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NASA thinks that the technologies needed to launch an interstellar probe to Alpha Centauri at a speed of up to 0.1c could be ready by 2069:
In 2069, if all goes according to plan, NASA could launch a spacecraft bound to escape our solar system and visit our next-door neighbors in space, the three-star Alpha Centauri system, according to a mission concept presented last week at the annual conference of the American Geophysical Union and reported by New Scientist. The mission, which is pegged to the 100th anniversary of the moon landing, would also involve traveling at one-tenth the speed of light.
Last year, Representative John Culberson called for NASA to launch a 2069 mission to Alpha Centauri, but it was never included in any bill.
Meanwhile, researchers have analyzed spectrographic data for the Alpha Centauri system and found that small, rocky exoplanets are almost certainly undiscovered due to current detection limits:
The researchers set up a grid system for the Alpha Centauri system and asked, based on the spectrographic analysis, "If there was a small, rocky planet in the habitable zone, would we have been able to detect it?" Often, the answer came back: "No."
Zhao, the study's first author, determined that for Alpha Centauri A, there might still be orbiting planets that are smaller than 50 Earth masses. For Alpha Centauri B there might be orbiting planets than are smaller than 8 Earth masses; for Proxima Centauri, there might be orbiting planets that are less than one-half of Earth's mass.
In addition, the study eliminated the possibility of a number of larger planets. Zhao said this takes away the possibility of Jupiter-sized planets causing asteroids that might hit or change the orbits of smaller, Earth-like planets.
(For comparison, Saturn is ~95 Earth masses, Neptune is ~17, Uranus is ~14.5, and Mars is ~0.1.)
Planet Detectability in the Alpha Centauri System (DOI: 10.3847/1538-3881/aa9bea) (DX)
(Score: 2) by c0lo on Wednesday December 20 2017, @07:36PM (3 children)
TFS just says there may be quite large rocky planets we don't know about.
Given that we've seen Pluto only in 1930, I have a hunch we may not see quite significant (in terms of gravitation, capable of sending our probes astray) bodies around Alpha Centauri stars, no mater how much we'll advance with the telescopes.
Large is relative when it comes to the cosmic void and longish (as in days/weeks) exposure times.
Look, a smallish 100kg probe at 0.1c will have a kinetic energy of 45e15J (relativistic kinetic energy).
If you start breaking on Pluto's orbit (40AU) from 0.1c you will have about 11 hours to reach almost zero kinetic energy for an orbit around the star (Earth around the Sun is 30500 m/s = 0.001c). Say that you shoot around the star and continue deceleration, lets make 22 hours for deceleration.
You will need to loose those 45e15J in 79200 seconds - assuming constant energy dissipation, this means you'll need to lose 568181818181 J/s.
Translation: dissipate the kinetic energy at a rate of 568 TW.
I really don't know any material able to support an energy exchange of 500TW using a mass of 100kg.
Seems very likely that you will need to apply the brakes a lot earlier than 40 AU
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Solar sail, you say? The radiation pressure goes down with the square of distance to the source - Earth gets about 1.4kW/sqm (in space), Pluto get 0.9W/sqm.
The area of the sail that will use 1W/sqm to put on a 568 TW brake is a square of 753 km each side.
Let's say the 99% from the entire probe is dedicated to the sail, with only 1kg of useful payload.
At 99 kg/568e9 sqm is a sail with nanometer thickness - I really doubt that sail can offer enough rigidity to connect and put a drag without tearing itself apart from your 1kg payload
If you start braking later, when the radiation pressure is higher? Well, yeah, but then you'll have less time to brake, which means higher power to dissipate.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2) by Immerman on Wednesday December 20 2017, @09:14PM (2 children)
Do you have any idea how hard it is to pass close enough to a planet to have your path substantially modified by its gravity? Especially something as tiny as Pluto? Meanwhile we're already beginning to directly image planets around other stars, and have telescopes in the works that will actually be designed for the job and do a radically better job of it.
That's not to say it's *impossible* that we'd just happen to graze close enough to something we hadn't spotted to foul up an insertion path enough that automated adjustments couldn't correct for it - just that the increase in cost of launching a probe capable of seeing it soon enough to be of any use would completely dwarf the cost of the original probe. You talk about a 100kg probe - while most every idea I've heard is closer to a few grams - a basic sensor package and a communication system that, with luck and an incredible receiver, we could just barely hear reliably.
Consider - that 45e15J (12.5 TWh) would have to be provided on this end to launch a 100kg probe, probably over the same 11 hours unless you have a secondary launching facility on Pluto to accelerate it toward the sun to begin with. That means that even at 100% efficiency you'd be talking about spending 11 hours dedicating 6% of global power consumption to launching the probe. Heck, can you even imagine the laser needed to deliver that kind of power? Or that anyone would want such a powerful space-based weapon to be built?
I think the usual plan is that most of the mass is an ultra-thin light-sail used for launching and solar-braking, which then gets ejected at closest approach while the sensor package micro-probe fires its own braking thrusters at the point of closest approach/maximum payoff, expelling most of its remaining mass in the process. Ideally you'd like to use the solar sail as reaction mass, but you'll probably be hard pressed to find an effective way to do so.
As for the solar sail - at 0.77mg/m^2, 99 kg of graphene would cover only 128e6 m^2 with only at 0.33 nm thickness - though of course at 42N/m it's probably far stronger than needed for most of the sail, so we've got plenty of overhead to work with to make it more reflective and/or larger. You can also turn a lot of that incredible tensile strength into rigidity by spin-stabilizing it - since you're diving straight at the sun, so you don't need to be able to reorient the sail. And the sail is mostly decelerating itself, only 1% of the total deceleration needs to be transmitted to the comparatively tiny probe
Meanwhile the trick to solar braking is embedded in that same inverse-square law - you have to get close to the sun. You did miss the fact that solar sails effectively double the energy, since it sends it back the way it came, not that it changes things dramatically. At Earth's orbital distance it'd take 12.5TWh/(1.4kWh/m^2 * 2 * 128e6 m^2) = ~35 hours. Not enough. At Mercury's orbit radiation is 6.7x denser - so only about 5 hours - better, but too little to late. At say, 10 solar radii though - there the radiation density is 467 times higher, and would require only 4.5 minutes. Way more than we need, since we've been decelerating all the way in, so we can actually stay at a safer distance. Meanwhile graphene melts at somewhere over 5000K, while the sun is only 5777K - so, with a nice reflective surface it should last long enough to do the job.
(Score: 2) by c0lo on Thursday December 21 2017, @01:17AM (1 child)
Spin stabilisation is a nice.... ummm... twist, so to speak ;)
Except...
We'll need that overhead and something more. At 0.33nm, the sail will be transparent for light.
Even a metallic film will be transparent - has to do with the skip depth [wikipedia.org] - the depth on which the electric/magnetic field intensity drops to e-1 in a bulk conductor. Optimally, you need something on the order of λ/2 film thickness to maximize the reflection by constructive interference [wikipedia.org] - so you'd be playing in the hundred nanometers range . You may go suboptimal, but in any case not to the level of 0.33nm; at that thickness, the light simply "flows" through the sail as if the sail is non existent.
Microsensors are fine.
What is not fine at the micro dimension is the "call-home" feature. 'Cause we do want to get some info back from that probe, something that's not drowned by the EM emission of 3 starts - one of which is a red dwarf, cooler, emitting with peaks IR and microwave.
If any of the planets there has atmosphere and auroras, that'll be another source of noise in longer range RF.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2) by Immerman on Thursday December 21 2017, @02:57PM
Actually, a single layer of graphene blocks ~2.5% of light, pretty much across the spectrum, and the effect stacks - so at 10nm (30 layers) you're blocking ~53%. Working out something more opaque (and reflective rather than absorbent) will no doubt be a challenge. We'll have to see see what material scientists can accomplish given a motive.
Transmission will indeed be a challenge, I've got no answers for that, except that RF is almost certainly out - as you say, there's not really any quiet place in the spectrum. I'd bet on a carefully tuned laser myself. And that communication would be strictly one-way - With the mass and power constraints I'd expect it'd be nigh-impossible for it to detect an interstellar signal. Though... perhaps the launching laser could be tuned and modulated to be readily detected at that distance.
And hey, even if we never heard back from the probe itself, we'd now have a big honking launching laser that would be of great help for moving stuff around the solar system as well...