Participants concluded that bringing the launch date forward from 2012, as now planned, would optimise the use of technology developed for Bepi Colombo, ESA's mission to Mercury due to be launched in 2009, and maximise synergy with solar missions planned by other agencies.

The workshop was the first opportunity since Solar Orbiter was formally approved last October, for the scientific community to discuss the mission plan and objectives. "With 150 or so participants, the workshop was very well-attended considering that the mission is so far in the future. This shows that there's a high level of interest in the scientific community," says Richard Marsden, a member of the Solar Orbiter study team from ESTEC and one of the workshop organisers.

Solar Orbiter will approach the Sun to within two tenths of the Sun-Earth distance (0.2 AU). This fact alone presents enormous scientific opportunities, but also technological challenges. The major opportunity will be to understand 'space weather' which is delivered to Earth by the solar wind streaming out from the Sun's atmosphere or corona.

"If you really want to understand the Sun, you need to have instruments that 'look' at it and 'feel' what comes out at the same time," says Richard Harrison from the Rutherford Appleton Laboratory in the United Kingdom, who was also on the Solar Orbiter study team and helped to organise the workshop.

That means combining key features of SOHO (the Solar and Heliospheric Observatory) and Ulysses, ESA's current highly successful solar and heliospheric missions. Like SOHO, Solar Orbiter will image the Sun and like Ulysses, it will sample the heliospheric environment from an orbit above the ecliptic plane. The close orbit will enable what is 'felt' to be linked to what is 'seen', leading to a better understanding of how coronal events cause space weather on Earth. Coronal structures just 35 km across will be resolved.

Assisted by a series of Venus swing-bys, the spacecraft's 150-day orbit will evolve gradually over the mission lifetime from an inclination of about 12 to 38 degrees to the solar equator. For several days around perihelion (closest approach to the Sun), the orbit will keep pace with the Sun's rotation thus allowing structures in the corona and at the surface of the star to be monitored as they develop.

When the spacecraft is above high latitudes, it will image the polar regions, enabling variations in the Sun's global emission to be measured accurately for the first time. SOHO or other Earth-orbiting satellites see the Sun's disc from a single perspective and so cannot provide global measurements. "How do we know that changes we see with SOHO are universal?" asks Harrison. "It could be that when the equator brightens, the poles dim. We just donít know, but Solar Orbiter should provide the answer," he adds.

A major technical challenge will be coping with high temperatures and a surfeit of sunlight, an unusual problem in space science missions which more commonly encounter low temperatures and scarce photons. "Twenty five times as much sunlight will fall on Solar Orbiter as on a spacecraft in Earth orbit. We need to get rid of 99% of this light so as not to damage the coating on the optics, or melt the glue that keeps the solar cells attached to the solar panels," says Marsden.

Another challenge is how to use propulsion and Venus swing-bys to brake the spacecraft gradually into the desired orbit. Space science missions more commonly need to accelerate away from the Sun. Solar Orbiter will use solar electric propulsion (SEP) which is under development for Bepi Colombo, ESA's other inner solar system mission. SEP will be tested on SMART-1, one of ESAís technology demonstration missions.

"The thrust level is lower with SEP than with conventional chemical propulsion, so you use thrust for weeks at a time instead of in short sharp bursts. This means that you can continually change the flight path and so manoeuvre into a complex orbit, like that of Solar Orbiter, more efficiently than with conventional thrusters," says Marsden.

Perhaps the biggest challenge of all will be to achieve the mission's objectives within the budget of a flexi-mission, about 200 million Euro. Clearly-defined and focused objectives are called for, as are well-designed instruments.

"We would like the measurement capability of SOHO, but with a payload of 100 kg instead of 650 kg, so we are looking at advanced technology and miniaturisation," says Marsden. The workshop established a payload working group to explore issues of instrument design further, including any technological developments that may be referred to ESA's newly-established Science Payload and Technology Division.

Solar Orbiter is attracting interest beyond Europe and the workshop made a strong plea for international coordination of future solar missions, which was swiftly accepted. The wide attendance from beyond Europe meant that the Inter Agency Consultative Group (IACG) on space science could meet and it was quickly agreed that Solar Orbiter should become a major element of NASA's 'Living with a Star' programme which begins in 2007. However, Solar Orbiter would complement appropriate missions in this programme, especially NASA's Solar Dynamics Observatory (SDO), if its launch date were brought forward to 2010.

For more information please contact:

Dr. Richard Marsden
Tel: +31 71 5653583
Email:Richard.Marsden@esa.int

Dr. Bernhard Fleck
Tel: +1 301 286 4098
Email: bfleck@esa.nascom.nasa.gov

ESA Science Programme Communication Service
Tel: +31 71 5653183