ESA Science & Technology - Solar Orbiter Executive Report

The Mission

The Solar Orbiter mission will provide the next major step forward in the exploration of the Sun and the heliosphere to solve many of the fundamental problems remaining in solar and heliospheric science. It incorporates both a near-Sun and a high-latitude phase.

The near-Sun phase of the mission enables the Orbiter spacecraft to approach the Sun as close as 48 solar radii (~0.22 AU) during part of its orbit, thereby permitting observations from a quasi-helio-synchronous vantage point. At these distances, the angular speed of a spacecraft near its perihelion approximately matches the rotation rate of the Sun, enabling instruments to track a given point on the Sun surface for several days.

During the out-of-ecliptic phase of the mission (extended mission), the Orbiter will reach higher solar latitudes (up to 35º in the extended phase), making possible detailed studies of the Sun’s polar caps by the remote-sensing instruments.

Science Requirements

The Sun's atmosphere and the heliosphere represent uniquely accessible domains of space, where fundamental physical processes common to solar, astrophysical and laboratory plasmas can be studied under conditions impossible to reproduce on Earth or to study from astronomical distances.  The Solar Orbiter mission, through a novel orbital design and an advanced suite of scientific instruments, will aim to help scientists achieve major breakthroughs in the understanding of solar and heliospheric physics by:

• Exploring the uncharted innermost regions of our solar system
• Close-up study of the Sun
• Fly by the Sun tuned to its rotation and examine the solar surface and the space above from a co-rotating vantage point
• Provide images & spectral observations of the Sun polar regions from out of the ecliptic

As a result, the following scientific goals for the mission have been identified:

• Determine the properties, dynamics and interactions of plasma, fields and particles in the near-Sun heliosphere
• Investigate the links between the solar surface, corona and inner heliosphere
• Explore, at all latitudes, the energetics, dynamics and fine-scale structure of the Sun’s magnetized atmosphere
• Probe the solar dynamo by observing the Sun’s high-latitude field, flows and seismic waves.

Mission Requirements

The main missions requirements are outlined below:

• Launcher vehicle - Soyuz Fregat 2-1B (from CSG)
• Total cruise duration - shorter than 3 years (goal)
• Orbital period in 3:2 resonance with Venus
• At least one orbit with perihelion radius < 0.25 AU and > 0.20 AU (science phase)
• Inclination with respect to solar equator increasing to a minimum of 30 deg
• During the extended operational lifetime, the Solar Orbiter operational orbit shall reach an inclination with respect to solar equator not lower than 35 deg (goal)
• Support a payload of 180 kg and 180 W (including 20% maturity margins)

Mission Profiles

The Solar Orbiter mission design is based on a cruise phase and a science phase. The cruise phase comprises the Earth escape and a trajectory that remains close to the ecliptic in order to bring the Orbiter into a Venus resonant orbit. The mission design for the science phase, which begins with a Venus Gravity Assist Manoeuvre (GAM) reducing the perihelion distance.  Later Venus GAMs increase the orbital inclination to more than 30 degree with respect to the solar equator.

Two possible mission scenarios are under consideration.  The originally preferred scenario uses Solar Electric Propulsion and the alternative approach uses traditional chemical propulsion in Deep Space Manoeuvers in the cruise phase. 

Based on initial analysis of these two mission scenarios the following conclusions have been reached:

• Both the baseline and the back-up profile can be considered as feasible, although with different levels of development risk and corresponding cost (higher in the case of the baseline and lower for the chemical profile)
• The chemical trajectory is characterised by a longer cruise time (3.3 against 1.8 yr) and by the fact that a slightly lower maximum heliospheric inclination would be achieved (33.9 against 34.9 deg) at a later time (requiring an additional Venus GAM during the extended mission phase)
• Although presenting a longer cruise time, in the case of the chemical profile science operations could start during cruise
• If selected, a chemical profile, while having a modest impact on the science objectives, would enable to lower the overall programme risk.

Conclusions

The assessment study of the Solar Orbiter has addressed all mission areas, from the scientific requirements to the payload complement, the space and ground segments, including all corresponding programmatic aspects.

Specific attention has been given to the reference payload, in the form of a dedicated industrial study as well as of internal activities, in order to prepare adequately for the future payload AO and maintain a certain degree of control over the corresponding spacecraft resources. These activities have indicated that, given the number and complexity of the instruments on board, such an attention should also be given in the following mission phases. The system level study has indicated that two mission profiles are viable:

a) Baseline profile, assuming Solar Electric Propulsion and a 1.8 year cruise phase (with higher development risk)
b) Back-up profile, using chemical propulsion, with a 3.3 year cruise phase (with lower development risk).

In both cases, all critical design drivers have been analysed and, while design challenges do exist, no major feasibility questions have been raised, showing a feasible mission, compatible with the nominal launch date of October 2013.