The University of Kent's Unit for
Space Sciences and Astrophysics:
"Dust and Small Bodies of the Solar System"

The University of Kent's Unit for Space Sciences and Astrophysics has two main parts- the Radio Astronomy Group, situated in the Electronic Engineering Laboratory, and the Space Sciences Group, based in the Physics Laboratory. The activities of the Radio Astronomy Group are beyond the scope of this article- see their web page (www-star.ukc.ac.uk) for more information. The Space Sciences Group currently numbers just over 20 people, including five academic staff and a dozen postgraduate students.

The group's activities can be summed up in the phrase "Dust and Small Bodies of the Solar System". Within this topic there are four main areas of activity:

• Hypervelocity impact physics of dust and debris particles on spacecraft and planetary surface materials
• Dust (& debris) flux environment in Earth orbit and in interplanetary space
• Ground-based astronomical observations of primitive solar system bodies (optical & IR properties)
• Physical properties of the surfaces of planetary satellites and minor bodies from in situ measurements

Impact physics experiments are carried out both within the Unit's laboratory and in space, with the goal of understanding the impact process. This allows us to 'decode' space-exposed surfaces to characterise the cosmic dust environment and leads to a better understanding of how spacecraft can be protected. The Unit's two main facilities are a two-stage 'light gas gun' which is able to accelerate mm-sized particles to around 5 km/s, and a 2 MV Van de Graaff accelerator for sub-micron-sized particles at 1-100 km/s. Common target samples for experiments in these facilities include solid metal and metal foil targets, glass, composite materials and ice. Surfaces returned from space for analysis are usually solid metal, metal foil or solar arrays. The Unit has several facilities for scanning impacted surfaces. The equipment includes a scanning electron microscope and a range of optical microscopes and CCD devices. A clean room is available for the storage and examination of space-flown hardware.

Recent space missions to have carried surfaces returned to Earth for analysis at the Unit include the following:

• The Long Duration Exposure Facility (LDEF) Micro-Abrasion Package (MAP). LDEF was deployed by the Space Shuttle in 1984 and retrieved in January 1990. Panels of metal foil were flown on LDEF and afterwards scanned for impact perforations. Several other surfaces from LDEF such as thermal blankets were also scanned.
• The EuReCa (European Retrievable Carrier) Timeband Capture Cell Experiement (TiCCE). EuReCa was deployed in August 1992 and retrieved 11 months later by the Shuttle. Like LDEF-MAP, EuReCa-TiCCE was an instrument developed by the Unit.
• The EuReCa and Hubble Space Telescope solar arrays. The EuReCa solar arrays and one of the HST arrays were retrieved by the Shuttle and scanned. Although solar arrays have a complex layered structure, their retrieval has provided more data on the dust and debris environment and the way in which solar arrays are damaged. Like the LDEF thermal blankets, solar arrays are 'opportunistic' experiments, utilising surfaces not originally intended for impact detection. EuReCa's 'scuff plates' (bumpers to protect the spacecraft as it is slotted into the Shuttle's cargo bay) were also examined for impacts in this way.
• EuroMir 95- ESA astronaut Thomas Reiter installed the European Space Exposure Facility (ESEF) experiment (built by a French-led team) on the exterior of Mir's Spektr module. The Unit provided small samples of metal foil and 'aerogel', a low-density material that can be used to capture cosmic dust particles intact. This experiment was called 'Dustwatch-P'. Most of the samples have now been retrieved but some remain on Mir for retrieval at a later date.

Computer models called hydrocodes are also available within the Unit to study impact scenarios not attainable using the current experimental facilities. These codes can also be tested and upgraded by comparing their results with laboratory experiments.

Dust and Debris
Results from missions such as LDEF, EuReCa and the HST solar array provide valuable information on the dust and debris environment in low Earth orbit. Following retrieval and analysis of surfaces from these missions, mass and velocity distributions of particles can be obtained. These are then useful in an engineering sense to quantify the potential hazard to spacecraft. The Unit also has a number of 'active' sensor designs able to record impacts as they happen and send the data back to Earth. This avoids the costly retrieval process and allows impacts to be resolved in time. This is important when trying to distinguish between the natural background and peaks of activity due to meteor showers. The STRV 1c and 1d spacecraft (built by the UK's Defence Evaluation and Research Agency) will both carry prototypes of the Unit's latest impact sensor design, the final versions of which will be built in Finland to fly on ESA's Proba spacecraft.

Beyond Earth orbit the Unit is studying the interplanetary dust environment- in particular using results from current missions such as Ulysses and Galileo which carry dust detectors. The Cassini spacecraft will also carry a dust detector, this time able to perform a basic chemical analysis of the impacting particle. The Cosmic Dust Analyser (CDA, built by a German-led team including the Unit) will be turned on in 1999 during the spacecraft's cruise towards Saturn.

Ground Based Observations of Primitive Objects in the Solar System
The Unit is active in the field of Earth-based astronomy of primitive objects in the solar system. In particular the Unit's astronomers perform optical and IR photometry of objects such as comets, Kuiper belt objects and Centaurs.

The Unit's interests in dust and comets were combined when the Giotto mission to Halley's comet in 1986 provided the opportunity to measure the dust density and mass distribution by attaching acoustic sensors to the spacecraft's bumper shield. DIDSY (Dust Impact Detection System) was developed and built by the Unit. Giotto went on to encounter a second comet, Grigg-Skjellerup, in 1992. The Unit also had collaborative involvement in OPE (Optical Properties Experiment), another instrument on board Giotto.

More recently the Unit has had the chance to exploit its experience with Giotto-DIDSY. NASA's Stardust mission to return a sample of cometary dust to Earth will carry a small acoustic impact sensor on its bumper shield. Stardust is due to be launched in 1999 and will encounter comet Wild 2 in 2004.

In Situ Measurement of Physical Properties
The next major mission in which the Unit is involved is Cassini/Huygens. As well as the CDA dust instrument on the Cassini orbiter, the Unit has also contributed instrumentation for the Huygens Titan probe. Dr John Zarnecki is the Principal Investigator of the Surface Science Package (SSP), built by the Unit and collaborating institutes in the UK and Poland. The Unit has also provided electronics and an accelerometer for the Huygens Atmospheric Structure Instrument (HASI).

As a result of the development of SSP, a Titan environment simulator was constructed to mimic a range of gaseous and liquid environments that may be encountered by Huygens during its descent and landing. The simulator is an insulated can where gases such as methane and ethane can be cooled and condensed. Individual sensors from SSP have been tested and calibrated in the simulator.

Another recent planetary mission containing the Unit's hardware was the ill-fated Russian Mars 96 spacecraft. Both surface penetrators contained cylindrical modules housing seismometer electronics and a tilt sensor.

The next major European planetary mission after Cassini/Huygens will be Rosetta, a mission to rendezvous with a comet nucleus and send a lander to its surface. The Unit has involvement in three experiments on Rosetta. On the orbiter, a Dust Flux Analyser (DFA) (being built by an Italian-led team) will measure the dust properties within the coma. On the lander, the Unit is a partner in two instrument teams. The first, MUPUS (a partnership between institutes in Germany, Austria and Poland as well as the UK), will measure thermal, mechanical and structural properties of the comet nucleus' surface material. The second, SESAME (led by an institute in Hungary) contains a small acoustic impact sensing plate to detect any dust particles falling back to the nucleus surface.

Future Developments
On the horizon the Unit looks forward to future cosmic dust and debris experiments in Earth orbit such as those proposed for the International Space Station, as well as missions to other bodies in the Solar System. Experimental programmes using the Unit's impact facilities will continue, as will ground-based observations of minor bodies.

The Unit's web address is wwwspace.ukc.ac.uk

Andrew J Ball, 1997.

Published in the 25th Anniversary Convention edition of Eclipse, the official journal of the South East Kent Astronomical Society, Vol. 26, No. 2, November 1997.