On January 2, 2004 the Stardust spacecraft completed a close flyby of comet Wild2 (P81). Flying at a relative speed of 6.1 km/s within 237km of the 5 km nucleus, the spacecraft took 72 close-in images, measured the flux of impacting particles and did TOF mass spectrometry

Anderson, J. D

Atkins, K.

Bhaskaran, S.

Brownlee, D. E.

Cheuvront, A. R.

Clark, B. C.

Duxbury, T. C.

Economou, T.

Green, S.

Hanner, M. S.

Hörz, F.

Kissel, J.

McDonnell, J. A. M.

Newburn, R. L.

Perkins, D. E.

Price, S.

Ryan, R. E.

Sandford, S.

Sekanina, Z.

Tsou, P.

Tuzzolino, A. J.

Villinga, J. M.

Williams, K. E.

Zolensky, M. E.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe Stardust – a successful encounter with theremarkable comet Wild 2Conference or Workshop ItemHow to cite:Brownlee, D. E.; Anderson, J. D; Atkins, K.; Bhaskaran, S.; Cheuvront, A. R.; Clark, B. C.; Duxbury, T. C.;Economou, T.; Hanner, M. S.; Ho¨rz, F.; Kissel, J.; McDonnell, J. A. M.; Green, S.; Newburn, R. L.; Perkins, D. E.;Price, S.; Ryan, R. E.; Sandford, S.; Sekanina, Z.; Tsou, P.; Tuzzolino, A. J.; Villinga, J. M.; Williams, K. E. andZolensky, M. E. (2004). The Stardust – a successful encounter with the remarkable comet Wild 2. In: 35th Lunarand Planetary Science Conference, 15-19 Mar 2004, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1981.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukTHE STARDUST- A SUCCESSFUL ENCOUNTER WITH THE REMARKABLECOMET WILD 2Brownlee1 D.  E., J. D. Anderson2, K. Atkins, S. Bhaskaran2, A. R. Cheuvront3, B. C. Clark3, T.C. Duxbury2, T. Economou9, M. S. Hanner4, F. Hörz5, J. Kissel6, J. A. M. McDonnell7, S.Green7, R. L. Newburn2, D. E. Perkins3, S. Price3, R. E. Ryan2, S. A. Sandford8, Z. Sekanina2, P.Tsou2, A. J. Tuzzolino9, J. M. Villinga3, K. E. Williams2, and M. E. Zolensky5, 1University ofWashington, 2Jet Propulsion Laboratory, 3Lockheed Martin Space, 4University of Massachusetts,5Johnson Space Center Systems, 6Max-Planck-Institut für Aeronomie, 7Open University, 8AmesResearch Center, 9University of Chicago (Dept of Astronomy 351580, U. Washington, Seattle,WA 98195- brownlee@astro.washington.eduOn January 2, 2004 the Stardust spacecraftcompleted a close flyby of comet Wild2 (P81).Flying at a relative speed of 6.1 km/s within 237kmof the 5 km nucleus, the spacecraft  took 72 close-inimages, measured the flux of impacting particles anddid in-situ compositional analysis of freshly releaseddust with a time-of-flight mass spectrometer.  Theprimary goal of the mission is to collect >500particles >15µm diameter and return them to Earth onJanuary 15, 2006.  The cometary particles ranging insize from a micron to ~100 microns were collected inlow density silica aerogel. After returning over ahundred 2x4x3 cm aerogel collection cells will beprocessed at the curatorial facility at the NASAJohnson Space Center and 5 to 100 micron sizeextracted cometary particles will be distributed toanalysts by a system that will be based on theallocation procedures for  cosmic dust, Antarcticmeteorites  and lunar samples.The flyby appears to be an unqualifiedsuccess.  Although there were considerablechallenges for imaging, including tracking thecomet at 1.5 degrees per second with a 200 mmlense, the optical navigation camera was usedfor scientific imaging with great success.  Theimages are sharp images, with little motionsmear and only minor effects due to scatteringwithin the camera optics.  The initial dust fluxdata indicate the flux was similar or greater thanpredictions, and that the expected quantity ofparticles was collected.  The data indicate that ahalf  dozen or more large particles penetrated thefirst layer of the multi-component “stuffed”Whipple shield that was designed to protect theleading edge of the Stardust spacecraft fromhypervelocity impacts of particles as large as 1cm diameter.  The dust impact data clearly showtime variability consistent with the spacecraftpassing through discrete jets of out-flowingparticles.Although imaging was a secondary scienceobjective, the images taken during the flybyprovided the best yet images of a cometarynucleus and revealed remarkable insight into thenature of Wild2, its evolution and its emission ofvolatiles and solids into space. The images weretaken at intervals giving stereo pairs and a widerange of phase angle coverage. Every otherexposure was a long exposure that tended tooverexpose the nucleus but provide goodconditions for detecting sunlight scattered fromdust entrained in gas jets emanating from activesurface regions.  Each long exposure imageclearly shows about a half dozen jets.  Stretchingthe images show many more and it is likely thatthe comet contains dozens of jet producingregions that are activated by solar illuminationduring a full spin period.  Many of the jets arehighly collimated and appear to be derived fromsmall regions (<<1km) on the surface.  To firstorder the jets appear to be predominantly radial.Lines drawn from jets back the surface alwaysintersect depressions but actually associatingfeatures with  jets is complicated by theabundance of features on the surface of  Wild2.The surface of the comet is quite remarkablein that it is covered with features ranging in sizeup to about 2km, nearly half the size of theobject.  Although detailed analyses have yet tobe done and the images have yet to be properlyprocessed, it is rather clear already that the Wild2 surface differs from imaged asteroids, and theother two imaged comet nuclei- Halley andBorrelly as well as the ice-rich surfaces ofCallisto and Ganymede.  Wild2 has anexceedingly rough surface covered with large,Lunar and Planetary Science XXXV (2004) 1981.pdfdeep flat bottomed depressions that are boundedby steep walls.  Some of these features are roundand may be impact craters while others are notround, have scalloped  edges and do not appearto be impact craters.  Most of the depressionsare  large and there does not appear to be apopulation of small impact craters similar to thecontinuum in crater size observed on other icybodies.  Some areas appear to be distinctlyblocky at scales of 10’s of meter size, and thereare are a number of scarps as well as flatregions. Some flat regions appear to containfractures.  Positive relief with high angle slopes(hills and cliffs) exist up to150 m high. Linearfeatures are seen that may indicate slumping.Scarps and high angle positive relief featuressuggest that the near surface  must have modestcohesive strength and they cannot be merelycomposed of loose powdery materials. Manysurface features are remarably crisp and sharp,unlike the subdued nature of regolith coveredasteroids.  It may also have albedo features buteven preliminary assessment therof will requireproper processing.  Thus the surface issurprisingly rich with topographic features andmay also have albedo features .The surface of Wild2 is dramaticallydifferent and more complex than availableimages of Halley and Borrelly.  The Deep Space1 images of Borrelly [1] show the best surfacedetail but most of the surface of that comet iscomparatively smooth without the large scalethe circular features and depressions seen onWild2. It is possible that the more complexnature of Wild2 is related to its orbital evolution.Wild2 is a Jupiter family comet that wasperturbed to its 5.24 to ~1.5AU orbit by a ~13Jovian radii flyby encounter of Jupiter on9/9/1974.  Prior to that time it was on a 25 to4.97 AU orbit with an orbital period of 57 years.Like other Jupiter family (JF) comets it ispresumed to have been derived from the Kuiperbelt and have spent billions of years in distantsolar orbits.Wild2 is a fresh comet in the sense that it hasonly had 5 perihelion passes in its present orbit.It is unknown if it has had an earlier period ofsolar irradiation and strong ablation in the innersolar system. Is likely that Wild2, as a Jupiterfamily comet, has been inside Jupiter’s orbitbefore in the past million years, about 10 timesis typical for a JF comet [2].   Typical activecomets are believed to survive only a fewthousand years before they either disintegrate orbecome inactive.  With high ablation ratesinterior to 2.5 AU, typical comets shouldprobably not retain ancient surface features suchas craters made by long term exposure in theouter solar system. Wild2 may have had only amodest solar ablation exposure in the inner solarsystem and it may retain ancient impact features,perhaps related to the early history of the KuiperBelt.The H2O production rates derived fromLyman alpha measurements for Wild2 indicatethat Wild2 ablates about 107 tons of waterduring each of its present orbits.  Using a densityof 0.5, for porous ice, this implies an averagesurface recession rate of only a quarter of ameter per orbit.  If the ablation occurs from only5% of the surface then the mean surfacerecession rate in active areas will be on the orderof 5 m/orbit.The apparent mix of possible impact cratersand possible ablation craters on Wild2 provide aremarkable opportunity to investigatefundamental aspects of gas production, crustproduction and temporal evolution of activecomets. It may also provide insight into a varietyof processes that occurred in the Kuiper Belt.Comparison of Wild2 with data on Halley andBorrelly [1] will provide a remarkableopportunity to investigate the diversity amongcomets and what such diversity might tell aboutthe nature and evolution of the solar system’sprimordial ice-rich bodies.[1] Britt et al (2004) Icarus in press. [2]Levison H. F. and Duncan M. J. (1997) Icarus127,13-27.Lunar and Planetary Science XXXV (2004) 1981.pdf

The Stardust – a successful encounter with the remarkable comet Wild 2

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The Stardust – a successful encounter with the remarkable comet Wild 2

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