Backward star ain't from around here

[n2doc]

Here's an apple that landed far from the tree. A dim star just 13 light years from Earth was born in a cluster 17,000 light years away.

Discovered in 1897, Kapteyn's Star is the 25th nearest star system to our sun, but it is no local, says Elizabeth Wylie-de Boer of Mount Stromlo Observatory in Canberra.

The cool star's composition is tricky to study, but astronomers can look at 16 other stars in the same "moving group", all of which orbit the galaxy backwards and are very old. The odd motion marks them as members of the Milky Way's ancient population of halo stars.

Of the stars, 14 had the same abundance of elements – such as sodium, magnesium, zirconium, barium – as Omega Centauri, the galaxy's most luminous globular cluster. The cluster emits a million times more light than the sun.

"It's long been thought that Omega Centauri is the left-over nucleus of a dwarf galaxy that merged with the Milky Way," says Wylie-de Boer, whose paper will appear in the Astronomical Journal. "During the merger, the outer regions of this dwarf galaxy were stripped."

more:

http://www.newscientist.com/article/dn18138-backward-star-aint-from-around-here.html

[Ian David]

1. According to the article, you would need a solar sail 1 kilometer in area for interstellar travel.

However, I bet you could do it with a much smaller sail if you were willing to orbit the sun a few times to build up momentum before heading out of the solar system.

Or, you could accelerate them with a laser beam.

In that case, the only problem I see with using a small sail would be decelerating once you reach your destination.

And if I'm right, there's no reason why we couldn't launch an interstellar nanoprobe within a decade or less.

Here's my plan:

Launch a whole series of solar sail nanoprobes, one after the other, to Alpah Centauri.

Each probe would transmit data to the next probe that was launched behind it, eliminating the need for a transmitter powerful enough to broadcast all the way back to Earth. Low-power signals would be relayed back to Earth, using something similar to Internet protocol.

Continue doing this until you have a constant "loop" of 50 or more nanoprobes circulating between Earth and Alpha Centauri.

If Alpha Centauri proves to be "boring," you could simply re-direct all (or some) of the nanoprobes to a different star system.

If we wanted to do a sample-return mission, we could decelerate the probes with a laser beam once they returned to our solar system.

But by the time the probes returned to Earth, hobbyists would be able to retrieve them with their family space ship.

[skoalyman]

2. I've often thought about that they could easly do this.

:toast:

[Fumesucker]

3. To decelerate on the far end you use two sails, one as a reflector

The detached reflector accelerates to even higher speeds while sending the laser beam back to the actual probe which decelerates at a somewhat lower rate.

Of course the reflector would have to have active focusing and pointing in order to aim the reflected beam back at the probe.

http://en.wikipedia.org/wiki/Solar_sail

[Ian David]

6. At the other end, on an interstellar mission, the probe would be out of range of the laser.

You'd need one hell of a laser to decelerate a solar sail 12+ lightyears away.

Of course, you COULD send ANOTHER probe with a solar sail to Alpha Centauri that sits in a lagrange point with a laser beam to decelerate or accelerate any other solar sail vehicles.

[Fumesucker]

7. It depends on the size of the mirror sail..

And yes, it would take quite a large laser, but if we're talking strictly theory, it's doable.

The thing is, you wouldn't need to build the deceleration laser until at least decades after the original launching since you are only going to get to a fairly small percentage of light speed anyway.

[Ian David]

8. Deceleration laser should be launched FIRST, so it will be parked at Alpha Centauri...

... as future probes arrive.

[Fumesucker]

9. No, a deceleration laser is going to be a massive device..

What you do is launch the probe with the laser you have and then build the far more powerful deceleration laser in the decades it takes the probe to arrive, complete with detachable deceleration mirror, at the destination.

If you can get a deceleration laser to the target system in the first place you don't need to be sending probes anyway, you already have an extremely well developed interstellar capability.

Any interstellar probes in the easily foreseeable future are likely to mass something more like grams than metric tons, a deceleration laser on site in the target system is going to be in the metric tons class at the very least.

Another method of probe deceleration would be the magnetic sail.

http://en.wikipedia.org/wiki/Magnetic_sail

The solar wind is a tenuous stream of plasma that flows outwards from the sun: near the Earth's orbit, the solar wind from the Sun contains several million protons and electrons per cubic meter, and flows at 400 to 600 kilometers per second. The magnetic sail introduces a magnetic field into this plasma flow, perpendicular to the motion of a charged particles, which can deflect the particles from their original trajectory: the momentum of the particles is then transferred to the sail, leading to a thrust on the sail. One advantage of magnetic or solar sails over (chemical or ion) reaction thrusters is that no reaction mass is depleted or carried in the craft.

In typical magnetic sail designs, the magnetic field is generated by a loop of superconducting wire. Because loops of current-carrying conductors tend to be forced outwards towards a circular shape by their own magnetic field, the sail could be deployed simply by unspooling the conductor and applying a current through it. For a sail in the solar wind 1AU away from the sun, the field strength required to resist the dynamic pressure of the solar wind is 50nT. Zubrin's proposed magnetic sail design would create a bubble of space of 100 km (62 mi) in diameter where solar wind ions are substantially deflected, using a hoop 50 km (31 mi) in radius. The minimum weight of such a coil is constrained by material strength limitations at roughly 40 metric tonnes, and it would generate 70N of thrust<1>, giving a mass/thrust ratio of 600 kg/N. It is not clear how such a coil would be cooled.

The operation of magnetic sails using plasma wind is analogous to the operation of solar sails using the radiation pressure of photons emitted by the Sun. Although solar wind particles have rest mass and photons do not, sunlight has thousands of times more momentum than the solar wind. Therefore, a magnetic sail must deflect a proportionally larger area of the solar wind than a comparable solar sail to generate the same amount of thrust. However it need not be as massive as a solar sail, because the solar wind is deflected by a magnetic field instead of a large physical sail. Conventional materials for solar sails weigh around 7 grams per square meter, giving a thrust of 1e-5 N/m2 at 1AU. This gives a mass/thrust ratio of at least 700 kg/N, similar to a magnetic sail, neglecting other structural components.

The solar and magnetic sails have a thrust that falls off as the square of the distance from the sun.

When close to a planet with a strong magnetosphere, e.g. Earth or a gas giant, the magsail could generate more thrust by interacting with the magnetosphere instead of the solar wind, and may therefore be more efficient.

[muriel_volestrangler]

4. I think the 1km sail is already with a laser powering it

The sail is 32 square metres when it's unfurled, 5.5 m on a side. The mass of the craft is 4.5 kilograms. (For interstellar travel), I would estimate you'd need a sail that's half a kilometre by half a kilometre in area and a craft that's 1 kilogram in mass. And instead of a 5-micron sail like we have, which is very thin, with really advanced technology you might have a 1-micron sail by the time you do interstellar travel. This would also need a high-powered laser to keep enough light energy focused on the sail.

http://www.newscientist.com/article/dn18141-propelled-by-light-the-promise-and-perils-of-solar-sailing.html

And I wonder if repeated gravitational slingshots would help much; the point is that an interstellar flight in a decent time needs a really high velocity, and you won't build that up by hanging around in orbits that repeatedly return near the Sun to find another planet. What you'd want to do is get going on the phase in which you can use the sail.

[Ian David]

5. You would circle the sun building up momentum from light pressure AND gravity slingshots. n/t

[muriel_volestrangler]

10. But you can't build up speed beyond escape velocity, by definition

apart from on the final outward leg. And reaching escape velocity for the solar system isn't itself that fast - the Pioneer and Voyager spacecraft are going to take thousands of years to reach anywhere. It's the continued acceleration from the sail that'll be important, raising the speed far above escape velocity.

[Ian David]

11. You can't build up enough speed from light pressure to exceed escape velocity...

... yet keep tacking your sail so you keep circling the sun?

[muriel_volestrangler]

12. What I'm saying is that while you're still circling the sun, you're not going very fast

eg at the same distance from the Sun as the Earth, the escape velocity for the solar system is 42.1 km/s. If you're going any faster than that, you aren't in orbit any longer - you're on a hyperbolic path that takes you out of the solar system for ever, so you'll only pass any planets once more, at most. And you'll actually have that 42.1 km/s subtracted from your speed as you leave the solar system by the force of gravity. It's the speed you build up during your final exit from the solar system that counts.

The nearest star is 40 trillion km away. To get there in 100 years, you'd have to average about 12,700 km/s. The velocities involved in going around inside the solar system are neglibigle in camparison.

[Ian David]

13. I can pilot a sailboat around an island faster than the escape velocity from the island's gravity...

... using wind energy.

Why can't I circle the sun using solar wind energy faster than the escape velocity from the sun?

[muriel_volestrangler]

14. Because then you'd need to be putting force, and energy, into changing your direction

and would be working against your existing momentum just to keep that circular (or elliptical) course. Just as circling an island doesn't actually enable you to go any faster once you've stopped circling it (if you were going due north at 5 knots at one point, by the time you've reached the other side, you've put work into getting rid of the northward momentum, and then some into southward momentum; you'd be better off just trying to increase your momentum northwards), using your rocket/propulsion system to keep you nearer the Sun than gravity would is counter-productive, if you want to eventually build up velocity away from the Sun.

The thing is, the force of gravity is fixed, for a given distance from the Sun. If you want to keep in a path around the Sun, and your speed exceeds the escape velocity for that point, you have to supply the extra centripetal force yourself, because it's more than gravity gives you. That means you're wasting your propulsion system on staying close to the Sun.

You can use gravitational 'slingshots' to get up to escape velocity, and then gain a bit more as you leave the solar system, but those velocities are trivial in comparison with the speed needed to get to any other star in a lifetime. Getting out of the solar system isn't the problem; it's designing a system that can accelerate continuously for years, to go hundreds of times faster than escape velocity. A laser-driven sail might be the answer (I'd wonder what happens with diffusion of the beam at the vast distances involved, though), but its success will depend on the efficiency of the sail, not the small change in velocity needed to exit the solar system.

[Ian David]

15. I'm not talking about accelerating with gravity. I'm talking about accelerating with light pressure.

[muriel_volestrangler]

16. Yes, that's the point; you'd be wasting the acceleration you can get from the sail

by having to use it to change direction, rather than to increase in speed. If you're going north at 10 m/s, and you have a change of velocity available to you of 25 m/s, you can either use it to end up going north at 35 m/s, or south at 15 m/s (if you applied it to go in a circle, it'd be even more wasteful - you'd have to use some of your available force to give you a velocity component to the west, and then use some more to get rid of that). You're better off using the force of the sail to add to the speed in the direction you're already going.

[Ian David]

18. Thanks! I get it now. I'm a little slow with these things. n/t

[Fumesucker]

17. You would need to accelerate *toward* the sun ..

And that is not possible with light pressure I think.

Light pressure and the effect of wind on a sailboat sail are only roughly analogous, a sailboat can sail into the wind to some extent because the sail is an aerodynamic device like a wing, there is no equivalent forces working on a light sail AFAIK.

[krispos42]

19. To keep circling the sun you either need to be in a natural orbit

(centripedal force equals gravity), or a thrust-aided orbit (centripedal force equals gravity plus thrust).


If you want to orbit the sun at a faster-than-natural orbit (more centripedal force needed), you would need to direct your thrust outward, pushing your acceleration vector in the same direction as gravity. This way, the sum total of gravity and thrust would match the amount of increased centripedal force needed to orbit.



If you want to orbit the sun at a slower-than-natural orbit (less centripedal force needed) you thrust away from the sun, so the sum total of gravity and (opposite) thrust would match the reduced centripedal force needed to orbit slowly.

If you wanted to hover over the sun, for example (zero centripedal force needed), the thrust would have to exactly match the sun's gravity.