Solar sailing seems like a simple concept—instead of being pushed along by the wind, as in a typical sailing ship, a spacecraft can use highly reflective said to be pushed along simply by sunlight. But as with almost all engineering challenges, that technique is much easier said than done. Sunlight can head up one side of a sail more than another, causing the ship to rotate unexpectedly. Other unforeseen situations could arise that can also have catastrophic consequences for any mission using this propulsion technology.

Luckily, there is a way to account for those situations, though it involves a lot of math. Control theory is common in system design, and now researchers at Beihang University have devised a control scheme that they think could help minimize the risk to solar sails.

Control systems are relatively easy to understand in concept. A system has a series of inputs; in the case of a solar sail, those inputs might be the solar force pushing on it from the sun and maybe the gravity of any nearby object.

Galileo has grown to become Europe's single largest satellite constellation, and the world's most accurate satellite navigation system, delivering meter-level positioning to more than 3.5 billion users around the globe.

It all began at ESTEC's Test Center, Europe's largest satellite testing facility. This is where the very first positioning fix took place in March 2013, after the launch into orbit of the initial four IOV satellites. Following that, all 34 Galileo Full Operational Capability satellites also passed by ESTEC for their pre-flight testing.

This 3000 sq. m environmentally-controlled complex, operated and managed by European Test Services for ESA, hosts an array of test equipment able to simulate all aspects of spaceflight, from the noise and vibration of launch to the vacuum and temperature extremes of Earth orbit.

The production line at manufacturer OHB in Germany completed one new satellite every six weeks. After integration each satellite was then shipped to the ESTEC Test Center for a three-month test campaign, after which it would be accepted by the Agency and declared ready for flight. Some facilities have had to be adapted specifically for Galileo, and the ESTEC Test Center had to institute new security protocols because this was the first time that satellites with security restrictions were being tested at the site.