European Solar Polar Orbiter Mission



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The Solar Polar Orbiter: A Solar Sail Technology Reference Study

Malcolm Macdonald,2 Gareth W. Hughes,3



University of Glasgow, Glasgow, Scotland.

Colin R McInnes,4



University of Strathclyde, Glasgow, Scotland.

Aleksander Lyngvi,5 Peter Falkner,Error: Reference source not found Alessandro AtzeiError: Reference source not found



European Space Agency, Noordwijk, The Netherlands.

Abstract


This paper presents an assessment of a Solar Polar Orbiter mission as a Technology Reference Study. The goal is to focus the development of strategically important technologies of potential relevance to future science missions. In this paper the technology is solar sailing, so the use of solar sail propulsion is thus defined a priori. The primary mission architecture utilizes maximum Soyuz Fregat 2-1b launch energy, deploying the sail shortly after Fregat separation. The 153 × 153 m square sail then spirals into a circular 0.48 AU orbit, where the orbit inclination is raised to 90 deg `with respect to the solar equator in just over 5 years. Both the solar sail and spacecraft technology requirements have been addressed. The sail requires advanced boom and new thin-film technology. The spacecraft requirements were found to be minimal, as the spacecraft environment is relatively benign in comparison with other currently envisaged missions, such as the Solar Orbiter mission and BepiColombo.

Introduction


The Science Payload and Advanced Concepts Office of ESA (European Space Agency) have introduced Technology Reference Studies (TRS) to focus the development of strategically important technologies of likely relevance to future science missions. This is accomplished through the study of technologically demanding and scientifically interesting missions, which are not part of the ESA science programme. This paper discusses one such mission, the Solar Polar Orbiter (SPO). The TRS cover a wide range of mission profiles with an even wider range of strategically important technologies. All TRS mission profiles are based on small satellites, with miniaturized highly integrated payload suites, launched on a Soyuz Fregat 2-1b.1
Science missions are technologically very challenging. It is important to define and prepare critical technologies far in advance, hence ensuring they are developed in a timely manner and that associated cost, risk and feasibility of potential future mission concepts can be properly estimated. The TRS are set up to provide a set of realistic requirements for these technology developments far before specific science missions are proposed by the scientific community. Through their study a set of detailed requirements for technology development activities can be determined. The TRS are a tool to focus technology development activities; they are not part of ESA’s science mission programme. A TRS is carefully selected to address a wide range of technologies that have to be applicable to many other scientific mission profiles.
Terrestrial observations of the Sun are restricted to the ecliptic plane and within the solar limb, thus restricting observations to within ± 7.25 deg of the solar equator. Close solar measurements at all latitudes are necessary to achieve a global three-dimensional picture of solar features and processes. Observations directly over the solar poles are imperative to understanding the Sun. Most previous missions to study the Sun have been restricted to observations from within the ecliptic. The Ulysses spacecraft used a Jupiter gravity assist to pass over the solar poles, obtaining field and particle measurements but no images of the poles.2 Furthermore the Ulysses orbit is highly elliptical; with a pole revisit time of approximately 6 years. It is desired that future solar analysis be performed much closer to the Sun, as well as from an out-of-ecliptic perspective. The Solar Orbiter mission scheduled for launch in October 2013 intends to deliver a science suite of order 180 kg to a maximum inclination of order 35 deg with respect to the solar equator and to a minimum solar approach radius of 0.22 AU using SEP (Solar Electric Propulsion).3 The inability of the Solar Orbiter mission to attain a solar polar orbit highlights the difficulty of such a goal with conventional propulsion. A 1998 study considered the use of solar sail technology to place a science payload into a solar polar orbit.4 Reference Error: Reference source not found defined a 164 kg spacecraft, using a 6 g m-2, 158 × 158 m solar sail and a cruise time of 4.6 yrs. Within this prior study the definition of solar sail technology requirements is imprecise due to the technology status of solar sail hardware at the time.
The primary objective of the mission presented in this paper is to deliver a spacecraft into an orbit at 90 deg inclination with respect to the solar equator, using a launch vehicle no larger than the Soyuz Fregat 2-1b. The spacecraft orbit should be phased such that once on-station it will remain near to the solar limb from a terrestrial perspective. The spacecraft should also be positioned on an orbit interior to Earth’s.
This paper summarizes the output from the SPO TRS, allowing definition of key technology requirements for this class of solar sail mission. The SPO TRS draws some significantly different conclusions from similar previous studies, many of which are due to the fundamentally different methodologies utilized; these will be discussed within this paper. In particular this paper develops the mission concept with realistic orbit trajectory generation to the actual science orbit rather than some approximation of this orbit. The SPO spacecraft systems are fully defined within the technology limits of the mission timeframe and with consideration of the limitations due to the use of solar sail propulsion, such as pointing accuracy due to sail flexing. The solar sail system and technology requirements are also fully defined. A full range of mission architectures have been investigated in order to ensure that an optimal reference mission is generated. Furthermore, the global effect of varying the solar close approach radius is considered for the first time through amalgamation of trajectory and spacecraft / sail systems into one complete analysis.




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