Watsisname, WOW, amazing explanation as always. I was going to answer that but it's much more helpfull when you give an order of magnitud crunching some numbers. A puny spin rate indeed.
Besides that I would like to remind you all that there is plenty of information in the oficial webpage of the project.
In the
Challenges section they have identified 29 important issues to solve for this mission to be accomplished. There is a little forum for each challenge where people have asked very intelligent questions, in my opinion, and some interesting ideas have been proposed.
Getting within photography range of proxima centuari b will require extreme precision. Can the really aim with such precision? Not to mention getting the timing right?
The question is adressed here:
In short,
It is expected that in the 20 years until launch Proxima B gets directly imaged and precission ephemerids are calculated that could make the spacecraft come closer than 1 AU from the planet. This seems a lot of distance still but the project heavily relies on the incremental nature of it. Many spacecrafts can be lunched (since the costs are going to be insignificant compared to current car-sized spacecrafts) with tiny differences in the angle of ejection as to make some of them arrive much more closer than 1 AU to the planet.
Some might even crash. Even with that another idea is to have the first nanosails to triangulate the position of the planet with high accuracy and beam the information to a year-appart second wave of nanosails that could maybe make small corrections
using the micro-thrusters.
If a planet-to-nanocraft distance of 1 AU and a velocity of 0.2c are assumed, an angular rate of 80 arcsec/s for the slew manouver is needed. This could very easily be acomplished using electrodynamic theters attached to the structure of the sail or the mini-thrusters mentioned before. Extremely efficient image compression and smart feature detection algorithms would be crucial.
Taking all of this into account (blurring, distance, etc...) a 40km per pixel image resolution could be accomplished. Using the mini-thrusters the pointing precission would be of
below 0.1 arcsecs.
I've taken photoshop to resize images of Earth, the Moon and Europe to match those 40 km/pxl resolution.
As you can see the expected resolution when taking into account the blur because of the errors in pointing around 1 AU distance is enough to show continents, even mountain ranges (look at the alps in Europe), large bodies of water, cloud patterns etc... Is enough to see detail also in an object the size of the Moon. You can see the lunar mare and even the ejecta streaks of the tycho impact crater.
Europe has been cleaned of clouds in that image so maybe some features would be hidden.
Consider also that Proxima B is a little bigger than the Earth so even if the detail is the same you would see more surface features in the image.
how will the probes be able to transmit back the data with such a small sender? The inverse square relationship must hurt badly at Proxima Centauri.
The question is adressed here:
In short,
The idea is to transmit the images using a 1W laser onboard. The sail would be used to focus the laser communication signal by shaping the sail into a Fresnel lens that would create a diffraction limit spot size on Earth of 1 km. To receive the weak signal the same laser beamer array used to lunch the probes would be reconfigured to a receiver mode. Current technology have demonstrated that it is possible to detect single photons emitted by lasers from very large distances. The current record holder is the LADEE Laser Communication system, which was able to operate from lunar distances. The current performance is of order 2 bits per photon, so a lot of redundancy in the information sent should be considered as to complete the information from the lost photons at arrival on Earth. It has been estimated that using the laser beamer as a phased array telescope would offer a sufficient collecting area to receive the signal from Proxima Centauri. Other solution contemplated are using a chain of communication relay probes launched in succession as to transmit the signal step by step across the 4 light years between both stellar systems.
Also in page 29 of
A roadmap to interstellar flight you can read about the details of similar projects.
if an interstellar dust speck hits the solar sail, it only punches a small hole in it.
But whenever this happens, the probe will still be knocked into a spin. Sounds like a job for that course-adjustment laser.
The question is adressed here:
Interplanetary dust around the stellar systems is way more frequent than interstellar dust. The nanosail would traverse a few hundreds of AU at most in the denser interplanetary medium so the danger is not so high. Nothing is known about dust in the vicinity of Proxima Centauri (
besides maybe some estimates based on this) but a lot is known on the dust in our Solar system. Thankfully Proxima Centauri is above the ecliptic plane from the Sun so the cruise through the dust of the Solar system would be much shorter. Also if the 45º inclination of the asteroid belts around Proxima centauri is true then we should expect also that the dust is accumulated in a plane that is not aligned with Sol and therefore the journey across this medium is also short at the arrival.
The idea of using the laser array on Earth to clear the way some AUs of the first part of the way has also been discussed. Maybe in that way we could peer through the zodiacal dust denser part minutes before the nanosail is lunched.
For interstellar dust each square centimeter of the frontal cross-sectional area of the nanosail would encounter about 1,000 impacts from dust particles of size 0.1 micron and larger. However, there is only a 10% probability of a collision with a 1 micron particle, and a negligible probability of impact with much larger particles. So for 10 nanosails 1 could be lost due to a large impact, thus not comprimising the mission. The rest of the sails would probably be hitted by smaller 0.1 micron objects so this is important. A 0.1 micron dust particle moving at 20% of the speed of light would penetrate and melt the StarChip to a depth of 0.4 mm. Traveling with the nanocraft’s edge facing parallel to the velocity vector would reduce the cross section to 0.1 cm[sup]2[/sup], for a 10 cm StarChip with a 0.1 mm thickness. A protective coating of beryllium copper could be added to the leading edge of the StarChip, as a sacrificial layer for additional protection from dust impacts and erosion. It could even be elongated to further minimize the cross-section (the sail folded like an arrow).
Also as
Watsisname has noted, the momentum kick from 0.1 micron dust particles is small, and its effect on the nanocraft’s trajectory might be compensated for by photon thrusters.
As for charged particles it has been speculated that maybe those could be defelcted using a magnetic field or electrically charging the sail. The problem is tu ensure that the galactic magnetic field does not impose deviations in the path of the spacecraft. In general the erosion due to electrons and protons can be handled with a protective coating over the entire sail. The 18MeV protons would travel several millimeters into the target before stopping. A protective layer sufficient to stop 18 MeV protons would be required to avoid damage to the electronics by proton implantation. The net loss of mass from the forward-facing surface due to sputtering would only amount to a few monolayers. Helium nuclei carry 72 MeV and are 10% as numerous as single protons. Hits by CNO group nuclei carry 200-300 MeV and are ~0.1% as common, and hits by iron nuclei carry ~1GeV of energy and are ~0.01% as common so the risk of damage is low and only a few nanosails would probably be lost because of this.