Why sailing to the stars has suddenly become a realistic goal

April 14, 2016 by Monica Grady, The Conversation
Are we soon to visit Alpha Centauri (left)? Credit: Skatebiker/ Wikipedia, CC BY-SA

It takes a bold person to declare that interstellar travel is now within our grasp. Physicist Stephen Hawking has shown that he is just that, taking part in the Breakthrough Starshot initiative. The project has announced a $100m research programme to investigate the technology of using light to propel spacecraft out of the solar system to explore neighbouring stars. For the first time in human history, interstellar travel is a realistic and achievable aspiration, and not just the playground of science fiction.

So what has changed that makes achievable? First of all, clear expectations. This is not about a great big spaceship with a colony of astronauts travelling for generations to settle a planet around a distant star. Neither is it about faster-than-light travel, tunnelling through wormholes to arrive at the other side of the universe in an instant of time. This is about technology that already exists, or nearly exists, being applied in new and exciting ways.

The technology is that of solar sails combined with the ability to miniaturise instruments. The idea behind solar sails is that the light that streams from the sun can be used to propel an object. Gradually, as the object is moved, it accelerates and builds up speed, eventually reaching values that are a significant fraction of the speed of light. So far, tests have been made on sails that are many metres across, carrying payloads of kilograms in weight. And the technology works – both the European Space Agency and NASA have developed systems that could be applied to spacecraft. To work efficiently, these sails need to be several metres across, just as a sail on a sea-going vessel needs to be a certain size to catch enough wind to push a ship forward.

Miniaturisation of instruments has been a significant driver of space exploration ever since the launch of Sputnik, the first artificial satellite to orbit the Earth. The ideal instrument for a has minimal mass and requires minimal power. These requirements led to the concept of CubeSat, a tiny satellite where a standard platform has been developed onto which different instruments could be fitted. The entire satellite can also be launched by a variety of different vehicles.

Artist’s view of an experimental solar sail satellite built by NASA, dubbed NanoSail-D2. Credit: NASA

We can all appreciate the rate at which equipment is getting ever smaller – a quick glance at a smart phone, with its powerful camera, ability to record and store gigabytes-worth of sound and images shows where technology sits at the moment. There have been proposals to use a CubeSat to launch multiple tiny probes carrying instruments to the moon and to Mars. Breakthrough Starshot, though, will be a single microchip – a spacechip that is a spaceship (and vice versa!).

The concept is to have individual microchips each propelled by a solar sail. Given that the sail is likely to be much, much bigger than the vessel – the spacechip is likely to be of the order of a centimetre long – there are practical as well as technological difficulties to overcome. But attaching the sail to the ship is probably one of the least serious potential problems.

Interstellar threats

Interplanetary and interstellar space are full of hazards for a tiny traveller. The two greatest hazards will be dust and radiation. Even though there have been great advances in production of thin but strong materials that would make appropriate solar sails, a dust grain travelling at speed could rip a sail apart. Also, microchips are very sensitive to high doses of radiation – and interstellar space is shot through by cosmic rays. While the chips are still within the , how vulnerable will they be to changes in the ?

Once all the technological development has taken place, and the chips are launched – which might be sooner rather than later, if the Breakthrough Starshot initiative is successful – where will they go and what will they do when they get there? The idea is that they will travel to our nearest neighbour beyond our Solar System, Alpha Centauri, taking about 20 years to get there, travelling at a speed of about 60,000 km per second (or 135m miles per hour).

This is not just an amazingly short time to travel a distance of 40 trillion km, it is also a reasonable length of time for a space mission to last – think of how long the Hubble Space Telescope has been operating and returning spectacular images. En route for Alpha Centauri, the chips will act as tourists – taking pictures, meeting the local inhabitants (planets? comets?), and looking at the scenery (such as more distant stars).

These findings in themselves are probably sufficient scientific justification for the project. For example, we have never seen a star or a supernova from interstellar space – we are always looking through the lens of the solar system. Once the tiny travellers reach Alpha Centauri, the instruments will make measurements of a stellar system, giving us – again for the first time – a detailed insight into stars that are different from our own. They might even find planets and moons.

We've had talk of surfing on gravitational waves to travel in time – which is still very much . Now, though, we are sailing in a solar wind, and interstellar travel is within our grasp. We might not yet be at the stage of interstellar travel for people, but being able to see what it is like to travel through while still in the comfort of our own home is certainly a first step.

Explore further: Stephen Hawking joins futuristic bid to explore outer space (Update)

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1 / 5 (1) Apr 14, 2016
So who is going to control a laser capable of vaporizing almost any existing satellite?
1 / 5 (1) Apr 14, 2016
After the tiny/miniature probes reach Alpha Centauri, how do they send data back to earth? Data transmission would require a large antenna with lots of power. What's the point of a tiny probe if it's packing a 100-foot-diameter antenna (WAG) that requires a MW of power (WAG) to send accessible information back to earth?

And if the probes just go there, but never send information back to earth, what's the point?
4 / 5 (1) Apr 14, 2016
Is this going to be a fly-by, like the New Horizons mission to Jupiter? I mean, at those speeds applying some brakes to enter a stable orbit around Alpha Centauri would be a big deal. Are they planning to try to use light pressure from that star for braking? Or is this just a matter of getting as much info as they can in the short time (days?) that will take for the probe to leave that system behind?
not rated yet Apr 14, 2016
So who is going to control a laser capable of vaporizing almost any existing satellite?

Mission Control, most likely.

The laser array would only be capable of vaporizing objects very close to its focus, and one presumes the launch team would have access to an ephemeris.
5 / 5 (1) Apr 14, 2016
a dust grain travelling at speed could rip a sail apart.
A dust grain at speed is going to clip a neat and tiny hole in the sail, the combined sail/grain material escaping as a high-speed jet without transferring much if any momentum to the sail. The real problem is dust hitting the chip itself, as the micron-sized dust particles will carve holes wider than the electronic circuits.
1 / 5 (1) Apr 14, 2016
Surely about halfway, the sail will encounter a roughly equal opposing 'headwind' of light streaming from the target star and essentially stall?
not rated yet Apr 14, 2016
Perhaps it will turn side-ways ??
5 / 5 (2) Apr 15, 2016
a dust grain travelling at speed could rip a sail apart.

A dust grain impac travelling at a fraction of c might do a tiiiiny bit more than punch a hole through it. We're talking energies like in a supercollider (much more than that since a dust grain has quite a few more atoms in it than the packets of protons/electrons used in supercolliders)

who is going to control a laser capable of vaporizing almost any existing satellite?

Somone who isn't interested in making it a weapon (read: scientists)

how do they send data back to earth?

The idea is that the sail can also harvest part of the energy shot at it to power transmissions. Whether that works over such distances is anyone's guess.

Is this going to be a fly-by

Yes...and a rather quick one. These sails can't decelerate.

the sail will encounter a roughly equal opposing 'headwind'

These aren't propelled by sunlight but by a powerful laser. No such laser at the destination.

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