Space probes to planets of other stars - 2
In April 2016 the scientific world was shaken by the news of the discovery of an extrasolar
planet of terrestrial type orbiting in the "habitable zone" of a small and cold star,
a red dwarf as there are many in our Galaxy and probably all over the universe.
Only this star was not just any star, but the closest star to Earth!
A star that is not for nothing called Proxima, or rather Proxima Centauri, only 4.22 light-years
away!
The little red dwarf is the third and weakest member of the triple system of Alpha Centauri.
Describing a very long ellipse that runs in about 547,000 years, Proxima orbits around
the two main stars of the system, much brighter and more similar in characteristics to the
Sun.
Currently, it is close to the point of maximum distance from the center of gravity of the
system, equal to about 13,000 astronomical units.
The planet, named Proxima b, has instead a minimum estimated mass equal to 1.17 times
that of the Earth, and orbits around its star in 11.2 days at a distance of about 7 million
kilometers.
This is a very small distance if compared with that of Mercury to the Sun (58 million
kilometers) or the Earth to the Sun (150 million kilometers).
This could make you think of a planet with a very hot surface...
But Proxima Centauri (with a surface temperature of 3000 degrees) has a brightness that is
just one-thousandth of the solar one, so that astronomers have estimated that Proxima b
receives 2/3 of the light that the Earth receives from the Sun, which is enough to ensure a
surface temperature compatible with the possible presence of liquid water.
In short, thanks to the strong motivations brought by the discovery of an extrasolar
planet so close, the idea of reaching another star no longer appears as mere science fiction.
In fact, a few months before the discovery of Proxima b, a group of entrepreneurs and
scientists had already taken the first steps towards a mission to the Alpha Centauri star
system.
We are talking about the Starshot Breakthrough project.
The Facebook patron Mark Zuckerberg, Steven Hawking (unfortunately recently deceased)
and the two Nobel physics laureates Steven Chu and Saul Perlmutter are just some of the
minds that have made possible a project that will cost between 5 and 10 billion dollars
and will be funded with 100 million dollars by Russian billionaire Yuri Milner.
The goal is to launch in 20 years a fleet of small probes equipped with laser-propelled
sails.
The total cost, hopefully those responsible, should be around 10 billion dollars, and the
company would take another 20 years to reach Alpha Centauri.
On the contrary, to reach Proxima b! (since the destination was obviously and immediately
changed after the discovery of the planet…).
But here is the mission plan.
The launch.
Breakthrough Starshot has as its goal the realization of a completely different type
of interstellar probe from those conceived so far.
All this is part of a mission where there will be a "mother ship" capable of conducting
in Earth orbit thousands of nanoprobes that will use the well-known solar sail technology
to move.
It's known for over a century that light carries momentum and can give a boost to objects.
But the sunlight is not powerful enough to accelerate a probe up Proxima Centauri; the
trip would require a huge sail and almost impossible to control.
So in the project sunlight has been replaced by radiation from powerful lasers placed on
Earth to generate beams of light so consistent that they "blow" into the sails for several
minutes without interruption, long enough to accelerate them up to 60,000 kilometers
per second.
Nanoprobes.
The microscopic Starshot probes will not look like anything that so far launched in space...
they will be in practice chips as big as a stamp on which will be implemented and miniaturized
batteries, sensors and cameras.
The chips will be mounted in the center of a square sail of 4 meters side, all for a
maximum weight of about one gram (!). To maximize speed and minimize laser damage,
the sail will need to reflect almost all incoming light.
During the acceleration phase, the sail will have to remain extremely flat and actively
detect, compensating them, the imperfections of the laser beam, so that the vehicle remains
on the right course: even a slight deviation in the initial phase could send it on a very
different trajectory.
However it is designed, the sail will have to be resistant.
The 100 gigawatt laser beam will hit it hard, generating acceleration tens of thousands
of times higher than the force of gravity that we experience on the Earth's surface.
The Starshot mission is obviously all about quantity.
There will have to be hundreds or thousands of small probes sent to Proxima b (about ten
a day), to compensate for the inevitable losses that will occur during the trip.
The development of the probes will proceed step by step.
The first step will be to build a prototype that accelerates to about 1000 kilometers
per second, less than 2% of the speed foreseen for Starshot, with a total cost between 500
million and billion dollars.
The journey.
The lasers will push for a few minutes on each sail to be accelerated, until each probe
will have reached a fifth of the speed of light and traveled a couple of million kilometers.
The next 20 years, the time it takes to reach Proxima b, will then be decidedly boring,
unless there are always possible accidents.
At this stage, the greatest risk will come from collisions with dust particles, hydrogen
atoms and other particles in the interstellar medium.
To this must be added the danger posed by cosmic rays, atomic nuclei that whizz through
space at speeds close to the speed of light that can damage electronics.
Even without destroying it, a collision could send the probe off course.
It will therefore have to have its own navigation and direction systems, powered by a generator
capable of exploiting a radioactive isotope such as plutonium-238; in practice, a nuclear
battery.
These systems will have to include a basic artificial intelligence that controls the
position of the stars and adjust the course using photon thrusters.
The arrival.
If all goes as expected, around 2060 the on-board computer of each of the incoming Starshots
will wake up, contact Earth for periodic check of its status, detect that it is approaching
Proxima Centauri and prepare for the flyby.
When the probe arrives at Proxima, there will be no way to slow it down: it will speed through
the star system in about two hours.
This is a challenge for the design of your measuring instruments.
Experts agree that the highest priority will be to send photographs to Earth.
It is estimated that even from a distance of an astronomical unit, a photo could reveal
whether the planet is watery and green like ours, or sterile like Mars.
And it could also detect features as big as mountains and craters.
A spectrometer onboard could probe the atmosphere of the planet, if it had one, indicating to
researchers on Earth will be the existence or not of molecules such as oxygen, methane
and more complex hydrocarbons, which are possible signatures of life.
The instruments could also try to measure the magnetic field of the planet or other
variables useful to find out if Proxima b has an environment compatible with life or
much more hostile.
However, no photo has ever been taken by a camera moving at one-fifth the speed of light.
And to find out more, you'll first need to find out how to do several simulations.
The cameras will have to rotate to keep the planet under observation, and on Earth computers
will have to correct image distortions caused by the effects of relativity and the change
of angle and distance of the camera from the planet.
Then there will come one of the most difficult challenges for Starshot, for which those responsible
admit they have not yet found a solution: how to transmit data from Proxima b to Earth
using a laser beam of 1 watt or less?
Basically, we will have to invent a way to amplify the signal to make it detectable from
Earth after a journey of 4.22 years.
It is being hypothesized that the nuclear battery on board could power capacitors to
make the beam as bright as possible, similar to a camera flash.
And to further amplify the signal, the sail could perhaps be used as an antenna.
But the problem obviously remains and does not seem easy to solve.
Well, this is what we can report on the more "serious" mission proposal towards Proxima
b currently under examination.
The Starshot Breakthrough project is certainly much cheaper and technically feasible than
the cyclopean Daedalus project.
But it is probably flawed for the opposite excess: too much simplification to remain
within mass limits and almost no ability to really explore the planets orbiting around
Proxima Centauri.
If they ever manage to get close to the red dwarf, the nanoprobes will speed through its
planetary system at over 40,000 km / s, without the ability to slow down and with scientific
equipment reduced to the bone.
It will be a great stroke of luck if the entire mission will give as its main fruit the blurred
image, at most a few pixels large, of Proxima b or some other planet orbiting the red dwarf.
Experts not involved in the project express a mixture of moderate optimism and skepticism.
"I think there are enormous challenges" in increasing laser power and other essential
technologies, says Gregory Quarles, director of research at the Optical Society in Washington
DC.
But adding that with the right level of public and private funding for optical and materials
science research, "there will be a return on this investment.
Others say that Starshot's minimalist approach has the merit of differentiating the mission
from previous and less plausible proposals.
"There is nothing that I can see that is completely impossible in an obvious way," says Caleb
Scharf, an astrophysicist at Columbia University: "We are not talking about a big spaceship
to send to another star.
Others fear that technological obstacles may prove impossible to overcome.
"I'm cautious about the short-term future of the project," says Bruce Betts, solar sail
expert at the Planetary Society.
"Every single piece of the problem seems to be surmountable, until you realize you have
to cram everything into one small, small mass object".
And even if some of the probes managed to get on Proxima b, according to Andreas Tziolas,
president of the Icarus Interstellar space exploration organization, it is unlikely to
provide any useful data.
"He has a very low probability, if anything, of being able to send a return image from
Alpha Centauri," he says.
"That small spacecraft can't carry enough energy to transmit a signal back."
Before any probe lifts off the ground, astronomers could learn a lot about Proxima b without
sending anything beyond the Earth's immediate vicinity.
By the end of 2021 is scheduled to launch the James Webb Space Telescope, and several
giant Earth telescopes will probably be in operation in the next decade.
Thanks to these instruments astronomers may be able to determine if the atmosphere of
the extrasolar planet contains signs that suggest the presence of life.
But, as any explorer would say, nothing is better than to verify on the spot.
In 2015, for example, Pluto's flyby revealed mountains of ice and nitrogen glaciers that
were invisible to Earth's most powerful telescopes.
Likewise, Proxima b - like every other exoplanet nearby - can hold surprises that will only
be visible in a close encounter.
There will also be more far-reaching returns, say mission supporters.
"I see Starshot as an opportunity to develop our capabilities," says Kelvin Long, director
of Interstellar Studies in London and member of the project's advisory board.
"It will be like when we went to the Moon.
Not to mention that a laser array capable of propelling a spacecraft to Proxima Centauri
- says Long - even if it fails it could easily send probes throughout the solar system in
a few days, or into the interstellar medium within a week or two.
Wouldn't it still be worth a try?