Venus sees the light

A measurement system has been developed that will help scientists track the Venus Explorer spacecraft with extraordinary accuracy. Venus is 41 million kilometres from Earth yet ESA's first mission to the planet will be pinpointed with an accuracy of better than one billionth of a second. And this will be achieved by monitoring the energy from a quasar hundreds of millions of light years away.

The craft is due to enter orbit this week, five months after being launched from the Baikonur Cosmodrome, Kazakhstan. Its mission is to gather information about the atmosphere, plasma and surface of the planet and it will orbit Venus for 500 days. But getting it into precisely the right orbit is vital.

The measurement technique, called Delta-DOR, will be used to make sure the spacecraft is slowed down just the right amount so that it enters its orbit correctly. It has been developed by BAE Systems, ESA and the University of Rome for the UK company's Intermediate Frequency Modem System equipment (IFMS).

'This is the first time ESA will be using our equipment with its newly developed and especially accurate Delta-DOR technique,' said Andy Baslington, BAE Systems programme manager. 'This works by measuring the difference in time that it takes for signals from the spacecraft to reach two different receiving stations and how this varies over a 30-minute period.'

The receiving stations with the IFMS boxes are in Spain and Australia. The clocks of both are synchronised by comparing the energy given off by a specific quasar, an astronomical body powered by a gigantic black hole. With that done, the different times at which the stations receive signals from the Venus Express can be accurately measured using Delta-DOR and the spacecraft's precise position and velocity can be calculated.

Location and velocity data will be used by the mission controllers at the ESA Space Operations Centre in Darmstadt, Germany, to prepare for orbit insertion. This is a series of telecommands, engine burns and manoeuvres to cut the spacecraft's velocity of 29,000km per hour relative to Venus, just before the first burn, to an entry velocity some 15 per cent slower, allowing the spacecraft to be captured into orbit around the planet.

The spacecraft will have to ignite its main engine for 50 minutes to achieve deceleration and place itself into a highly elliptical orbit around the planet. Most of its 570kg of on-board propellant will be used for this manoeuvre. The craft's solar arrays will be positioned so as to reduce the possibility of excessive mechanical load during engine ignition.

Over the subsequent days a series of additional burns will be carried out to lower the orbit apocentre and to control the pericentre. The aim is to end up in a 24-hour orbit around Venus early in May.

Scientists hope the information gathered during the Venus Express mission will shed further light on the mechanisms of climate change on our own world. The main body of the spacecraft is 1.65m x 1.7m and 1.4m high, on to which all the payload instruments are integrated. The solar arrays, with a collecting area of 5.7m2, will provide 1,100W of power once the spacecraft is in orbit around Venus.