# Are we actually that close to techniques of accelerating probes to speeds like a quarter $c$?

As recent research has been started by a team around Stephen Hawking under the name Breakthrough Starshot they are going for accelerating little probes up to 0.2~0.25$c$. There isn't that much information about it available yet, but they say there will be different obstacles before they really can start launching the probes. While I read it as its questionable the accelaration method will even work as expected, it seems no one else is having any doubt about it.

Even critics just see problems in challenges like:

How they want to make probes the size of 1cm3 be traveling for 30 years through space, while not hitting any particles at said 0.25$c$ or other stuff from that region.

But no one (or at least I was not able to find) was stating:

They will never ever be able to accelerate the probes to that speed because of [$reason]". So is it really that certain, that we have knowledge available how to accelerate microcontroller like electronics of that size to a quarter of speed of light? • People in a group can 'hype' the project for many reasons. When hype enters the room, cool calm reason leaves. – Andrew Thompson Jul 12 '16 at 11:03 • Are you asking only about the ability of electronics and other hardware to survive the extreme acceleration? (Because I think that question might be answerable) – Andy Jul 12 '16 at 11:53 • @Andy: I'm asking if the weak point of that project really just is about how to protect the probes, and not as I have thought its questionable if we even will be able to accelarate cargo to 0.25$c\$ – Zaibis Jul 12 '16 at 12:00
• @Zaibis IMHO, SF's answer does not answer your question any better than mine. You asked if we have knowledge, not if we have money and political clout. – called2voyage Jul 13 '16 at 18:04
• @Zaibis That said, I will respect your decision to accept the answer that is most helpful to you. – called2voyage Jul 13 '16 at 18:05

The solar sail idea just works, tested and true.

Very powerful lasers are a reality. Very accurate, narrow-beam lasers are a reality too. Bringing these two together is absolutely doable.

Powering that up requires just lots and lots of solar cells. We have these.

Very reflective mirrors (so that the probe is accelerated and not obliterated by the laser) are a reality too, used with laboratory lasers extensively.

These are the essentials to perform the acceleration. It's completely doable from technological and scientific point of view.

There are two important reasons why we might never see it done:

• money

• politics.

First, while expensive, development and creation of these probes will not be outrageously expensive. But the laser propulsion device, while much simpler to build, will need a lot of power, and simply will weigh a lot. Putting that in orbit is going to cost a small fortune. Less than the ISS or the LHC, but still a lot.

And then - a lot of people may be very unhappy about someone putting an extremely powerful laser of exquisite accuracy in Earth orbit. That thing CAN be misused. And as result, politics kicks in and the whole project may be blocked due to its military potential.

• Yeah, and the fact that the laser they need would be powerful enough to break down the Earth's atmosphere is something they don't mention. So add "Placing the laser on the moon" to the list. – Phiteros Jul 12 '16 at 15:13
• @Phiteros: Why not just in Earth's orbit high enough? – SF. Jul 12 '16 at 15:26
• That would probably work as well. I was just thinking that, if you're going to have that powerful of a laser, it will need to be quite large, and it will draw a substantial amount of power. I do not know that the solar panels of a satellite could provide the necessary amount of power. Plus, the laser itself would exert a force on the satellite, accelerating it and changing its orbit. – Phiteros Jul 12 '16 at 16:00
• @Phiteros: In case the satellite accelerates the crafts away from the Sun (located between them and it) the light pressure from the Sun will perfectly balance with acceleration from the laser. Worse if it's half a year later... still, being quite massive, it shouldn't be affected too badly. – SF. Jul 12 '16 at 17:03
• I'm very skeptical about this answer, which contains no numbers or references to sources of information. In particular: Very powerful lasers are a reality. Very accurate, narrow-beam lasers are a reality too. I need some major convincing on these points. Over what distance are we talking about accelerating the solar sails? 100 AU? a light year? What about the diffraction limit? – Ben Crowell Jul 14 '16 at 3:42

Given that the interstellar medium (ISM) has a density of about 1 atom per cubic centimeter and given that laser propulsion could, in theory, accelerate a spacecraft to 30% of the speed of light in ten minutes, I'm going to say this one is plausible. They are talking about using several probes, so even if a few happened to get destroyed by particles, some should remain.

To be clear, we are talking about technology that will be available within the next couple of decades. They are not talking about launching today. Here is a list of the challenges that the team has identified that they have to overcome. For example, one is in fact that protection is needed for the spacecraft from collisions.

I don't want to understate the challenge of collision protection. One commenter (David Theil) on the Breakthrough Initiatives page says:

At a typical interstellar medium density of 1 atom per cubic cm these can be ignored from a momentum consideration. Dust particles of order 10^-14 g in mass have density of roughly 10^-12 per cubic cm in the local bubble. A cm sized spacecraft could expect to encounter a few million of these beasties on the way to Alpha Cen (roughly 1 parsec.) At a speed of 6x10^9 cm/s (0.2c) each one will deposit about 10^5 ergs into our little spacecraft...not enough to raise the temperature all that much if averaged over the whole 1 gram mass (assumed heat capacity of silicon), BUT presumably enough to sputter away some protective coating. I would want to do some lab experiments to see how different coatings respond to such collisions. Even the LHC can't produce 10^16 eV particles. This is going to be a tough thing to test and will have to rely on modeling.

• Just for personal curiosity, could you include a citation or link with more info on photonic propulsion? – Timpanus Jul 12 '16 at 12:07
• @Timpanus Included--but there is an abundance of information out there. – called2voyage Jul 12 '16 at 12:10
• @Zaibis The "interstellar space [could be] rather filthy" portion of the site I just linked is mostly a concern for larger spacecraft. Something as small as these probes is unlikely to sweep across anything like a chunk of comet ice. – called2voyage Jul 12 '16 at 13:14
• @Zaibis You should note, however, that protection from collisions is one of the identified challenges that needs to be dealt with. – called2voyage Jul 12 '16 at 13:20
• At 0.20 to 0.25 times c, interstellar hydrogen is effectively a 20 MeV/A particle beam. You need 200mg/cm^2 of material to stop it, and that material is subject to damage and sputtering. At 1 proton per cm^3, a light-year has 10^20 protons/cm^3 or about 100 micrograms, or about 1 part per thousand of the mass of the "radiation shield" used to stop the protons. It's a very interesting materials problem. You do damage and annealing at the same time, and the H and He will diffuse back out by itself. The more I look at it, the BtSs project really is an interesting engineering challenge. – uhoh Jul 12 '16 at 13:46