The dust mass distribution of comet 81P/Wild 2

The Dust Flux Monitor Instrument (DFMI) made direct measurements of the dust environment in the mass range 10-14 m -5 kg at comet 81P/Wild 2 during the Stardust flyby on 2 January 2004. We describe the techniques for derivation of the particle mass distribution, including updated calibration for the acoustic subsystem. The dust coma is characterized by "swarms" and "bursts" of particles with large variations of flux on small spatial scales, which may be explained by jets and fragmentation. The mass of the dust coma is dominated by larger particles, as was found for comets 1P/Halley and 26P/Grigg-Skjellerup. However, almost 80% of the particles were detected many minutes after closest approach at a distance of ~4000 km, where small grains dominated the detected mass flux. The mass distribution varies on small spatial scales with location in the coma, consistent with the jets and fragmentation inferred from the highly heterogeneous dust spatial distribution. The cumulative mass distribution index α (where the number of particles of mass m or larger, N(m) α m -α) in the coma ranges from 0.3 to 1.1. It is possible that jets and fragmentation occur in all comets but have not previously been well observed due to the limitations of detectors and flyby geometry. We estimate that 2800 ± 500 particles of diameter 15 μm or larger impacted the aerogel collectors, the largest being ~6— 10-7 kg (diameter ~1 mm), which dominates the total collected mass. Of these, only 500 ± 200, representing just 3% of the collected mass, originated in the far postencounter region

Interstellar Dust Inside and Outside the Heliosphere

In the early 1990s, after its Jupiter flyby, the Ulysses spacecraft identified interstellar dust in the solar system. Since then the in-situ dust detector on board Ulysses continuously monitored interstellar grains with masses up to 10e-13 kg, penetrating deep into the solar system. While Ulysses measured the interstellar dust stream at high ecliptic latitudes between 3 and 5 AU, interstellar impactors were also measured with the in-situ dust detectors on board Cassini, Galileo and Helios, covering a heliocentric distance range between 0.3 and 3 AU in the ecliptic plane. The interstellar dust stream in the inner solar system is altered by the solar radiation pressure force, gravitational focussing and interaction of charged grains with the time varying interplanetary magnetic field. The grains act as tracers of the physical conditions in the local interstellar cloud (LIC). Our in-situ measurements imply the existence of a population of 'big' interstellar grains (up to 10e-13 kg) and a gas-to-dust-mass ratio in the LIC which is a factor of > 2 larger than the one derived from astronomical observations, indicating a concentration of interstellar dust in the very local interstellar medium. Until 2004, the interstellar dust flow direction measured by Ulysses was close to the mean apex of the Sun's motion through the LIC, while in 2005, the data showed a 30 deg shift, the reason of which is presently unknown. We review the results from spacecraft-based in-situ interstellar dust measurements in the solar system and their implications for the physical and chemical state of the LIC.Comment: 10 pages, 2 b/w figures, 1 colour figure; submitted to Space Science Review

Galileo dust data from the jovian system: 2000 to 2003

The Galileo spacecraft was orbiting Jupiter between Dec 1995 and Sep 2003. The Galileo dust detector monitored the jovian dust environment between about 2 and 370 R_J (jovian radius R_J = 71492 km). We present data from the Galileo dust instrument for the period January 2000 to September 2003. We report on the data of 5389 particles measured between 2000 and the end of the mission in 2003. The majority of the 21250 particles for which the full set of measured impact parameters (impact time, impact direction, charge rise times, charge amplitudes, etc.) was transmitted to Earth were tiny grains (about 10 nm in radius), most of them originating from Jupiter's innermost Galilean moon Io. Their impact rates frequently exceeded 10 min^-1. Surprisingly large impact rates up to 100 min^-1 occurred in Aug/Sep 2000 when Galileo was at about 280 R_J from Jupiter. This peak in dust emission appears to coincide with strong changes in the release of neutral gas from the Io torus. Strong variability in the Io dust flux was measured on timescales of days to weeks, indicating large variations in the dust release from Io or the Io torus or both on such short timescales. Galileo has detected a large number of bigger micron-sized particles mostly in the region between the Galilean moons. A surprisingly large number of such bigger grains was measured in March 2003 within a 4-day interval when Galileo was outside Jupiter's magnetosphere at approximately 350 R_J jovicentric distance. Two passages of Jupiter's gossamer rings in 2002 and 2003 provided the first actual comparison of in-situ dust data from a planetary ring with the results inferred from inverting optical images.Comment: 59 pages, 13 figures, 6 tables, submitted to Planetary and Space Scienc

One year of Galileo dust data from the Jovian system: 1996

The dust detector system onboard Galileo records dust impacts in circumjovian space since the spacecraft has been injected into a bound orbit about Jupiter in December 1995. This is the sixth in a series of papers dedicated to presenting Galileo and Ulysses dust data. We present data from the Galileo dust instrument for the period January to December 1996 when the spacecraft completed four orbits about Jupiter (G1, G2, C3 and E4). Data were obtained as high resolution realtime science data or recorded data during a time period of 100 days, or via memory read-outs during the remaining times. Because the data transmission rate of the spacecraft is very low, the complete data set (i. e. all parameters measured by the instrument during impact of a dust particle) for only 2% (5353) of all particles detected could be transmitted to Earth; the other particles were only counted. Together with the data for 2883 particles detected during Galileo's interplanetary cruise and published earlier, complete data of 8236 particles detected by the Galileo dust instrument from 1989 to 1996 are now available. The majority of particles detected are tiny grains (about 10 nm in radius) originating from Jupiter's innermost Galilean moon Io. These grains have been detected throughout the Jovian system and the highest impact rates exceeded $\rm 100 min^{-1}$. A small number of grains has been detected in the close vicinity of the Galilean moons Europa, Ganymede and Callisto which belong to impact-generated dust clouds formed by (mostly submicrometer sized) ejecta from the surfaces of the moons (Kr\"uger et al., Nature, 399, 558, 1999). Impacts of submicrometer to micrometer sized grains have been detected thoughout the Jovian system and especially in the region between the Galilean moons.Comment: accepted for Planetary and Space Science, 33 pages, 6 tables, 10 figure

Four years of Ulysses dust data: 1996 to 1999

The Ulysses spacecraft is orbiting the Sun on a highly inclined ellipse ($i = 79^{\circ}$, perihelion distance 1.3 AU, aphelion distance 5.4 AU). Between January 1996 and December 1999 the spacecraft was beyond 3 AU from the Sun and crossed the ecliptic plane at aphelion in May 1998. In this four-year period 218 dust impacts were recorded with the dust detector on board. We publish and analyse the complete data set of both raw and reduced data for particles with masses $\rm 10^{-16} g$ to $\rm 10^{-8}$ g. Together with 1477 dust impacts recorded between launch of Ulysses and the end of 1995 published earlier \cite{gruen1995c,krueger1999b}, a data set of 1695 dust impacts detected with the Ulysses sensor between October 1990 and December 1999 is now available. The impact rate measured between 1996 and 1999 was relatively constant with about 0.2 impacts per day. The impact direction of the majority of the impacts is compatible with particles of interstellar origin, the rest are most likely interplanetary particles. The observed impact rate is compared with a model for the flux of interstellar dust particles. The flux of particles several micrometers in size is compared with the measurements of the dust instruments on board Pioneer 10 and Pioneer 11 beyond 3 AU (Humes 1980, JGR, 85, 5841--5852, 1980). Between 3 and 5 AU, Pioneer results predict that Ulysses should have seen five times more ($\rm \sim 10 \mu m$ sized) particles than actually detected.Comment: accepted by Planetary and Space Science, 22 pages, 8 figures (1 colour figure

A technique for the detection and determination of the velocity, mass, radiant, charge and flux of micrometeorite particles in space

The potential attained by micrometeorite particles in space is considered and it is shown that it is sufficient for the particles to be detected by an electronic technique. The preliminary design of a suitable sensor is discussed, and it is shown that the system may be extended to measure the velocity and mass of the particles

The role of comets and asteroids in solar-system development - space exploration

Exploration by space missions of the near-nucleus regions of comets Halley and Grigg-Skjellerup has resulted in valuable but expensive snapshots of cometary phenomena. The 'ground truth' from such missions, which can be established only by this means of dedicated space exploration, provides essential inputs to models of cometary processes. It also gives calibration data for a very wide base of cometary and asteroidal observations, past, present and future. Seen as objects which are both eroded by impacts from interplanetary dust and also the progenitors of interplanetary dust, we find both asteroids and comets are needed to contribute to this population. Contrary to expectations, as new data on the asteroids and comets is analysed, Rie find the differences between the two classes of primordial body is very much less distinct; accounting for the interplanetary distribution and properties of dust mass requires not only both classes of object but also a distribution of mixed classes. ESA's newly selected cometary mission Rosetta will offer a unique opportunity, during a rendezvous encounter from aphelion to perihelion, for the extended and detailed in situ observations of a target comet. It will also act as a valuable focus on the nature and role of comets in both the origin and development of the Solar System

Impact Cratering From Ldefs 5.75-Year Exposure - Decoding Of The Interplanetary And Earth-Orbital Populations

The IDEF multiple-foil microabrasion experiment (MAP) was exposed in five pointing directions stabilized relative to the orbit motion vector. Penetration records from this exposure provide an excellent opportunity for the decoding of the possible contribution from Earth orbital (bound) components and hyperbolic (unbound) particulates of extraterrestrial origin. Complemented by other experimental data at larger dimensions on LDEF, a preliminary flux distribution is derived for the nominal east- (ram), west-(trailing), and space-pointing detector surfaces. Modeling of the orbital dynamics for these two classes of population, and their collisional probabilities with IDEFs near-circular orbit, demonstrates a high anisotropy in the flux rate for the different detector locations. Bound and unbound particulates are also seen to have quite different signatures regarding anisotropy. According to the modeling, the west- and space pointing flux distributions must dominantly represent the unbound extraterrestrial populations. The different impact velocities on these two faces also permits, with computer modeling, the derivation of the average geocentric particle impact velocity, an extension of the same modeling permits, further, a transformation to predict the flux distribution of the same unbound particulates on the east face. Hence we can identify any excess flux observed on the east face as a possible component in Earth orbit. The east-to-west flux ratio for the MAP data is 34 +/- 7 for the penetration of aluminum at 5 mum and 7.3 +/- 1.7 at 30 mum; the space-to-west ratio is 4.9 +/- 1.0 at 5 mum (Niblett, 1991). These data demonstrate, using the modelling developed and that of Zook (1991), that LDEF impacts on all detectors are dominated by unbound and hence extraterrestrial particulates above particulate masses of 6.4 x 10(-10) g mass. However, for small particulates an orbital component is clearly identified on the east and side (N,S) faces that exceeds the interplanetary flux distribution by a factor of around 4 on the east face. The source is not yet identified, but the possible role of space debris and aerocaptured natural interplanetary dust is discussed. The data are compared to craters reported on the Solar Maximum Mission (SMM) louvres (Laurence and Brownlee, 1986). The SMM data are consistent with IDEF MAP data as a crater distribution, but very significant revisions to the interpretation of the SMM data are identified. The application of a new dimensionally scaled penetration formula, incorporating hypervelocity impact calibration to 16 km s-1 velocity, shows that the interplanetary component measured on the LDEF foils is consistent with interplanetary sources measured at 1 AU heliocentric distance

Impact comminution of icy targets in the outer solar system: application of hydrocodes

The availability of hydrocodes with crack growth modelling provides an opportunity to study the extended spallation region for brittle targets such as ices, where conchoidal fracture dominates the volume of crater excavation. Autodyn version 4.1.0.9 using Johnson-Holmquist crack modelling is explored in this role to investigate ejecta size and velocity distributions following validation in quasi-static runs which mimic laboratory measurements of glass and water ice. It is found that the code can model, with realistic values, bulk material properties and fracture patterns. Ejecta size and velocity distributions are presented which indicate low ejecta velocities which would lead to regolith development on 500m diameter objects in the Jovian or Saturnian system. The code is also able to track momentum reaction during impact and establish catastrophic collision limits of icy or rocky bodies under meteoroid impacts