks to being at higher pressure than the surrounding vacuum.

      The beauty of a skyhook is that you don't *need* to reach escape velocity (or more accurately, orbital velocity - once you're in orbit reaching escape velocity is relatively simple). Picture the cable as one pair of spokes describing a line through the center of a wheel. A wheel which is spinning at just the right speed so that the lowest point on the rim perfectly matches speed with the top of the atmosphere. Essentially you get a wheel rolling across the top of the atmosphere. And if you're in, say, a stationary hot air balloon directly beneath it you'll see a point on the rim come almost straight down above you, stop, and go back up again. Like this, only the wheel would be rolling around a circle, rather than along a flat plane: https://en.wikipedia.org/wiki/Cycloid [wikipedia.org]

      So, you're sitting in your hot air balloon (which does indeed work even better in the dense atmosphere likely on a Super-Earth), floating at the top of the atmosphere. And when that cable tip comes down and momentarily stops right above you - you grab it. You can then ride the "wheel" up to the top of the arc, at which point you'll be traveling at over twice orbital speed. For a skyhook in low Earth orbit (~8km/s) , it that means you'd be traveling at ~16km/s, about 5km/s faster than escape velocity from orbit (escape velocity from orbit is sqrt(2) times the circular orbital speed), more than fast enough to reach the orbits of Pluto or Mercury without any further propulsion. So probably you want to let go before you reach the top.

      That is to say - skyhooks can be *insanely* effective. Basically you're transfering angular momentum from the skyhook to the ship, but the skyhook's presumably much greater mass will keep it in orbit, where you can use ion thusters or other slow, high-efficiency propulsion to regain the lost momentum over a much longer time period. Or, you can rendevous incoming ships near the top of the wheel to slow them down and drop them gently into the atmosphere at negligible relative speed, in which case you transfer their angular momentum to the skyhook. A 100% efficient atmosphere-to-orbit momentum transfer bank with no moving parts.

      As for launching from super-Earths, the big problem is that the superlinear rocket equation gets worse much faster, to the point that you just can't carry enough chemical propellant to reach orbit with a "simple" rocket. Nuclear rockets would still do the job nicely, as might some of the more clever and efficient launch systems we've dreamed up. But it means a civilization that started out on the surface of a planet would have to develop powerful nuclear rockets before they could reach orbit. If you start in orbit though, where you can build orbital space elevators of various types, then you simplify things immensely (for sufficiently obtuse definitions of "simplify" - we were building nuclear rockets within decades of our first orbital chemical rockets - while we're still nowhere close to building a skyhook. Theoretically we have the basic materials and technology to do it easily, but the cost and details of attempting an orbital project of that scale have kept us from doing so.

      • (Score: 2) by edIII on Friday November 16 2018, @05:48AM (3 children)

        by edIII (791) on Friday November 16 2018, @05:48AM (#762559)

        You really got me thinking about the viability of this. Fun to think about.

        So where do you put the second fuel tank so that it's not destroyed when the first tank explodes? Not to mention, even if you can pull that off you almost certainly need ALL that fuel to make it home, so destroying one tank is just as deadly as destroying all of them.

        Think of an engine as a module that contains a thruster, antimatter storage pod, and then whatever matter we are mixing with (Deuterium?). The modules are separated. I do imagine the module can split in half with smaller thrusters to allow it to come apart and navigate back to the rear of the ship keeping maximum distance while rotating backwards, being dragged in half at the rear. Antimatter on one side, Deuterium on another.

        That would have to be a phenomenally large torus, as the exhaust plume expands rapidly thanks to being at higher pressure than the surrounding vacuum.

        You got me there. It would need to turn into a super structure at 100k feet behind if the plume expands that rapidly. Although, that also makes me wonder just how much heat is actually experienced along the surface area of the torus? The ineffiency you mentioned of being directly behind was blocking some of it right? A torus, or series of them, should allow most of it to pass right around them. Which brings up an interesting idea depending on the level of heat, and that is directly feeding off it for greenhouses and other needs. You would have backups, but imagine the steam baths.

        I do imagine the torus would be as large as possible. Resources are difficult to get in the middle of nowhere, so I was assuming a ship that was more of a biodome. I would imagine several miles in diameter, and as many as we could reasonably tow while still being nimble enough to get around safely. The inside of the torus could be hundreds of feet in diameter. I'm thinking a generational ship since nobody mentioned we were escaping the speed of light, just using thrusters with a lot of energy. Even at a reasonable fraction of C, it would be several generations to get to this star and back.

        The skyhook on a cycloid sounds pretty damn neat too. It would be one hell of an interesting ship if it could be built.

        --
        Technically, lunchtime is at any moment. It's just a wave function.
        • (Score: 2) by Immerman on Friday November 16 2018, @03:23PM (2 children)

          by Immerman (3985) on Friday November 16 2018, @03:23PM (#762713)

          I think keeping your antimatter and hydrogen (no benefit to deutrium outside of fusion, and it's a lot rarer) separate is fairly pointless. After all, the antimatter containment system, engine, ship, etc. is after all made entirely out of matter that will react just as strongly as the hydrogen would. In fact, a dense liquid or powder would likely be a much better choice than hydrogen since it's only the mass that matters, and the denser it is, the easier it is to store the same amount of mass.

          As for the temperature of the exhaust - it's going to be high - ridiculously high. You'll probably be using the antimatter annihilation to power an ion drive since the matter-antimatter reaction will leave you with only massless photons, mostly gamma radiation, and photon drives suck. (Also, you *really* don't want to stand in an exhaust stream composed primarily of gamma radiation) When you get right down to it temperature is a measure of average molecular speed - and the entire point of an antimatter powered drive is that it can expel the reaction mass at a much higher speed than is possible with chemical propellant. The faster (hotter) the exhaust when it leaves the rocket, the more momentum it has, and thus the more it causes your ship to accelerate for the same amount of reaction mass. For reference, the exhaust temperature of a Merlin engine is apparently 1500C, or 3km/s - and you probably want the ion drive exhaust to be at least 10-100x higher than that. The VASMIR ion drive in comparison has an exhaust speed of about 50km/s. Not sure how that translates to temperature - it's not a straightforward conversion, and depends on the mass of the molecules involved.

          • (Score: 2) by edIII on Friday November 16 2018, @10:50PM (1 child)

            by edIII (791) on Friday November 16 2018, @10:50PM (#762880)

            Thanks for explaining all of this, btw. I'm not very strong in physics at all.

            --
            Technically, lunchtime is at any moment. It's just a wave function.
            • (Score: 2) by Immerman on Saturday November 17 2018, @02:12AM

              by Immerman (3985) on Saturday November 17 2018, @02:12AM (#762922)

              My pleasure.