Comet 81P/Wild2 and the Stardust Mission
On January 15th 2006 at 3 am a green fireball shot across the sky of West USA and its cause, a 1 m capsule, parachuted safely onto the Utah desert. This capsule had just completed a 7 year, 5 billion km journey to Comet 81P/Wild 2 and back to Earth. The space mission is called Stardust and we are learning about the composition of comets from it.
The Stardust re-entry over the Utah desert in January 2006
Comet Wild2 (pronounced ‘Vilt 2’) has had a very long and very complicated life. It started in the earliest stages of the Solar System 4.57 billion years ago as the rocky, metallic and icy components that ultimately formed the planets started to condense. The ices of water, ammonia, carbon dioxide and carbon monoxide condensed in the cold outer parts of the Solar System. Most of these ices ended up as part of the gas giant planets but some remained in 1-10 km sized solid bodies called planetesimals. As the gas giant planets formed their massive gravity caused these early comets to disperse. Some were dispersed to surround the furthest reaches of the Solar System into what is called the Oort cloud of comets. However, such comets have very long orbital periods (e.g. hundreds or thousands of years) so their passages are difficult to predict and were of no use for the Stardust mission. Other comets with short periods (e.g. under 200 years) lie within Neptune’s orbit. These are part of the Kuiper Belt. Comet Wild 2 was in this region of the Solar System until 1974 when it passed close to Jupiter, causing it to be deflected into the inner Solar System – hence Wild-2 is classified as a Jupiter Family Comet. It now has an orbital period of 6 years with perihelion (nearest point to Sun) just beyond the distance of Mars and an aphelion (farthest point to Sun) near Jupiter’s orbit. In 1978 it was discovered by astronomer Paul Wild and this famous comet now takes his name.
Since Wild 2 hasn’t passed near to the Sun many times, it is in a relatively pristine condition with little sublimation of the ices and dust having taking place at this point in the comet’s life: ideal for the Stardust mission.
After launch in 1999 the Stardust craft was put into an orbit around the Sun such that in 2004, on the second orbit, the spacecraft encountered Comet Wild 2 for several hours, passing through the dust ‘coma’ given off the nucleus. As it passed through the coma the rack of foils and aerogel collected tens of millions of grains of cometary dust. Aerogel has a density of around 0.01 grammes per cubic centimetre - that is one hundredth the density of water. However it was able to stop Comet Wild 2’s particles, which when they hit the aerogel were travelling at 6 kilometres per second.
Tracks #41 within Aerogel Collector. Width of image 2 mm.
The Stardust craft also took pictures of the 4 km Wild 2 nucleus during it close approach at 300 km distance. That is the most famous part of Stardust’s life but it has also encountered asteroid Annefrank in 2002 in a planned flyby. Another part of the Stardust mission was to collect grains which originated from outside the Solar System – interstellar grains. This was done in 2000 and 2002 by exposing collector surfaces to the direction of interstellar particles: these were collected at very high velocities (‘hypervelocities’) – around 25 kms-1.
Impact crater formed at 6 kms-1 on aluminium foil strips of the Stardust collector tray. Analysing the residue within such craters is one of the ways of determining what Comet Wild 2 was made of. This image was taken with a scanning electron microscope OU/UL.
After its third orbit around the Sun, Stardust pointed for Earth and its successful landing in Utah.
Stardust capsule lands in Utah Desert, 2006
After landing the shell containing the collectors was quickly retrieved: landing in water or being rained on could have spelt ruin for the mission as the aerogel would have dissolved. Samples were unpacked in clean rooms to avoid dust contamination. Aerogel with cometary grains was cut into blocks less than 1 mm across and prepared for electron microscope analyses. Foils were cut up and distributed to selected labs around the world. In February 2006 Fedex packages arrived in the UK bringing Comet Wild 2 - which was born in the Kuiper Belt at the start of the Solar System - into our laboratories. At Leicester we are now studying Stardust grains and impact residues using electron microscopy. Some of the foils that were exposed to the interstellar flux have now also arrived at Leicester (Aug. 2010) for study.
Samples arrive at Leicester for Study
CometaryTrack #162 has now been analysed at the Diamond synchrotron showing its composition by using an intense beam of X-rays which can be focused to a few microns. In addition to X-ray fluorescence we use a range of synchrotron X-ray diffraction and X-ray Absorption techniques.