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Femto-Spacecraft Could Travel to Alpha Centauri

Earth’s nearest exoplanet twin orbits a star about four light years from here. Now scientists say it’s possible to visit this system in our lifetimes by propelling a tiny spacecraft on the tip of a laser beam.

Last year, a small team of astronomers announced the discovery of an Earth-like planet orbiting the red dwarf star Proxima Centauri, one of our nearest neighbors in the Alpha Centauri system. This exoplanet, called Proxima Centauri b, sits in the habitable zone around its host. Any water there should exist in liquid form, making this planet an important candidate in the search for extraterrestrial life.

Consequently, Proxima Centauri b has generated intense interest. It is about 40 trillion kilometers from Earth, a distance light travels in just over four years. A spacecraft traveling at about a tenth of light speed could make the trip in about 50 years.

And that raises an interesting question. Is it possible to build a spacecraft that we could send to Proxima Centauri within the lifetimes of people living today?

Today we get an answer of sorts, thanks to the work of Andreas Hein and pals at the Institute for Interstellar Studies in London, U.K. These guys have drawn up plans for a gram-sized spacecraft that could make the journey, equipped with a suite on instruments that could make rudimentary observations of the star system and send the results back to Earth. They call their femto-spacecraft the Andromeda probe and say it could be on its way to Proxima Centauri, and its sister stars, in just a few years.

But there is a caveat. While some of the technologies required for this journey are available now or in the near future, others are vastly more speculative.

The basic design is straightforward. The Andromeda Probe is essentially the souped-up innards of a smartphone camera. It consists of a black and white 12 megapixel camera, a lens, some inertial sensors, and a magnetometer. It also has a nuclear battery, rudimentary steering, and a communications system. “The total mass of the proposed spacecraft is 23 grams,” say Hein and co.

And it is propelled by laser light. The idea, one that several others have explored before, is to fit the Andromeda Probe with a light sail and accelerate it toward Proxima Centauri on the tip of a hugely powerful laser beam. This laser will sit in Earth orbit with a continuous power output of 15 gigawatts.

Hein and co give a thorough rundown of the technology challenges involved in building such a spacecraft (if not the laser system).

One of the most critical challenges is deep space navigation. Much of the probe’s navigation accuracy will depend on the pointing accuracy of the laser. The team say that nanoradian accuracy would do the trick and that several current spacecraft have similar requirements. The James Webb Telescope, for example, has a pointing accuracy of 24 nanoradians.  

But the spacecraft will still have to make minor adjustments from time to time. And this will only be possible if it knows where it is with high accuracy. This could be done using the onboard camera and inertial sensors for star tracking but it is still a challenging task.

The spacecraft will have to orient itself and track objects accurately, too. Travelling at 0.1 light speed, the spacecraft will pass through the Proxima Centauri system in about six days. In that time, it will need to take as many photographs and other observations as it can manage.

So the spacecraft will have to track its targets and point itself at them with high accuracy, otherwise the images will be useless. And it will have to do this autonomously, because the eight-year round time for communications places severe constraints on what help ground control can give.

Hein and co identify various ways of pointing the spacecraft and changing its orientation. Crucially, several of these do not require internal power sources. One idea is to change the reflectance of parts of the light sail so that the laser exerts an uneven force that causes it to turn.

Another, more power hungry idea is to move a mass on a beam, causing the spacecraft to rotate. But the team’s favorite is to fit the sail with moveable flaps that can generate torque and so turn the spacecraft.

One of the problems with interstellar travel is the risk of hitting a dust particle. At a tenth of light speed, such a collision could vaporize the spacecraft.  So Hein and co plan to cover the probe with a graphene Whipple shield consisting of several layers designed to break apart any particle as they pass through the layers, thereby spreading their energy.

The risk of destruction raises another interesting idea—sending a swarm of spacecraft. That increases the redundancy of the mission and the data gathering capability. It also makes better use of the laser beam, which will spread out, so much of the energy will be lost. For these reasons, a swarm of femto-spacecraft  make sense.

Then there is the communication system with Earth, which must be capable of sending home whatever pictures the spacecraft takes. With power at a premium, and at distances of several light years,  that’s likely to take significant time.

But it could be improved in various ways. One idea is to set up a data rely system by sending femto-spacecraft at different times to intercept the signals and pass them on. Another is to use the gravitational field of the sun as a lens to focus signals from Proxima Centauri. That would mean positioning a spacecraft behind our sun in a direct line with the target, another difficult and expensive task.

Onboard power is a little more straightforward. This would have to be a nuclear battery of some kind. This generates heat as its radioactive contents decay and is one of the few components that are well within modern engineering limits.

Finally, there is the tricky question of how much all this would cost and when such a mission could be sent. Hein and co settle on the remarkable figure of $11 million of the development costs of the first spacecraft. That seems ambitious, even if it is just for the spacecraft.

It also fails to account for the much more significant costs associated with developing and launching a laser propulsion system of gargantuan proportions. A 15 GW laser is a significant beast. That’s about an order of magnitude more power than the capacity of the Three Gorges Dam in China, currently the world’s most powerful power station.

While some of the technologies behind this idea seem feasible in the short to medium term, there are several that are not, particularly the laser propulsion system. Neither do the costs or timetable seem fully anchored in the real world.

Nevertheless, Hein and co say their report assumes a launch date of somewhere between 2025 and 2035.

Perhaps the most significant shortcoming of this mission is the scientific payoff. The promise here is a few smartphone images of Proxima Centauri or its sister stars in the Alpha Centauri system.

There is a high probability that these images will not be high quality and may not show anything at all, least of all Proxima Centauri b. In all likelihood, we will probably be able to take better pictures of this exoplanet more quickly using space telescopes in our own solar system, such as the James Webb Telescope.

But that is to discount the spirit of adventure in this mission. And that’s why it would be unwise to rule it out.

A new space race is heating up as private space enterprises battle it out over access to orbit. The targets that are most clearly in their sights are: sending human into orbit, sending them around the moon, nearby asteroids, and perhaps onto them. Beyond that, there is Mars.

But for an aspiring space business wanting to make a name for itself, Proxima Centauri and its sister stars could make a challenging proposition. Hein and co are surely crossing their fingers.

Ref: The Andromeda Study: A Femto-Spacecraft Mission to Alpha Centauri

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