'50 nano spacecraft proposed to explore 300 asteroids'

Finnish scientists have proposed a fleet of 50 tiny spacecraft, propelled by innovative electric solar wind sails, which could visit more than 300 asteroids in just over three years.
The Asteroid Touring Nanosat Fleet concept comprises 50 spacecraft equipped with instruments to take images and collect spectroscopic data on the composition of the asteroids, according to scientists at the Finnish Meteorological Institute (FMI) in Helsinki.
Each nanosat would visit six or seven asteroids before returning to Earth to deliver the data, they said.
"Asteroids are very diverse and, to date, we have only seen a small number at close range. To understand them better, we need to study a large number in situ," said Pekka Janhunen, scientist at FMI.

"The only way to do this affordably is by using small spacecraft," said Janhunen, who presented the concept at the European Planetary Science Congress (EPSC) in Latvia.
In the mission scenario, the nanosats flyby their target asteroids at a range of around 1,000 kilometres.

Each nanosat carries a four-centimetre telescope capable of imaging the surface of asteroids with a resolution of 100 metres or better.
An infrared spectrometer analyses spectral signatures in light reflected or emitted by the asteroid to determine its mineralogy, researchers said.
The instruments can be pointed at the target using two internal reaction wheels inside the nanosats, they said.

"The nanosats could gather a great deal of information about the asteroids they encounter during their tour, including the overall size and shape, whether there are craters on the surface or dust," said Janhunen.
"They would also gather data on the chemical composition of surface features, such as whether the spectral signature of water is present," he said.
Electric solar wind sails (E-sails) make use of the solar wind - a stream of electrically charged particles emitted from the Sun - to generate efficient propulsion without need for propellant.
Thrust is generated by the slow rotation of a tether, attached at one end to a main spacecraft carrying an electron emitter and a high-voltage source and at the other to a small remote unit.

The spinning tether completes a rotation in about 50 minutes, tracing out a broad, shallow cone around a centre of mass close to the main spacecraft.
By altering its orientation in relation to the solar wind, the nanosat can change thrust and direction.
The thrust generated by E-sails is small - a five kilogramme spacecraft with a 20-kilometre tether would give an acceleration of one millimetre per second at the distance of the Earth from the Sun.
However, calculations show that, on top of the initial boost from launch, this is enough for the spacecraft to complete a tour through the asteroid belt and back to Earth in 3.2 years, researchers said.