Interstellar probes

I get annoyed when something a little unexpected hits the news, and immediately after, a number of prominent people come up with proposals that look great, but somehow miss what seems to be obvious. In my last post (, I discussed the newly discovered exoplanet Proxima b, which is orbiting a red dwarf about 4.25 light years away. Unfortunately, that is a long way away. A light year is about 9.46 x 10^12 km, or nearly nine and a half trillion km. The Universe is not exactly small. This raises the question, what do we do with this discovery? The problem with such distances is that if we sent something at the speed of light it would take 4.25 years to get there. Apart from electromagnetic radiation, we cannot get anything up to light speed. If we got something up to a tenth of light speed, it would take 42.5 years to get there, and if it sent back messages through electromagnetic radiation, say light or radio, it would take a further 4.25 years to get back, and we would get information in 46.75 years. In other words, it is close enough that in principle, someone could get information back in his/her lifetime, provided they started young. So, how could one get even up to one tenth light speed? This is where the experts jump in.

What, they propose, with about one day’s thought at most, is to send small, almost micro, probes that have a sail attached. The reason for small is that we can get more acceleration for any applied force. The idea then is that these miniprobes can be accelerated up to a significant fraction of light speed by directing a laser at the sail. Light has momentum. There is the classic way to demonstrate this: little paddles with one side light and the other dark are suspended with bearings so the paddles can rotate, then these are enclosed in a glass jar and the air evacuated. If light is shone on it, the paddles rotate. It is very easy to show from Maxwell’s electromagnetic theory why this must be. Maxwell showed that electromagnetic radiation is emitted if charge is accelerated. The reason why light must carry momentum: if the body carrying the charge is accelerated, its momentum changes, and conservation of momentum requires the radiation to carry that momentum away. For our sail, the great advantage is all the energy is generated on Earth. The normal problem with space travel is so much weight has to be carried just for propulsion, in the fuel, the engines, the piping, and the structure connecting the engines to the rest. Here all the power is generated somewhere else, so that weight can be left behind. All the weight needed for acceleration is the sail and a connection to the probe. This proposal seems superficially to be a great idea, but in my opinion, there will be great difficulties in making this work.

The first reason is aim. If the probe has motors, they can correct for faulty aim, but if all the power comes from Earth, you have to get it right from Earth. The probe may fly by, but it has to get reasonably close. Assume you can tolerate it being anywhere on a 1000 km arc (you can put in your own number) where the arc is part of a circle with the centre at Earth. The length of the arc is r θ, where θ now represents the maximum angle that the “aim can be wrong. That means θ has to be less than 1000/40.2×10^12, or 1 part in 40 billion. Do you really think you can aim that well? Of course, the aim is for where the planet will be then. If you get the speed slightly wrong, the planet could be on the other side of the star when the fly-by happens. The velocity control has to have similar accuracy. Besides the planet orbiting, the star is also moving, so the whole system has to be there when required.

The next problem is that the laser beam must strike the sail exactly above the centre of mass of the probe. Any deviation and there is an applied torque to the probe, and the sail starts to spin. Can we be that accurate?

The final problem is the sail has to be exactly at right angles to the beam. Suppose it deviates by φ. Now the accelerating impulse is p.cos φ, where p is the mometum available to be transferred to the probe, and there is a sideways impulse of p.sinφ. Now, if the probe moves even slightly sideways, as noted above it starts to spin, and it is out of control. Alternatively, if it starts to spin in any way at all, the sail will give the probe a lateral nudge. It does not need much to exceed one part in forty billion. It will move out of the laser beam, and it would be seemingly extremely difficult to devise a means of correcting for such errors becaus eyou have no way of knowing wher eit is, once it has got underway for a reasonable distance. In short, a great idea in principle, but geometry is not going to make this at all easy. What I find hard to understand is why the proponents of this scheme do not seem to have stated how they can get around this obvious problem.