New Horizons: RALPH visits Pluto

Having started on 19 January 2006, spacecraft New Horizons - as part of the NASA New Frontiers Program - passed by Pluto at a rather close distance in order to learn more about the dwarf planet and its moons. This was the first Pluto exploration mission. The project is led by the Applied Physics Laboratory of the Johns Hopkins University in Baltimore, Maryland, USA. Parts of the spacecraft are optical components developed and realized at the Williamsburg, Virginia LEONI Fiber Optics Inc. production site. The costs including development and production of the spacecraft, its instruments, launching rocket and the mission procedure until 2016, amount to around 700 million US$.

Even the strongest telescopes can hardly detect details on its surface as Pluto is very far from the sun. The resolution of the qualitatively best pictures taken by the Hubble Telescope is only 500 km per pixel! Therefore, you have to get closer to know more about Pluto. By the way, the exploration of Jupiter - which New Horizons passed by in February and March 2007 - is also one of the mission aims. The spacecraft observed cloud movements, investigated the magnetosphere of the planet, and searched for polar lights and lightings in Jupiter’s atmosphere.

As early as 2005, the Williamsburg Team had been involved in the development of the Solar Illumination Assembly (SIA), at that time under RoMack Inc. two SIAs were built. One is at the Physics Lab of the John Hopkins University and one was installed in the spacecraft’s instrument assembly Ralph. After the launch in January 2006 the teams involved had to be patient until the spacecraft had flown around Jupiter in 2007 to gather momentum for its journey to Pluto. Only then, after the first pictures had been successfully taken, were they able to clarify whether the system installed really worked accurately.

The spacecraft carries seven scientific instruments. Some instruments are clustered into groups. Together, they weigh around 30 kg and need little less than 28 Watts of electric power.

Ralph operates in VIS-IR range and is supposed to provide color maps of Pluto’s and Charon’s surface at a resolution of up to 250 m per pixel, and to map the surface composition of both bodies. SIA was delivered to Ball Aerospace by colleagues in Williamsburg who developed Ralph in cooperation with the Goddard Space Flight Center, NASA and the Southwest Research Institute. The SIA sun sensors are used for aligning the spacecraft with the sun (and from a great distance also with the earth) to ensure communication in case of a failure of the navigation systems.

The Solar Illumination Assembly’s (SIA) field of view runs at a 90° angle from the antenna, and, among others, continuously checks if all image points still work (flat-fielding). The SIA is made of a small quartz glass lens (4 mm aperture, 10 mm focal length) which projects the sun onto the input end of a 125 µm fiber. The decoupling end of the fiber illuminates a pair of lenses that are approximately 10 cm away from focus level. To execute an SIA measurement, the spacecraft is aligned in a way that the SIA lens depicts the sun on the fiber. Arriving at Pluto, the image of the sun on the fiber should be about 50µm. The fiber is about 10 cm long, is protected by a stainless steel tube, and has more than 50% transmission in the entire spectral range from 0.4 to 2.5 µm. Inside SIA there is a second fiber with an attenuation factor of 40 which can be used for flat-fielding to the closer the sun (e.g. up to Jupiter).

The biggest challenge in development was the adjustment to the other measurement systems and obviously the requirements regarding reliability. By no stretch of the imagination would we have been able to send a service technician in the case of problems. Accordingly, we worked with quality assurance - apparently with success.

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