Comet 81P/Wild, also known as Wild 2 (pronounced "vilt two") ( ), is a

comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...

with a period of 6.4 years named after Swiss astronomer Paul Wild, who discovered it on January 6, 1978, using a 40-cm Schmidt telescope at Zimmerwald, Switzerland. For most of its 4.5 billion-year lifetime, Wild 2 probably had a more distant and

circular orbit A circular orbit is an orbit with a fixed distance around the barycenter; that is, in the shape of a circle. In this case, not only the distance, but also the speed, angular speed, Potential energy, potential and kinetic energy are constant. T ...

. In September 1974, it passed within of the planet

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

, the strong gravitational pull of which perturbed the comet's

orbit In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

and brought it into the inner Solar System. Its

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

changed from 43 years to about 6 years, and its

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

is now about .

Orbit

Prior to its encounter with Jupiter in 1974, the comet had an orbital period of around 43 years with an aphelion at around 25 AU and a perihelion of just under 5 AU. The encounter reduced the aphelion and perihelion to its present value of around 5 and 1.5 AU, respectively.

Exploration

NASA's Stardust Mission launched a

spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...

, named '' Stardust'', on February 7, 1999. It flew by Wild 2 on January 2, 2004, and collected particle samples from the comet's

coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to Nociception, respond normally to Pain, painful stimuli, light, or sound, lacks a normal Circadian rhythm, sleep-wake cycle and does not initiate ...

, which were returned to Earth along with

interstellar dust Cosmic dustalso called extraterrestrial dust, space dust, or star dustis dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and , such as micrometeoroids (30 μm). Cosmic dust can ...

it collected during the journey. Seventy-two close-up shots were taken of Wild 2 by ''Stardust''. They revealed a surface riddled with flat-bottomed depressions, with sheer walls and other features that range from very small to up to across. These features are believed to be caused by impact craters or gas vents. During ''Stardust''s flyby, at least 10 gas vents were active. The comet itself has a diameter of . ''Stardust''s "sample return canister" was reported to be in excellent condition when it landed in Utah, on January 15, 2006. A NASA team analyzed the particle capture cells and removed individual grains of comet and interstellar dust, then sent them to about 150 scientists around the globe. NASA is collaborating with

The Planetary Society The Planetary Society is an American internationally-active non-governmental nonprofit organization. It is involved in research, public outreach, and political space advocacy for engineering projects related to astronomy, planetary science, a ...

who will run a project called " Stardust@Home", using volunteers to help locate particles on the Stardust Interstellar Dust Collector (SIDC). As of 2006, the composition of the dust has contained a wide range of organic compounds, including two that contain biologically usable nitrogen. Indigenous aliphatic hydrocarbons were found with longer chain lengths than those observed in the diffuse interstellar medium. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust. Very few pure carbon ( CHON) particles were found in the samples returned. A substantial amount of crystalline silicates such as

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

anorthite Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

diopside were found, materials only formed at high temperature. This is consistent with previous observations of crystalline silicates both in cometary tails and in circumstellar disks at large distances from the star. Possible explanations for this high temperature material at large distances from Sun were summarised before the ''Stardust'' sample return mission by van Boekel et al.: :"Both in the Solar System and in circumstellar disks crystalline silicates are found at large distances from the star. The origin of these silicates is a matter of debate. Although in the hot inner-disk regions crystalline silicates can be produced by means of gas-phase condensation or thermal annealing, the typical grain temperatures in the outer-disk (2–20 au) regions are far below the glass temperature of silicates of approx 1,000 K. The crystals in these regions may have been transported outward through the disk or in an outward-flowing wind. An alternative source of crystalline silicates in the outer disk regions is in situ annealing, for example by shocks or lightning. A third way to produce crystalline silicates is the collisional destruction of large parent bodies in which secondary processing has taken place. We can use the mineralogy of the dust to derive information about the nature of the primary and/or secondary processes the small-grain population has undergone." Results from a study reported in the September 19, 2008 issue of the journal ''

Science Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...

'' has revealed an oxygen isotope signature in the dust that suggests an unexpected mingling of rocky material between the center and edges of the Solar System. Despite the comet's birth in the icy reaches of outer space beyond Pluto, tiny crystals collected from its halo appear to have been forged in the hotter interior, much closer to the Sun. In April 2011, scientists from the

University of Arizona The University of Arizona (Arizona, U of A, UArizona, or UA) is a Public university, public Land-grant university, land-grant research university in Tucson, Arizona, United States. Founded in 1885 by the 13th Arizona Territorial Legislature, it ...

discovered evidence of the presence of liquid water. They found iron and copper sulfide minerals that must have formed in the presence of water. The discovery is in conflict with the existing paradigm that comets never get warm enough to melt their icy bulk. Either collisions or radiogenic heating might have provided the necessary energy source. On August 14, 2014, scientists announced the collection of possible

particles from the ''Stardust'' spacecraft since returning to Earth in 2006.

Gallery

File:Wild2 3.jpg, Photograph taken by ''Stardust'' spacecraft File:Comet wild 2 jet plumes.jpg, Details of the plume jets File:Comet Wild2 Anaglyph.jpg, Red/green stereo anaglyph File:Comet Wild2 .jpg, ''Stardust'' approach image File:Comet wild 2.jpg

References

Notes

Citations

External links

*
NASA/JPL homepage for Stardust project

Stardust@Home Volunteer Particle Analysis Project

81P/Wild orbit and observations at IAU Minor Planet Center
{{DEFAULTSORT:Wild, 081P Periodic comets 0081 081P Comets visited by spacecraft 081P 19780106