Graphene could be a suitable material for use as a solar sail with which to propel lightweight space probes. However, little is known about how the material reacts to a laser, required for the propulsion. Scientists of the GrapheneX team from Delft University of Technology have therefore conducted tests in a German drop tower. Under microgravity conditions, they will direct a strong laser at a test piece of graphene.

For a moment, forget the rocket motors and the rockets needed to transport people to the International Space Station, the moon or Mars. Optimistic scientists and financiers are dreaming of launching small, lightweight space probes to the closest star system.

One of the ways of propelling such a probe is by using a solar sail, a large sail made from an ultralight material onto which laser light is shone from Earth. The accumulated impulse of the photons in the light propels the sail – and therefore the probe – towards the stars. Calculations have shown that it should reach around a fifth of the speed of light.


Graphene

The first step is to develop a usable solar sail. A team of four scientists at TU Delft is currently studying one of the possible materials: graphene. This is an ultra-thin and relatively extremely strong material comprising only carbon atoms, in a single layer of just one atom thick. Graphene has been the firm favourite of material researchers over the last few years.

The tests are being conducted in the drop tower of the Centre of Applied Space Technology and Microgravity (ZARM) at the University of Bremen, from a height of 146 m (see photo). The team won the experiment time and financing by participating in the Drop Your Thesis! programme of ESA Education.


Nine seconds

The idea of the tower is that a capsule is catapulted vertically upwards, to subsequently fall just as quickly. The capsule is weightless during the 9.3 seconds of the flight. ‘That's almost perfect, comparable to one millionth of the Earth's gravitational force, due to the vacuum in the tower', explains Vera Janssen, one of the GrapheneX team researchers and experimental physics doctoral student at the University of Leiden.


Pilot

Janssen and her colleagues designed a pilot for in the capsule, in order to test the effect of a strong laser shining on a sheet of graphene. That graphene has been secured to a copper frame in the capsule. Without being secured, the graphene would crease or fold. The test piece is circular and is 2 mm in diameter.

During testing, a 1 W laser is shone on this test piece, to determine how far it is transported during the nine seconds of weightlessness. An identical reference test piece alongside it is not propelled by the laser and will therefore remain stationary. ‘We expect to see around 2 mm difference between the two test pieces', says Janssen. Due to the great sensitivity of the feather-light test piece, the vacuum in the capsule is even more effective, at 0.003 mbar. The force exerted by the laser on the test piece is minuscule: 5∙10-5 N/kg, comparable with the force exerted by an object weighing 0.5 mg.


Measurements

The initial testing of the set-up took place on Monday, and the GrapheneX team has conducted measurements all week. ‘As far as we know, we're the only ones conducting such tests', says Janssen. The solar sail is actually an exotic application of graphene for the team members. They generally concentrate on more down-to-earth applications of graphene membranes, such as pressure sensors.

More information in this press release from TU Delft.

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