Mission

A first example of a dust telescope has been described in the proposal for the Galactic DUNE mission which is being proposed by an international team of 66 scientists (http://mpi-hd.mpg.de/galileo/DUNE/). The goal of the mission is the in-situ characterization of galactic interstellar dust, in order to provide crucial information not achievable with astronomical methods.

In order to meet this scientific goal, the Galactic DUNE mission was designed to allow the distinction between ISD and interplanetary dust, to measure the chemical and isotopic composition of ISD, and to monitor the ISD flux for various directions and masses over the duration of the mission. To minimize the contamination by man-made solid particles near Earth, Galactic DUNE's trajectory lies outside the debris belts at the distance of geosynchronous orbit. The selected trajectory is a high Earth orbit close to one libration point of the Earth-Sun system (Figure 2).

The Galactic DUNE satellite exploits the effect of the Earth's motion (vE = 30 km/s) on the local ISD velocity (visd = 26 km/s, assuming (3=1) and flux. This can be used to confirm the interstellar origin of the measured dust population. During the Earth's upwind motion, the velocities add up to 56 km/s. When the dust telescope points into this direction, the flux of ISD grains is almost an order of magnitude larger than the flux of interplanetary grains (Figure 3). During the downwind motion the relative velocity and consequently the ISD flux are low. At least two years are needed in order to characterize the ISD flux along the Earth's orbit about the Sun in a statistically significant way and to determine the various components of interplanetary dust particles. During a two-year mission more than 1,000 galactic and interplanetary grains will be recorded, 60 of which with high resolution mass spectra.

In Figure 3 (bottom) we suggest an observation strategy for the dust telescope. The average impact velocity measured by the charge sensors of D2S or CDA can then be analyzed and compared to the local escape velocity. An additional identification criterion for small ISD grains is provided by the annual variation of the ISD flux, due to the Earth's upstream and downstream motion in spring and autumn respectively.

During the first two years of the mission the interstellar dust flux along the Earth's orbit around the Sun, as well as the interplanetary dust flux, will be determined. In order to provide statistically meaningful data, the dust telescope will perform blocks of observations, each of duration 4 weeks, with a fixed instrument pointing in inertial space. The observations will be linked by maintenance periods of about two days.

The observation strategy for the second year can be modified depending on the findings from the first year. In particular, deviations of the flux direction of big interstellar grains from the nominal direction are important to characterize the coupling of big galactic grains to the LIC, or larger structure s, and to identify potential sources of these particles.

Observations during the later phases of the mission could address other related scientific questions. Potential targets are specific interstellar phenomena. For example, sampling could be undertaken, of the directions from which an interstellar dust flux is suspected (e.g. ß-Pictoris). In addition, interplanetary phenomena (e.g. dust in meteoroid streams, cometary dust or dust from the Moon), or even debris studies (fine grains from solid rocket burns) could be studied.

Figure 2. Galactic DUNE mission scenario. Earth's orbit around the Sun (vE = 30 km/s), and the direction of the interstellar gas and dust flow at speed vKo = 26 km/s are shown. Two positions of the Earth and the satellite are shown (right: late summer, and left: late winter,^not to scale). The corresponding fluxes are Fw = 4.5 10" m"2 s"1 in winter and Fs = 6.6 10" m" s"1 in summer. We have assumed that radiation pressure and solar gravity for ISD grains cancel each other, therefore ISD traverses the solar system on straight trajectories.

Orbits about the collinear LI or L2 points are of particular interest for Galactic DUNE, as they meet the requirement of being at large distance from Earth's orbital debris belts, and they provide a very stable thermal environment. An additional advantage of orbits about LI or L2 is that they allow easy orbit manipulations by means of lunar flybys as demonstrated by the Japanese Hiten and Nozomi missions. This feature of LI and L2 orbits is particularly interesting for possible follow-up missions to return samples of ISD to Earth.

Fig. 3. Model dust fluxes and observation strategy. We assume an ISD upstream direction of ^ed = 259°, Ped = 8°. The top figure shows the interplanetary dust flux as derived from the interplanetary dust flux model (divinel993, Griinl997a). The assumed spacecraft orbit is a circular orbit about the Sun at 1 AU with the spacecraft position characterized by its ecliptic longitude. The assumed detector sensitivity is 10"15 g and the detector has a 50° wide aperture. Azimuth 90° corresponds to the solar direction (S) and 180° is the apex direction of the Earth's motion (A). The middle figure shows the flux of ISD grains with an assumed p value of 1. Center flux directions of ISD grains of different p values (0, 0.5, 1, and 1.2) are shown by white lines. The bottom figure displays the combined fluxes. Overlain is a sample of possible observations represented by the diamonds. Individual observations last about a month (the months indicated represent the start of an observation at about the 20th day of the month). The azimuth range corresponds to ± 20°. It is assumed that during each observation the spacecraft with the dust telescope, points to a fixed direction in inertial space. Within 30° from the solar direction is the Sun avoidance zone.

Fig. 3. Model dust fluxes and observation strategy. We assume an ISD upstream direction of ^ed = 259°, Ped = 8°. The top figure shows the interplanetary dust flux as derived from the interplanetary dust flux model (divinel993, Griinl997a). The assumed spacecraft orbit is a circular orbit about the Sun at 1 AU with the spacecraft position characterized by its ecliptic longitude. The assumed detector sensitivity is 10"15 g and the detector has a 50° wide aperture. Azimuth 90° corresponds to the solar direction (S) and 180° is the apex direction of the Earth's motion (A). The middle figure shows the flux of ISD grains with an assumed p value of 1. Center flux directions of ISD grains of different p values (0, 0.5, 1, and 1.2) are shown by white lines. The bottom figure displays the combined fluxes. Overlain is a sample of possible observations represented by the diamonds. Individual observations last about a month (the months indicated represent the start of an observation at about the 20th day of the month). The azimuth range corresponds to ± 20°. It is assumed that during each observation the spacecraft with the dust telescope, points to a fixed direction in inertial space. Within 30° from the solar direction is the Sun avoidance zone.

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