mission designed to orbit as close as 7 million km (9.86 solar radii) from Sun center. WISPR employs a 95 • radial by 58 • transverse field of view to image the fine-scale structure of the solar corona, derive the 3D structure of the large-scale corona, and determine whether a dust-free zone exists near the Sun. WISPR is the smallest heliospheric imager to date yet it comprises two nested wide-field telescopes with large-format (2 K × 2 K) APS CMOS detectors to optimize the performance for their respective fields of view and to minimize the risk of dust damage, which may be considerable close to the Sun. The WISPR electronics are very flexible allowing the collection of individual images at cadences up to 1 second at perihelion or the summing of multiple images to increase the signal-to-noise when the spacecraft is further from the Sun. The dependency of the Thomson scattering emission of the corona on the imaging geometry dictates that WISPR will be very sensitive to the emission from plasma close to the spacecraft in contrast to the situation for imaging from Earth orbit. WISPR will be the first 'local' imager providing a crucial link between the large-scale corona and the in-situ measurements.

Aims. We present the design and pre-launch performance of the Solar Orbiter Heliospheric Imager (SoloHI) which is an instrument prepared for inclusion in the ESA/NASA Solar Orbiter mission, currently scheduled for launch in 2020. Methods. The goal of this paper is to provide details of the SoloHI instrument concept, design, and pre-flight performance to give the potential user of the data a better understanding of how the observations are collected and the sources that contribute to the signal. Results. The paper discusses the science objectives, including the SoloHI-specific aspects, before presenting the design concepts, which include the optics, mechanical, thermal, electrical, and ground processing. Finally, a list of planned data products is also presented. Conclusions. The performance measurements of the various instrument parameters meet or exceed the requirements derived from the mission science objectives. SoloHI is poised to take its place as a vital contributor to the science success of the Solar Orbiter mission.

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The technology for a steerable spacecraft radiator with a fully functional rotary joint capable of operating with anhydrous ammonia would be very beneficial for thermal control systems onboard future spacecraft. Different slipring rotary joints capable of continuous 360 deg rotation and a flexible pressure line joint or "twist capsule" with oscillatory motion were investigated. A rotary joint with commercial-off-the-shelf ethylene propylene diene monomer O rings and one with ethylene propylene diene monomer O rings that were preconditioned to be compatible with ammonia were designed and tested, and they were determined to not be viable candidates for a rotary seal with ammonia working fluid. A rotary joint with ultrahigh-molecular-weight polyethylene lip seals was also explored and has demonstrated at least an equivalent 15 year life in low Earth orbit. The ultrahigh-molecular-weight polyethylene lip seal rotary joint has a dynamic sealing capability from at least −15 to 70°C. A flightlike unit is being built with enhanced fidelity to be tested and qualified for flight. In parallel to the 360 deg rotary joints, a twist capsule rotary joint was also designed and tested. The twist capsule oscillates between 0 and 270 deg, and the first-generation model demonstrated a life capability equivalent to 27 years in low Earth orbit. An improved flightlike twist capsule rotary joint is in development to be tested and qualified for flight. Nomenclature Q L = leak rate of the rotary joint, standard cm 3 ∕s V = volume of rotary joint ΔP = pressure drop of rotary joint, bar Δt = change in time, s