New solar sail may travel to Alpha Centauri
earthsky.org
earthsky.org
It says the probe itself is the size of a microchip. Let's be generous and suppose it's the size of a modern Intel x86 processor. How does it communicate with Earth? There has to be some kind of antenna dish; so OK, maybe the sail can serve dual functions. How does it do attitude adjustment?
Presumably some of this chip-sized probe is a sensor of some kind; perhaps a camera. Without a sensor, it's hard to see how one could think of it as a "probe" at all. Does this camera have a lens? How big? A lens the size of an Intel processor won't capture much light, and I'm not sure the bandwidth from Alpha Centauri is going to be good enough for digital imagery.
How is the probe's transmitter to be powered? A battery no larger than a microchip seems a little inadequate for communicating between Alpha Centauri and Earth.
OK, so the probe doesn't start transmitting until it can harvest energy from Alpha Centauri itself. But now it needs a solar panel! A solar panel up to the job simply can't be made the size of a microchip.
Maybe it's not supposed to send any data back at all; maybe it's just meant as a proof of concept. But even our best telescopes can't detect something 3m wide, in the region of Alpha Centauri. So the only concept this could ever prove is that you can accellerate something to c/5; there's no point in aiming it Alpha Centauri.
The article doesn't say how long these lasers are supposed to keep running; it seems unlikely they'll stay focused on the sail much past the orbit of Jupiter. I haven't tried to do the maths, but to accellerate something the size of an x86 to c/5, I imagine the lasers will have to run continuously for over a year; how much is that going to cost, for a probe that can't send back data?
Hmm - the article is bylined "Deborah Byrd". So I click on the byline, and get a photo and bio of astronomer Theresa Wiegert. Who is Deborah Byrd?
I've never heard of earthsky.org before, but my confidence in this publication is immediately diminished on my very first visit.
You think? It's at the level of coming from someone's head written down on a napkin told at a cocktail party. Only, now, they've added a friggin' laser.
> I've never heard of earthsky.org before
As for the time to accelerate, you may overestimate. Constant acceleration for months on end brings you to very high speeds indeed. If you’re accelerating at 1G a full year I think it’s closer to 0.9c than 0.2, I’d have to do the math to check.
So, it could do a round-trip only if you had a similar laser set up at the destination. If you can do that, why build this tiny spacecraft?
I'll go into hiding now.
I see - thanks! But how is the sail supposed to modulate the reflected signal? At ~2 baud, you could do it with a minute attitude adjustment; but nobody's said this probe has any attitude-adjustment capability. Maybe the innovative sail material has reflectivity that can be altered electronically? But they didn't mention that in the article.
But our closest star is much further away and it’s an interesting challenge to send a powerful enough transmitter that far - power being the main problem- and have an antenna on Earth or locally in space that can receive the data.
There’s also a question of bandwidth. Voyagers downlink is 160 bits per second. Webb is about 25Mbps. What bandwidth is achievable at that range?
It’s doubtful the probe would be able to receive because you wouldn’t want to send a massive antenna that far out.
There’s propagation delay which would be 4.2 years in one direction in the case of our closest star Proxima Centauri.
If we take a long view - as in, the original team would be long dead when the mission completes - I suspect this is achievable.
Check out the Lucy mission which is impressive in its orbital complexity and long time horizon if you’re into this stuff.
It’s still 400,000 times less distance to transmit a signal so it would help.
I guess you build in redundancy, but it's not obvious to me that the added complexity - probes have to send and receive instead of just send - is worth the benefit. I really have no idea about this space though.
nuclear power perhaps? Not now, but maybe in 15-20 years?
I see that https://en.wikipedia.org/wiki/Alpha_Centauri has a much better caption for the same image.
Given the shape of the solar sail, a new sci-fi story makes this the Veeger where our solar sail collects enough dust due to some unforseen negative ionic charge developing that pulls in galactic dust to make it look like generic rock floating from the Centurian Oort cloud and written off as actual contact from another planet.
Also, a mirror flying through space is very hard to detect. It reflects very well but only in some directions meaning it is basically black when looking at it from any other direction.
It was a low brow sci-fi premise where the little probe that could collected all sorts of space dust on the 20 year journey increasing its size.
If it's 20 years on earth how much time would have passed for the probe, and visa versa?
What does this mean? The only quantity I can see with an obvious meaning is that the probe will arrive somewhere after experiencing a certain amount of time since leaving Earth. But the time of the probe's arrival is not easily matched to any time on Earth.
Suppose the probe arrives and dispatches a message which travels back to Earth at the speed of light. For simplicity, the distance is exactly 4 light years.
- How much time did the probe experience between leaving Earth and sending the "I made it" message?
- How much time did Earth experience between the probe leaving and the "I made it" message arriving?
It's kind of unsatisfying that these two questions don't share both their endpoints, but at least they both have well-defined answers.
In this case it means that if the probe arrives at Mission Year 20 Week 0 according to a reliable onboard clock, and broadcasts it 4 light years back to Earth, then Earth will get this signal on Year 24 Week 5.
The probe itself doesn’t need to turn around and come back.
20 years to come close enough, and 4.5 for the signal to come back at the speed of light.
That's why I think it's not a worthy effort right now. You won't be able to aim correctly so you may speed by objects at large distance at an incredible speed with very small cameras. I wonder if the results ill wbe any better than what you can from earth (or will be able to do if you invest some money)
It doesn't sound like you could fit this kind of system in the size of a microchip, but what do I know, I'm just a random Internet speculator.
If simple and small enough, railgun grains-of-rice sized probes at every "near" object in the visible galaxy and create a million node network to relay and repeat back to earth.
Get the cost down to $100 each.
Time the launching to say once a day so they travel behind each other as repeaters.
Then they only need small transmitters/receivers with small power sources.
I wonder if it will even slow down, is there any braking manoeuvre possible, or will it shoot through the target solar system at 0.2c ?
What would be nice, would be if a signal - data, information about the target solar system could be sent back during the fly-by. But I see no sign of that.
I've seen pictures of a barren desert landscape today, taken recently on Mars. It's marvellous, and I don't need the rover to come back for that.
But without even information coming back, what's the point?
There is no manuever which can help with 0.2c velocity drop. Individual gravitational assist manuever usian a planet of the solar system can add/remove hundreds of meters/second (depends on the planet mass and how close to its center we can pass). Plus the vehicle flyies-by the object used for such assist. Since Alpha Centauri is literally the closest star system, there is nothing on the way to be used for such an assist (preferably a star).
Or the local "Space Coast Guard" will board the vessel.
Maybe not gravitationally, but there's always the chance the planet will get in the way.
Nowadays, I think what’s the point. Life in the universe may be extremely rare but we are going about the process of spreading it all wrong. Rather than sending humans to far off worlds we should focus on a simpler task of inseminating worlds with microbial multi-cellular organisms that can survive and evolve over the course of millions of years into full first class citizens that can thrive on these worlds and their environments. Humanity may die off soon but if we kick off a process on several worlds that triggers new forms of life to grow and reach sentience, then perhaps we will have done our part in the universe, and all of the human race could rest in peace knowing we have passed on the most precious resource the universe has to offer.
I assume there's some fatal flaw to this, but it takes more domain knowledge to identify it than I'm familiar with.
> I assume there's some fatal flaw to this
It would take massive expenditure of energy to create and preposition all of those nukes.
A direct contact blast (as in Project Orion) would obliterate a thin radiation sail. Radiation effects might impart a propulsive force to a more distant sail, but probably not enough to generate the sustained acceleration required. Pulsed acceleration blasts would dynamically disrupt the ultralightweight sail rigging (compared with a continuous gentle pressure from the sun or a launch laser). EMP effects might fry any electronics on an unshielded probe.
[Edit] However, this idea is perhaps conceptually less completely barking than his strategy for recovering important hard drives from a defended ship passing through the Panama Canal in The Three-Body Problem. Spoiler ... ... you simply cut the ship (and crew) into tiny pieces using a nano-filament wire ambush, then recover the sliced hard drives from the sea bed and reassemble them. I basically stopped reading at that point.
But in any case, keep reading! I promise the rest of the book is good :)
[PDF] https://web.archive.org/web/20160318223348/http://ccrg.rit.e...
https://en.wikipedia.org/wiki/Alcubierre_drive
(ala Futurama "The engines don't move the ship at all. The ship stays where it is and the engines move the universe around it.")
The kinetic energy formula is mv^2/2. At the speed of 0.2c, the relativistic correction is only about 3%, so you can ignore it. Let's say you want a probe of only 2kg. At a speed of 60000km/s = 60 million m/s, the energy is 3600 x 10^12 joules or watt-seconds, which is the same as 1000 GWh. So, roughly the output of 1000 nuclear power plants for one hour. And this only if by some miracle we achieve 100% efficiency in converting electricity here on Earth in kinetic energy far out in space. We'll get back to this in a moment.
How long does the acceleration phase take? At a gentle 1g acceleration, this would be 60 million m/s divided by 10 m/s2, which is 6 million seconds, or 69 days, so a bit more than 2 months. At 1000g (mentioned in the article) it would only take 6000 seconds, or 100 minutes (1h40m). The average speed over this period is 0.1 c, so at the end of the acceleration phase, the spacecraft will be 10 min-light away from us, which is 180 mill km, or a bit more than 1 AU.
Now, lasers don't produce a perfectly collimated beam (i.e. parallel rays). The best one can achieve is the optical diffraction limit, which means an angle of divergence of 2.44 x lambda/pupil diameter. Let's say our laser has a huge diameter of 1.22 meters and we use green light (500 nm wavelength). We end up with an angle of 1 microradian. For such small angles the tangent is equal to the angle, so it's going to be 1e-6. In other words, for each 1 million meters, the beam spreads out by 1m. At 100 million kilometers, the beam spreads out by 100 kilometers. Since our spacecraft only has a diameter of 10m, it captures only 10^(-8) of the beam. Let's say you fiddle with the numbers (you use a bigger diameter laser, with a shorter wavelenght), and you reduce the divergence angle by a factor of 100. That means you still capture only 10^(-4) or less of the beam for most of the trip.
So that 1000 nuclear power station just went to 10 million power stations. By the way, you better build them in space, otherwise the poor Earth atmosphere will not be so happy about this whole business.
Ok, but let's now say you overcome all these issues. Congratulations, you just accelerated a spacecraft to 0.2c. What's out there in the vast empty space? Mostly molecules of hydrogen, and from time to time a speck of cosmic dust . Now that speck of dust is not so innocent. It can weigh as much as 100 mg [1]. 1 mg specs of dust are quite abundant. And such a tiny speck of dust is nasty, really nasty. Because it hits you at a speed of 0.2c, i.e. with an energy of mv^2/2 = 10^(-6) x (60 x 10^6)^2/2 = 0.5 x 3600 x 10^6 = 1800 MJ. For comparison, a modern American armor piercing tank shell has a weight of about 9 kg and a velocity of about 1600 m/s, so a total energy of 9 x 1.6 ^2 x 1e6/2 which is about 10 MJ. So one tiny speck of 1mg will hit you with the energy of 180 rounds of M1 Abrams.
You only have to spend 20 years with these little fellows.
Eventually we could build lasers in the Centauri system to let us also slow down as we approach.
> Use of a laser-pushed lightsail was proposed initially by Marx in 1966, as a method of interstellar travel that would avoid extremely high mass ratios by not carrying fuel, and analyzed in detail by physicist Robert L. Forward in 1989
Edit: Here’s a better one: https://what-if.xkcd.com/20/
They have a better grasp of how many unsolved problems they face than you or I have: https://breakthroughinitiatives.org/challenges/3
(Genuine question; my physics isn’t at that level).
As the goal here is to get as much power as possible on a very small thing up to 3 million km away, surely you can just work backwards from the wavelength to the mirror size you need to get that focus. I think they’re planning to use a phased array of laser light here because no single mirror is big enough, but the same principle applies.
Isn’t the limiting factor the aperture anyway?