Dawn mis­sion in­stru­ments

The Dawn spacecraft is carrying a total of three scientific experiments on board. Alongside the camera system, which will acquire images of the surface of the asteroids, measurements will be carried out with a spectrometer as well as a gamma ray and neutron detector. In addition, measurements of the spacecraft’s motion using radio signals will allow researchers to gain a better understanding of the asteroid’s gravity field.

The Visual and Infrared Spectrometer

The Visual and Infrared spectrometer (VIR) is – alongside the German camera system – the second European contribution to the Dawn mission. The spectrometer measures the thermal radiation (infrared) of the asteroids themselves, as well as the sunlight reflected in visible light and near infrared wavelengths. This data can tell the scientists a lot about the chemical and mineralogical composition of rocks, dust and ice on the surface of the asteroids.

At the heart of the spectrometer is an arrangement of mercury cadmium telluride photodiodes cooled to minus 203 degrees Celsius. These diodes are capable of detecting infrared radiation between wavelengths of 0.95 and 5.0 microns – that is the range containing most 'colour fingerprints' of rock-forming minerals. A CCD chip further records the visible light adjacent to ultraviolet and infrared wavelengths between 0.25 and 1.0 microns.

The Dawn VIR spectrometer was developed under the management of the Italian Instituto Nazionale Di Astrofisica (INAF) in Rome, with the support of the Agenzia Spaziale Italiana (ASI). VIR is a technical modification of the spectrometer on board Rosetta and Venus Express. It further uses key technical details of the spectrometer for visible and infrared light on the Cassini-Huygens spacecraft (studying Saturn and its moons).

The Gamma Ray and Neutron Detector

Dawn's Gamma Ray and Neutron Detector (GRaND) will provide new information about how Ceres and Vesta formed and evolved. GRaND is detecting strong neutron and gamma ray signals that will be analysed to map the elemental composition of the entire surface of Vesta. These signals can be the by-products of the bombardment by cosmic rays, radiation that pervades space, or emitted through the decay of radioactive elements. With GRaND, the scientists particularly expect to find out more about the abundance of radioactive elements on the two asteroids as well as their respective water content.

Gamma rays intercepted by GRaND are detected by an array of semiconducting cadmium zinc tellurium (CZT) crystals and/or interactions with a bismuth germanate (BGO). The neutrons are captured by means of special scintillators: if a neutron passes through such a scintillator, gamma radiation is created which is also captured by means of the crystals of the detector.

GRaND's Neutron Detectors consist of four boron-loaded plastic scintillators surrounding the BGO crystal and the CZT array. The neutrons interact with the scintillators; the BGO and CZT detectors measure the gamma rays generated by this interaction.

The Dawn GRaND instrument was developed and built by the Los Alamos National Laboratory (LANL). Its technical configuration is based on similar instruments, which were previously used on the NASA Lunar Prospector and Mars Odyssey missions.

Gravimetry will be used to determine, among other things, the mass, gravitational field and rotational axis of Vesta and Ceres. Receiving stations will be using special procedures to analyse the spacecraft's radio signals from the ground.

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Falk Dambowsky

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Freie Universität Berlin
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Ulrich Köhler

German Aerospace Center (DLR)
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