20 July 2016
Perihelion Distance (q)
Aphelion Distance (Q)
Click for NASA orbit diagram
Hills Observatory: 1 January 2013 to 2 December 2018
The comet, also known as Wild 2 (pronounced "vilt two") was discovered by Swiss astronomer
Paul Wild 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. In
September 1974, it passed within one million kilometers of the planet Jupiter, the strong
gravitational pull of which perturbed the comet's orbit and brought it into the inner Solar System.
Its orbital period changed from 43 years to about 6 years, and its perihelion is now about 1.59
astronomical unit (AU).
NASA's Stardust Mission launched a spacecraft, named Stardust, on February 7, 1999. It flew by
Wild 2 on January 2, 2004, and collected particle samples from the comet's coma, which were
returned to Earth along with interstellar dust it collected during the journey. 72 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 2 kilometres 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 5 kilometres.
Stardust's "sample return canister" landed in Utah, on January 15, 2006. 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, anorthite and 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 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 discovered evidence for the presence of
liquid water. They have 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 interstellar dust particles
from the Stardust spacecraft since returning to Earth in 2006.
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