Inspirations: To sail upon a star

  • Project Breakthrough Starshot plans to send probes to Proxima Centauri powered by lightsails.
  • The lightsail was conceived decades ago, but the IKAROS probe is the only spacecraft to use it.
  • IKAROS is powered by the sun, but Breakthrough Starshot will use a laser to accelerate the probes to 20% of lightspeed.
  • As well as using light, the sun can drive a ship that generates its own magnetic field.

Artist’s impression of Proxima b, with Proxima Centauri in the background (ESO)

Proxima b is a planet a little bigger than earth, it’s close enough to its star, Proxima CCentauri, to have liquid water, it’s about 40 trillion kilometres away and Stephen Hawking says it’s due a visit.

Astronomer Guillem Anglada-Escudé, who led the team that discovered it in August this year, put it succinctly: ‘The search for life starts now”.

The catch is that we can’t see if there’s any life on it from four light years away. In fact, we can’t even see the planet. The European Southern Observatory was able to deduce its existence by its effect with the light of Proxima Centauri. To see if there’s anything alive on it, we need to get a much closer look.

Of all the planets discovered around stars other than our own, Proxima b is not the most likely to harbour life. It’s so close to its star that it may well be tidally locked, with one hemisphere permanently facing the star. If it is, that hemisphere would be blisteringly hot while the other side would be desolately cold. Water would only be liquid in a narrow band between them, so it wouldn’t be a particularly friendly place. Worse, its star has probably dimmed relatively recently so its orbit has only been in the ‘habitable zone’ for a fairly short period of time.

Visiting our galactic neighbour

The difference between Proxima b and other exoplanets is that it’s much closer, and a hundred million dollars has already been put forward to sending a robot probe there. Close, in interstellar terms, is a relative term. The fastest spacecraft to leave earth is NASA’s New Horizons probe, which took nine years to reach Pluto. It would take it nearly 80,000 years to reach Proxima b.

Yet there is a plan to send a probe there in 20 years. The Breakthrough Starshot mission to the Proxima Centauri system was announced by internet entrepreneur Yuri Milner in April, actually before Proxima b was discovered. Milner has stumped up $100 million of his own money, and he’s got the backing of leading experts in astronomy and physics


Artist’s impression of a spacecraft powered by a solar sail (Kevin Gill [CC / Flickr])

including Stephen Hawking and Freeman Dyson. The list of technical challenges is formidable but the heart of it is the question of how they intend to get a spacecraft moving at 20% of the speed of light, or 0.2c to use the technical notation.

Milner’s plan is to harness the power of light itself to power a fleet of probes that would only weigh a few grams each. The concept of the lightsail, which captures photons from solar wind, has been around for several decades and was popularised in Arthur C Clarke’s short story Sunjammer in 1964. When the most expensive part of any mission is getting the spacecraft off our own planet, the idea of a propulsion system that doesn’t need fuel has an obvious advantage. Once unfurled, a solar sail could use the largest energy source in the solar system.

IKAROS takes flight

NASA conducted several theoretical studies on solar sails in the late 20th century, culminating in a 1999 paper by Dean Spieth and Robert Zubrin laying out the possibilities and problems. They described a lightsail made of aluminium, which would capture the photons flung out by the sun, embedded in a plastic substrate to maintain its shape. Their sail could achieve an acceleration of around 0.3m/s2. That’s not much compared to what a rocket engine could achieve, but a rocket engine can only accelerate for as long as it has fuel to burn. A solar sail could keep accelerating for as long as the sun is bouncing photons off it. After a month, it would be whipping along at 671km/s, which is about forty times


Artist’s impression of IKAROS (JAXA)

faster than New Horizons. At that speed, it would reach Pluto in a little over three months.

The concept was taken further by JAXA (Japanese Aerospace Exploration Agency), who designed a solar sail purely out of polyimide, and built in an ability to manoeuvre with liquid crystal panels that change how reflective different parts of the sail are. Their 14m2 sail was used to power the IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) probe, which launched in 2010 and was able to make a close flyby of Venus, which is actually closer to the sun than earth. It was the first, and so far only, spacecraft to be powered by a solar sail, although NASA has deployed a lightsail in orbit to test the technology.

Spieth and Zubrin took the concept further, suggesting that rather than plastic and aluminium, a sail could be made out of carbon nanotubes which would be considerably lighter, allowing for greater acceleration. Because a solar sail needs to be relatively close to the sun to accelerate, greater acceleration means greater speed. The plastic and aluminium sail that whizzes by Pluto at 671km/s won’t get any faster because the sun is little brighter than any other star from that far out, so there wouldn’t be many photons pushing the sail along. Spieth and Zubrin calculated that a carbon nanotube sail could accelerate as fast as 100m/s2, which would allow it to reach a maximum speed of nearly 40,000km/s. That’s about 0.13c, which would get it to Pluto in less than two days and to Proxima Centauri in 32 years. It’s a long trip, but it’s not so far from the 20 years Breakthrough Starshot is talking about.

There’s always a catch

There are, to put it mildly, some technical problems to be solved before anyone can build a 40,000km/s spacecraft. Spieth and Zubrin’s calculation is based on the mass of the sail


Robert Zubrin in 2011 (Penn State [CC / Flickr])

alone but to be of any use, there would have to be some sort of spacecraft attached to the probe, which will have mass of its own and reduce acceleration. The acceleration is governed by the ratio of the mass of the sail to the mass of the payload, so the larger the payload, the larger the sail required to reach that acceleration and subsequently that speed. It wouldn’t need a huge payload before it would need tens of square kilometres of sail to get anywhere near that 40,000km/s.

A bigger problem is that while research into carbon nanotubes is progressing rapidly, it’s still a long way from being able to make a solar sail out of them. Spieth and Zubrin did not believe they would be available until well into this century, and so far, they appear to be right.

The Breakthrough Starshot plan

Yet Breakthrough Starshot proposes to sail probes to Proxima Centauri faster than the fastest estimate offered by Spieth and Zubrin. Powerful as the sun is, it’s a long way from any probe that starts from earth and we wouldn’t be here at all if the photons didn’t spread out and lose their intensity before they got here. Milner proposes a power source that will provide a much more intense blast of photons: a giant laser.

In an interview with the Atlantic, Milner proposed a massive array of solar panels charging capacitors, then unleashing the lot in a torrent of photons that would accelerate the probes up to 0.2c in a few minutes. Paul Gilster of the Centauri Dreams blog, which functions as a discussion forum for serious discussion about spaceflight, calculates that such a laser would need a power of around 100 gigawatts. That’s vastly more powerful than


Artist’s impression of a lightsail being boosted by a laser on earth (Kevin Gill [CC / Flickr])

any laser in regular use, although it’s only about a thousandth of the power of Osaka University’s LFEX, the most powerful laser ever fired. However, LFEX can only fire for around a trillionth of a second while Milner is talking about sustaining power for several minutes. That creates a whole suite of problems around storing enough energy to sustain the burst and preventing the laser from melting itself while firing, as well as designing probes that won’t be vaporised by the burst or tear their precision instruments to pieces as they accelerate from zero to a fifth of the speed of light in those few minutes.

Nobody ever said interstellar flight was easy.

If Breakthrough Starshot works, the robot probes won’t stop to admire the view once they get to Proxima Centauri. They’ll be moving so fast that they’d cover the distance between earth and sun in less than 45 minutes. They’ll have time to snap a few pictures, make a few readings with their sensors and send the data back to earth before they whizz out of the other side of the Proxima Centauri system. Four years later, we’ll receive the data and for the first time, we’ll have close-up observations of another star and the planet or planets orbiting it.

So much for what may be done. As a science fiction writer, I can’t look at things like this without asking myself whether sails could be used for a manned mission. At the moment,


NASA’s NanoSail-D was lost on launch, although NASA has subsequently deployed lightsails successfully (NASA’s Marshall Space Flight Center [CC / Flickr])

the limitation is that to accelerate a large enough spacecraft to keep a crew alive and sane for any length of time better than a rocket, a sail would have to be larger than is manageable with current technology.

The magnetic sail

In a different paper, Zubrin suggested an alternative approach to solar sailing. The sun doesn’t only emit photons, but also protons and electrons. As they have a charge, they would interact with a magnetic field. Zubrin and his colleague, Andrew Martin, suggested that a spacecraft could be powered by a loop of cable carrying an electrical current, generating an electromagnetic field for those charged particles to push against. Unlike the solar sail, the sail wouldn’t have to physically cover the space it was collecting solar particles from and for a manned mission, it would have the added advantage that it would protect the crew from the radiation that scours everything outside the earth’s magnetosphere.

Unlike photons, protons and electrons move at a mere five hundredth of the speed of light, so they would only accelerate the magnetic sail at 0.02m/s2 and couldn’t push it along any faster than 650km/s. That’s at the lower end of Spieth and Zubrin’s estimates for the speed of a lightsail and it wouldn’t get near Proxima Centauri any time this millennium, but it’s still around forty times faster than New Horizons so it could be useful for missions within the solar system.

Better still is that the magnetic coil could be angled so that the spacecraft need not fly


Zubrin & Martin’s schematic of a spacecraft powered by a magnetic sail (NASA Institute of Advanced Concepts)

directly out from the sun but could manoeuvre from one planetary orbit to another. Zubrin and Martin describe a 283-day mission to Mars, which is considerably shorter than the two-year transit time that is the current minimum. That mission would be carried out using a coil with a radius of 32km, weighing around six tons. It would tow a 42-ton payload, which is a little less than the Apollo missions that put men on the moon.

The superconductivity problem

As we have yet to see a spacecraft wafted to Mars by the power of protons and electrons trapped in a magnetic field, we may surmise that there is a catch. The catch is that Zubrin and Martin’s concept was based around a superconducting coil. The cables and wires you’re using to power the screen you’re reading on carry electricity reasonably well, but they do lose some of the power of that electricity to heat. That’s good enough for everything we need electricity for right now, but it’s not good enough for a magnetic sail. That would need to carry electricity with perfect efficiency, losing none of its energy in the process. Such a material is called a superconductor, and they do exist but they only work at very low temperatures.

Interplanetary space is very cold. At 77k (-196°C, -321°F), anywhere on earth that was that cold would cause nitrogen to condense out of the air and flow like water. Unfortunately, that’s not cold enough for superconductivity.

When Zubrin and Martin published their paper, the most promising superconductors were based on copper oxides which need substantially colder temperatures. Since then, a new class of superconductors based on fluorine iron arsenide has been discovered, which superconduct at 52k (-221°C, -366°F). That’s still not cold enough for a magnetic sail, but

Artist's impression of the planet orbiting Proxima Centauri

Artist’s impression of the surface of Proxima b, with Proxima Centauri on the horizon (ESO/M. Kornmesser)

it’s closing in on the right range and researchers are still trying out the full range of compounds so they may come up with something better.

The sails of the future

So will we see more robots sailing around the solar system, or out of it, any time soon or will IKAROS be the closest we ever came to realising Arthur C Clarke’s vision of solar yachts?

Milner’s hundred million says it won’t. He’s talking about sending his starshot to Proxima Centauri in around 20 years. To do that, he’ll need prototypes so if he’s serious, we’re likely to see them zipping around our solar system in the next decade or so.

We haven’t had a good look at Neptune since Voyager 2 took a few photographs in 1989. Perhaps it’s time for a closer look.

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Posted in Inspirations, Wednesday Pontification

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