Introduction

A dust telescope is a dust analyzer that provides chemical and physical information on dust particles together with directional information that allows the reconstruction of the trajectory of the dust particle, all the way back to its source region. Already some of the first dust detectors in space on the OGO (Orbiting Geophysical Observatory) satellites and on the Lunar Explorer 35 (launched in 1966 and 1967, [1]) were time-of-flight (TOF) systems aiming at the trajectory analysis of the recorded particles. They consisted of a thin film front sensor and a rear sensor 10 cm apart. However, misled by the early false reports of a very high micro-meteoroid flux in the Earth's environment only 5 cm2 as sensitive area of a single detector were chosen. This had as consequence that no or only very few impacts were recorded during these missions. Berg and Richardson [2] extended this idea by combining 16 TOF tubes into a single 100 cm2 dust detector flown on Pioneer 8 and 9. This detector made the first important dust observation in interplanetary space and discovered new dusty phenomena [3], however, the total number of TOF events for which trajectories could be derived was low [4].

Because of the very low dust flux in space, later dust detectors had a wide field-of-view (FOV > 1 sterad which corresponds to an acceptance cone of > 60° half-angle) and a detection area of 100 cm2. The number of dust impacts recorded by these detectors at 1 AU from the Sun were less than 100 per year. Modern dust detectors, like the ones on the Galileo, Ulysses and Cassini missions employ a detection area of 1000 cm2. Despite their wide FOVs these detectors provided useful directional information on dust streams, like interstellar grains [5] and Jupiter dust streams [6], Because of the large numbers of particles in the stream (some 100 interstellar grains and some 10,000 Jupiter stream particles) statistical methods could be employed and the stream directions were derived with 10° and better than 2° accuracy, respectively.

In the early 1990s a Cosmic Dust Collection Facility (CDCF, [7]) was considered for implementation on the International Space Station. This CDCF combined the idea of trajectory analysis together with the subsequent intact collection of cosmic dust particles in low-density aerogel. For trajectory analysis several approaches were studied: segmented thin-film penetrations and pick-up charge measurements of the electrically charged dust particles [8] for position determination in at least two planes. The design goal was the determination of the impact direction with an accuracy of about 1° for tracing back the trajectories from low-Earth orbit to their cometary or asteroidal sources. However, because of NASA's budget cuts this project was cancelled before the technology was fully developed.

It was Cassini's Cosmic Dust Analyzer, CDA, that proved for the first time an important technique of accurate non-contacting trajectory measurement in space: the measurement of the pick-up charge of dust particles that entered CDA. Another important element was initially developed for the Halley missions: an impact ionization mass spectrometer [9] that provided compositional information on the impacting grain. A follow-up version of it is currently flying on the Stardust mission to comet Tempel 2. These instruments provided mass spectra of the impact generated ions from the dust particles at mass resolution M/AM of better than 100 atomic mass units. Thereby, these instruments are able to resolve most elements relevant for the characterization of extra-terrestrial materials, like meteorites.

It is the purpose of this paper to show how all these elements are brought together forming a true dust telescope that employs only state-of-the-art technology (section 3). However, before we do that, we will discuss the science questions that can be addressed with such an instrument (section 2). In section 4 we describe a specific mission proposal to study interstellar dust near Earth. This "Galactic DUNE" (Galactic DUst measurements Near Earth) mission is being proposed to space agencies for future implementation. Section 5 concludes by summarizing the major new features of the exciting opportunity for novel dust research.

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