The Electronics Research Laboratory at the University of Helsinki has successfully constructed a 1-km-long electric sail (ESAIL), which would interact with the solar wind (charged particles from the sun) to produce long-distance propulsion power for a spacecraft.

Using ultrasonic welding, the feat proves that manufacturing full-size ESAIL tethers is possible. Experts previously considered it impossible to weld together such thin wires.

Invented by Dr. Pekka Janhunen at the Finnish Kumpula Space Centre in 2006, the ESAIL consists of long, thin (25–50 micron) electrically conductive tethers manufactured from aluminum wires. A full-scale sail can include up to 100 tethers, each 20 kilometers long.

How it works

The electric field of the charged tethers will extend approximately 100 meters into the surrounding solar wind plasma. Charged particles from the solar wind crash into this field, creating an interaction that transfers momentum from the solar wind to the spacecraft.

In addition, the craft will contain a high-voltage source and an electron gun that creates a positive charge in the tethers.

Compared with other methods, such as ion engines, the electric sail produces a large amount of propulsion considering its mass and power requirement. Since the sail consumes no propellant, it has, in principle, unlimited operating time.

The deployed tethers are kept straight in space by a centrifugal force of five grams in a full-scale electric sail.

According to the Electronics Research Laboratory, a 1000 kg spacecraft with 100 electric-sail wires, each 20-km long, could produce acceleration of about  1 mm per square second. After acting for one year, this acceleration would produce a final speed of 30 km/s (67108 miles per hour), and would continue to acccelerate.

Note that the ESAIL has nothing in common (aside from the Sun as a power source) with the traditional solar light sail, which is propelled by photons.

How it’s made

A single metal wire is not suitable as an ESAIL tether because micrometeoroids present everywhere in space would soon cut it. So the tether must be manufactured from several wires in parallel, joined together every centimeter; which will allow micrometeoroids to only cut individual wires without breaking the entire tether.

The wire is produced with a fully automated, precision mechanical device under computer control, developed and constructed by the team. The device at the Kumpula Science Campus in Helsinki is integrated into a modified commercial ultrasonic welding device.

Space testing

The theoretically predicted electric sail force has not yet been experimentally measured, but it will be tested on the ESTCube-1 satellite, an Estonian small satellite to be launched in March 2013. ESTCube-1 will deploy a 15-meter-long tether in space and measure the force it is subjected to.

The Aalto-1 nanosatellite from Aalto University, to be launched in 2014, will deploy a 100-meter-long tether.

The EU funding contribution to the ESAIL project is 1.7 million euros.

Applications

According to Wikipedia:

• Fast mission (>50 km/s or 10 AU/year) out of the Solar system and heliosphere for small or modest payload
• As a brake for small interstellar probe which has been accelerated to high speed by some other means such as laser lightsail
• Inward-spiralling mission to study the Sun at closer distance
• Two-way mission to inner Solar System objects such as asteroids
• Off-Lagrange point solar wind monitoring spacecraft for predicting space weather with longer warning time than 1 hour

Electric Solar Wind Sail (E-sail)