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13th February 2019
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Fuelled by photons

Richard Corfield, 13 February 2019

018 Photons web

It took the Cassini-Huygens spacecraft 12 years to reach Saturn and Titan. With nanocraft-equipped space sails and a photon-engine, instruments could well be delivered to the gas and ice giants within hours or days, Richard Corfield reports

Spaceflight is a high-risk business. Spacecraft break down all the time and when that happens funding and careers evaporate. Back in the late 1960s, NASA decided to double the odds of success and send two spacecraft on one mission. Voyagers 1 and 2, for example, were the spacecraft that returned the first detailed pictures of the outer planets of our solar system and introduced us to the neighbourhood. Launched in 1977, both are still flying.

Any spacecraft must have three components: a payload, an engine and a fuel supply – by far the heaviest component. But what if we could do away with the onboard fuel supply and replace it with an external fuel supply? Say light itself?

The idea of solar sail technology has been floating around for decades. Indeed, the notion of a solar pressure can be traced back to 1610 in a letter that Johannes Kepler wrote to Galileo. But it was only in the 20th century that solar sails began to be considered as an achievable engineering reality. Broadly, solar sails fall into two categories: those using light from natural sources – ie the sun and ambient starlight in space; and those using coherent light from lasers.

The Japanese Aerospace Exploration Agency (JAXA) has already proved that the solar sail works as an engineering reality.1 The IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun) expedition was launched in 2010 and proved that the space sail, weighing 2kg, and measuring 14m x 14m, was successfully imparting momentum to the spacecraft. The sail is made of polyimide resin with a solar cell array embedded in it for momentum and onboard power generation. By varying the reflectance of the sail using embedded LEDs, the IKAROS team was able to steer it. The maximum velocity reached by the IKAROS spacecraft was 400m s-1.

Cube-Sats
Les Johnson, of NASA’s Marshall Space Flight Centre in Alabama, US, is involved in the agency’s own work on solar cells. ‘[We have] just completed our testing of the 86m2 flight sail for the Near Earth Asteroid [NEA] Scout solar sail mission and are integrating the spacecraft now for delivery early next year to the launch site,’ he says. The NEA mission is scheduled to deploy in 2020 from the first engineering test flight of the new Ares/Orion spacecraft.2

The square sail, about 12m along each edge, will harness passive solar energy and, like IKAROS, will gain momentum from it. For a solar sail to work, the mass of the spacecraft needs to be very low. The NEA will use an array of CubeSats, a module of electronics and sensor-containing cubes only 10cm on a side.

<1g
To reach relativistic speeds necessary to reach Proxima Centauri B within 20 years, the Starshot light sail should have an area of ~10m2 and a mass of under ~1 gram, which translates to a thickness of approximately 100 atomic layers.

Solar sails fall into two categories: those using light from natural sources – ie the sun and ambient starlight in space; and those using coherent light from lasers.

In the future, solar cell spacecraft are envisaged as being nanocraft with large sail areas for photons to catch the photon ‘wind’ while carrying a tiny payload.

Sailing high
But even these small spacecraft may soon be rendered obsolete by developments in nanoengineering. In the future, solar cell spacecraft are envisaged as being nanocraft with large sail areas for photons to catch the photon ‘wind’ and carry a tiny payload. The big advantage of solar sails is that they are capable of infinite acceleration; as long as photons keep hitting the sails, the craft will continue to pick up momentum. Under such conditions, very high velocities indeed are achievable.

In 2016, Project Starshot was founded with the objective of sending a probe to Proxima Centauri B, an exoplanet orbiting Proxima Centauri, that appears from spectroscopic measurements capable of supporting life. The project is supported by senior scientists including the late Stephen Hawking who was present at the launch in April 2016.

Harry Atwater and his research group at California Institute of Technology (Caltech), US, have set themselves the ambitious goal of reaching Proxima Centauri B within 20 years of launch, a task that will require the spacecraft to reach about 20% of the speed of light – an almost unimaginable velocity of 40,000 miles/second.

To date, the fastest moving spacecraft was the New Horizons spacecraft to Pluto, which reached speeds of 90,000 miles/hour.

‘The major disadvantage of ambient solar cells is that they are dependent on the intensity of the local photon flux,’ says Artur Davoyan of Project Starshot. ‘At [the location of] Earth the solar pressure on a sail with a surface area of 6400m2 will be 5N, but this decreases rapidly with increasing distance from the sun.’

To make solar sails more effective, it would be useful to make use of coherent light – where the photons are in step; in other words, lasers. There are three main engineering challenges: what material to use for the light sail; how it is deployed and supported; and how to make sure it rides the beam of laser light without falling off.

In a recent paper, Atwater and his team pointed out that, in order to reach relativistic speeds necessary for them to reach Proxima Centauri B within 20 years, the Starshot lightsail should have an area of ~10m2 and be kept to a mass of under ~1 gram, which translates to an equivalent thickness of approximately 100 atomic layers3. This means using nanophotonic structures. Nanophotonics is simply the interaction of particles of light with each other or objects on the nano scale: 0.000000009mm or less.

Nanophotonic structures can be used to harness light radiation and turn it into other forms of energy, such as motion or kinetic energy and electricity. In short, they are ideal for spaceflight.

To design such a sail, several competing factors need to be balanced. These are the reflectivity of the sail, which needs to be high to enable efficient photon momentum transfer; a large bandwidth, because as the spacecraft accelerates the impinging laser light will Doppler shift; and a low mass. The holy grail is what is termed the Figure of Merit (FOM), which must also take into account the total mass of the spacecraft, light sail and payload.

The group has investigated a range of materials and the two that meet these three criteria best are crystalline silicon (c-Si) and molybdenum disulfide (MoS2). Surprisingly, they found that absolute reflectivity was not altogether central to the light sail’s design. Piercing the sail with holes resulted in a minor loss of reflectivity and a major loss in mass.

But the material also has to hardy enough for the environments of space. Studies by Thiem Hoang and colleagues4 at Korea Astronomy and Space Science Institute have shown that the light sail is unlikely to suffer from interstellar particles such as hydrogen and helium, which represent the predominant form of gas found in outer space. Indeed, they calculate these particles may pass through the light sail with little or no interaction. But they do not minimise the problem. ‘The most critical challenge to send a relativistic-speed nanocraft to probe the outer planets in our solar system is the potential damage by big interplanetary dust grains – larger than a few micron – and small objects in the asteroid belt,’ says Hoang. ‘At speeds of v~ 60,000km/s (v ~ 0.2c), a big dust grain behaves like a mini-atomic bomb when hitting the spacecraft, which can completely destroy the spacecraft.’

However, the news is slightly better once the star sail has left the solar system. ‘For sending a relativistic nanocraft outside the solar system, the spacecraft can avoid the asteroid belt, but it still can be damaged by interstellar dust. Larger particles like dust in the interstellar medium are a greater problem. However, modelling studies suggest that even at the relativistic velocities required to reach Proxima Centauri B in 20 years, the space craft should arrive there with 90% of the light sail still functional.’

Tommaso Ghidini, adds a note of caution, pointing out that even though the deep space environment is depleted in potentially damaging space debris, the risk is at present unquantified5. Also, the Project Starshot light sail will start its journey by picking up laser light beamed from the laser array on the Earth’s surface and the area of near earth orbit has a significantly higher density of particles than deep space.

021 Alpha Centauri webImage: NASA / JPL-CALTECH

40, 000 miles/sec
Caltech researchers have set themselves the ambitious goal of reaching Proxima Centauri B (a planet orbiting Proxima Centauri, the nearest star of the Alpha Centauri system, a Red Dwarf named Alpha Centauri C) within 20 years of launch, a task that will require the spacecraft to reach about 20% of the speed of light – or ca 40, 000 miles/sec.

It took 35 years to reach interstellar space, but it will take 40,000 years for Voyager 1 to be closer to the star AC +79 3888 than our sun. Alpha Centauri is the closest star to our own right now, but because stars are moving, Voyager 1 will actually get within 1.7 light years of AC +79 3888 (aka Gliese 445) in 40,000 years.

Shaping up
Once in orbit the space sail will be deployed and as yet, there is no consensus about what shape the sail will take. It may well not be flat but curved, to better focus the laser light from the photon engine on Earth; it may even be spherical for the same reason.

Once deployed, however, it will have to be held rigidly by light weight construction materials. However, the impulse from the photon engine is only for 1000s, after which it will become largely redundant as part of the propellant system. It will, though, still have to be able to transform solar energy into electricity to power the nanocraft hitching a ride on the sail.

An attitude control system is essential for a sail craft to achieve and maintain a desired orientation. Attitude control is achieved by a relative shift between the craft’s centre of pressure and its centre of mass. In the case of a passive space sail, this can be achieved with control vanes, movement of individual sails, movement of a control mass or altering reflectivity.

Not everyone is enamoured with the idea of using a ground – or even space-based photon engine. By combining natural solar sails with the well known gravitational assist manoeuvre invented at NASA’s Jet Propulsion Laboratory in the 1960s, some consider that a spacecraft could be accelerated to interstellar velocities using solar photons alone rather than using additional expensive technologies such as ground-based laser launch systems.

‘I think the solar sail is more realistic,’ says Rene Heller of the Max Planck Institute for Solar System Research in Germany. ‘By using the sun rather than a ground-based laser system at departure from the solar system, the energy required to launch the sail would be limited to the energy required for a usual lift-off of a small satellite. This avoids the need for a > $10bn 100GW laser array completely.’

There are other problems that need addressing with solar sails, not least that of slowing down the spacecraft, especially one that has reached relativistic velocities. Heller, working with Michael Hippke6, points out that spacecraft accelerated to relativistic velocities will cross the Proxima Centauri system in hours or days and then continue into interstellar space. What is needed is a way of slowing the spacecraft down once it reaches Proxima Centauri. Heller and Hippke calculate that this could be achieved using the prevailing photon environment of Proxima Centauri. Specifically they show that the photon pressures from the stellar triple α Centauri A, B, and C – Proxima Centauri itself – can be used, together with gravity assist manoeuvres, to slow the light sail to orbital velocities.

Other places to go
There are other reasons to invest in solar sail technology, not the least of which is exploring our own solar system in real-time. It took the Cassini-Huygens spacecraft 12 years to reach Saturn and Titan. With nanocraft-equipped space sails and the photon-engine, instruments could be delivered to the gas and ice giants within hours or days.

‘The ultimate goal of the relativistic-speed nanocraft is to visit the closest stellar system, Alpha Centauri,’ says Hoang. ‘Such relativistic-speed nanocraft can be of course used to probe outer planets in our own solar system. At speeds of 0.2c [20% of light speed], it only takes 3 hours to reach Jupiter and one day to reach the furthest planet, Neptune.’ It is, to put it mildly, a very exciting time to be a space explorer.

References
1 Science Direct, doi.org/10.1016/j.actaastro.2012.03.032
2 NASA, 2018 solar-sail-test-will-study-near-earth-asteroid
3 H. Atwater et al, Nature Materials, 2018, 17, 861.
4 T. Hoang et al, The Astrophysical Journal, http: iopscience.iop.org/article/10.3847/1538-4357/aa5da6/meta
5 T. Ghidini, Nature Materials, 2018, 17, 846.
6 R. Heller and M. Hippke, The Astrophysical Research Letters, 2017, 85, (2).

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