P.R. Ratcliff

Ablation and Chemistry of Meteoric Materials in the Atmosphere of Titan

We compute the input of meteoric materials expected on Titan, and integrate this dust model with an ablation model and a comprehensive chemical model, investigating the effects on the atmosphere and surface. We find that a water deposition of approximately 10-100 times the expected interplanetary dust flux, or a recent large impact, is required to produce the observed CO2 abundance. Ionisation due to meteoric activity is not likely to be higher than that due to other sources.

Characteristics of the plasma from A 94 kms−1 micro-particle impact

The energies of the positive ions produced by the impact of a 70 nm boron carbide particle on a silver-doped aluminium target at a velocity of 94 kms(-1) have been derived for each ion species observed in the resulting time-of-flight mass spectrum. The results allow major conclusions to be drawn about the plasma energetics, and more tentative conclusions about the energy partitioning in the event.

Near earth environment

Planet Earth provides an interface to the interplanetary environment; its atmosphere forms a protective shield against direct impacts and erosion and is a medium in which to observe the approach of meteoroids and even to capture intact smaller meteoroids. The Earth’s gravitational well enhances the flux of interplanetary dust and modifies its velocity distribution. We consider the effect of the Earth on the dynamical properties on the interplanetary dust population, the relative contribution of sporadic meteoroids and annual streams, the efficiency of the atmosphere in capturing and fragmenting meteoroids and the effect of space debris on in situ experimental results. We review the range of modelling tools necessary to interpret the complex interaction of these populations with spacecraft, with particular emphasis on the improved calibration of impact detectors and the application of software models. Analysis of the available data from 30 years of in situ impact experiments, and more recent recovered samples reveals evidence of the relative contributions from space debris and various astrophysical sources. While temporally and spatially averaged fluxes are well represented by existing isotropic interplanetary models for meteoroids responsible for penetrating experimental foils (of thickness F max) greater than approximately 30 μm, at smaller sizes a high degree of anisotropy is apparent in resolved data. An Earth apex component is observed for particles larger than a few microns in size whereas at smaller sizes, β-meteoroids from the solar direction dominate. Space debris forms an increasingly significant proportion of the LEO population at F max < 30 μm in addition to its dominance in the centimetre size range and above.

Velocity thresholds for impact plasma production

Abstract Experiments have been performed on the dust accelerator facilities at the University of Kent at Canterbury (UK) and the Max-Planck-Institut fur Kernphysik (Germany) in which the production of plasma from impacts of micron and sub-micron particles at velocities from 1 to 90 km s −1 has been measured. Various projectile and target materials have been investigated. Time-of-flight mass spectrometry of the positive ions in the plasma allows their atomic species to be identified. By accumulating large amounts of data over a range of impact velocities it has been possible to identify the threshold velocities required to produce ions of different species, whether present in the system as the nominal projectile and target materials or as contaminants. The results obtained have been compared with theoretical predictions based on the principles of molecular dynamics and with the results of hydrocode simulations.

Use of combined light flash and plasma measurements to study hypervelocity impact processes

We present new measurements concerning generation of light flash during hypervelocity impacts. We use iron particles (10−13 to 10−17 kg) with velocities over the range 1 to 42 km/s impacting semi-infinite targets (aluminium and molybdenum). The main results of previous work in the field are found to be reproduced with some slight deviations. For iron projectiles with given mass and velocity the energy of the flash (normalized to mass) is proportional to velocity to the power of 3.5 for aluminium targets and 3.9 for molybdenum targets. The duration of the flash is of order 1 microsecond. Simultaneous measurements of the generation of impact plasma do not change this. The onset of plasma generation of the bulk target material does not affect the total light flash energy. We discuss the duration of the flash compared to a simple calculation of temperature in the target and plasma vs time.

First results of particulate impacts and foil perforations on LDEF

Abstract The results of an initial examination of the LDEF MicroAbrasion Package (MAP) and limited results from other onboard hardware are presented. The intriguing tasks of interpreting these data in terms of the dynamics of a particulate distribution of natural and artificial origin are discussed. It emphasises the unique aspects of the mission and especially the attitude stabilisation which may be exploited to extract a greater range of information compared with that previously derived from space collections and exposure of similar passive sensors.

Plasma production by secondary impacts: Implications for velocity measurements by in-situ dust detectors

Abstract A number of in-situ cosmic dust detectors derive the dust particle velocities from measurement of the risetimes of the impact plasma signal. Extensive calibration of these instruments has established a reliable empirical relationship but a quantitative explanation has not been available, with the result that confidence in flight data outside the range of the calibration data is hard to assess. Recent measurements taken at the dust accelerator facilities at the University of Kent (UK) and at MPI-K (Germany), supported by a theoretical analysis, have demonstrated that the relationship results from the time-spread of secondary impacts coupled with the mobility of ions in the impact plasma cloud, which is in turn determined by the magnitude and geometry of the applied electric field and on the ion species present. Results of the current investigations are presented, and the implications of measurements based on this principle at high particle velocities, at masses unobtainable in calibration studies, and for other instrument geometries, are considered.

The LEO microparticle population: Computer studies of space debris drag depletion and of interplanetary capture processes

Abstract The effect of the Earths atmosphere on micron-sized particles is quantified by showing that 1 μm particles in circular orbits below altitudes of approximately 500 km complete less than one orbit before being absorbed by the atmosphere, and that particles generated at low altitudes only survive for a significant time if they are in orbits of at least moderate eccentricity. Thus, micron-sized particles at low altitudes must generally be in eccentric orbits, rather than circular. In addition to acting as a very efficient sink for small particles in LEO, the atmosphere also enhances the flux of natural micrometeorites by three processes; atmospheric focusing, aerocapture , and aerofragmentation capture . The first two of these processes in combination provide an enhancement by a factor of ∼1.7 in addition to the calculated weighted mean gravitational focusing of 2.9. However, this still leaves natural material accounting for only ∼ 5% of the 1 μm particle population in the LEO environment.

Atmospheric drag modelling for orbital micro-debris at LEO altitudes

Abstract Data from satellite impact experiments and the scanning of recovered spacecraft offers an extended timebase to examine, using a consistent methodology, the microparticle fluxes. New penetration data from the TiCCE experiment on Eureca /1, 2/ adds to this database and shows that — despite an expected growth in the micro-debris flux — the observed flux is not greater than either LDEF or SMM. The question arises: “is this consistent with the micro particle flux being dominated by space debris or by meteoroids”. To assist this assessment, numerical modelling using the Gear method /3/ of explicit time integration of the atmospheric drag lifetime of micron dimensioned orbital debris in both circular (LEO) and eccentric (GTO) orbits has been performed for the relevant space exposures. Results are applied to the data to examine whether the recent variations in flux can be attributed to varying levels of, orbital micro-debris caused by atmospheric drag and its changes during the solar cycle.