Stevenage firm to keep ESA solar mission on target

Tessella in Stevenage is to assist in the design and development of the Attitude and Orbit Control Subsystem (AOCS) for the European Space Agency’s (ESA) Solar Orbiter mission.

Due to launch from Cape Canaveral in 2017, the Solar Orbiter spacecraft is the first mission in ESA’s Cosmic Vision programme to start its implementation phase.

During its seven-year mission it will help further understanding of the Sun and its effects on the solar system by carrying a payload made up of a suite of in-situ and remote-sensing instruments that will measure the particles, fields and waves of the plasma through which it travels, and simultaneously make observations of the Sun’s surface and outer atmosphere, photosphere and corona.

At its closest point to the Sun Solar Orbiter will be at a distance of 0.28AU (42 million kilometres), in an orbit that takes it out of the ecliptic plane. To position itself in this orbit, the spacecraft will make a series of gravitational-assist fly-bys past Earth and Venus.

Solar Orbiter’s sensing instruments will be protected by a heat-shield that is designed to give each one the required field-of-view and the spacecraft itself will have to point them permanently at the sun whilst it travels along precise orbits dedicated to specific science investigations.

‘If we go more than a few degrees away from the Sun the mission will be lost,’ said Dave Dungate, Tessella’s senior technical consultant.

The contract to deliver the AOCS was awarded to OHB Sweden, said Dungate.

OHB will procure the software and hardware required for the AOCS, including sensors (star tracker, inertial measurement unit, fine sun sensor) and actuators (reaction wheels). They’ve also been contracted to supply the Chemical Propulsion System (CPS) for Solar Orbiter.

OHB will also be responsible for the algorithms for fault detection, isolation and recovery, providing the test bench facility on which the AOCS will be verified.

Under a four year contract worth over €4m, Tessella will design, analyse, simulate and develop all the algorithms and demonstrate that performance requirements are met.

Dungate explained that Tessella’s algorithms will help detect and correct any anomalies that cause Solar Orbiter to point away from the Sun or out of the correct orbit.

He said, ‘We have a mode that we have to design for the spacecraft to control its pointing.

‘If there’s a failure on board, or another issue, it goes into this mode and there are very stringent requirements about not pointing away from the sun. If something goes wrong we have to detect it, kill the problem and switch to alternative suites of sensors or actuators and then bring it back and keep it safe.

‘We need to do that in the context of not becoming unstable and not exciting flexible modes on the spacecraft too much. It comes down to the algorithm design to do very careful mathematical analysis and trade-offs to optimise the balance between those things.’

Solar Orbiter is a collaboration between ESA and NASA – a launcher will be provided by NASA with one instrument and one sensor contributed from the United States.

During its mission Solar Orbiter will provide scientific data that will further the understanding of events such as coronal mass ejections, which are bursts of high-energy particles that can disrupt electrical power distribution systems, cause computers to crash, damage satellites and endanger astronauts.