Distinguishing Between Comets Tempel 1 and Wild 2
By Derek Blackway
Confused over NASA's multiple comet missions? You're not alone....
During life's busy schedule, it's tough keeping up with all that's in a day's work, so it's
understandable that facts about comets Wild 2 (pronounced "Vilt 2") and Tempel 1 get mixed up.
Yet there are easy ways to differentiate between the two, or any other comets.
Resembling human fingerprints, no two comets are exactly alike.
During a five-year period, NASA launched two missions, Stardust and Deep Impact, to explore
two different comets for scientific studies. Both are short-period comets with orbits
taking them between Jupiter and Mars, and were believed to be prime specimens of study
due to their positions in their natural environments. It turns out that their histories,
locations, sizes and shapes were found to be very different.
Comets are named after their discoverers. In 1867, Tempel 1 was discovered by German
astronomer, Ernst Wilhelm Leberecht Tempel. Comet Wild 2 wasn't even a twinkle in
Earth's eye back then. It took over 100 years for Wild 2 to make its appearance in
1974 when the pull of Jupiter's gravity changed its orbit. It was four years later
in 1978 when Swiss astronomer, Paul Wild, discovered the glow of a new celestial body.
Comets are visible for two reasons. First, dust driven from a comet's nucleus reflects
sunlight as it travels through space. Second, certain gases in the comet's coma,
stimulated by the Sun, give off light like a fluorescent bulb. Over time, a comet
may become less active or even dormant. When a comet comes close enough to the Sun
starts to heat up, it loses some of its material through the process known as sublimation.
This happens when a solid turns to vapor, without first melting into a liquid.
After about 1,000 trips past the Sun, a comet loses most of these volatile materials
and no longer generates a coma, which is made up of the gases that sublime off its
surface. Since it is the escaping gases that drive the dust particles from the nucleus -
the solid part of the comet - the comet no longer creates the long beautiful dust tail
that we can sometimes see in the night sky.
Encountering a "Wild" Comet
Wild 2 Mosaic
JPEG Image (458K)
This image and diagram show the comet Wild 2, which NASA's Stardust spacecraft flew by
on Jan. 2, 2004.
The principal goal of the Stardust mission is to capture solid material from comet Wild 2
in the form of dust particles and bring them back to Earth for the first-ever analysis
in terrestrial laboratories.
Stardust's aerogel collector grid was deployed and caught particles of the comet in January 2004.
When Stardust's sample return capsule returns on Jan. 15, 2006, the world will have its
first comet samples, which will contain detailed compositional results.
Stardust's secondary goal was to capture images of Wild 2 during encounter. Principal
Investigator, Dr. Donald Brownlee, comments on the encounter photos. "The surface features
we discovered on Wild 2 were unexpected and are different from other comets," he said and
went on to provide an example. "A crater named 'Left Foot' has a flat floor with vertical
cliffs around the edges and is 140 meters deep and 1 km wide. The comet itself is kind of
Making an Impact
This spectacular image of comet Tempel 1 was taken 67 seconds after it obliterated Deep Impact's impactor spacecraft.
The principal goals of the Deep Impact mission were to impact comet Tempel 1, observing how
the crater forms and measure the crater's depth and diameter. Tempel 1 was cratered by
Deep Impact on July 4, 2005. To date, only ground observation data along with a few other
comet mission flyby data are available. More about Deep Impact.
Preliminary findings for both mission are currently available, however, will be refined
and improved through more detailed studies by many scientists during the years to come.
Stardust Imaging Lead, Ray Newburn commented on what the team discovered when the
photographic data was analyzed. "In general, one can say that at least Wild 2 has surface
material with more strength and cohesiveness than most of us expected."
Wild 2's morphology (form and structure) includes: steep, near vertical cliffs;
house-size boulders; pinnacles; flat-floored craters with near vertical walls;
haloed pit craters; overhangs and materials with varying albedo (reflectivity).
Wild 2 is a short-period (orbital period around the Sun is less than 20 years) comet
that orbits the Sun once every 6.39 years. Since it has passed the Sun only a few times,
it still has most of its dust and gases and is in relatively pristine condition. This is
important because comets are made up of material left over from the solar nebula after
the planets were formed. Unlike the planets, most comets have not changed very much since
the formation of the solar system. Therefore, comets may hold the key to understanding the
early development of the solar system.
"Deep Impact's Infrared Spectrometer has detected water, carbon dioxide and
carbon-based molecules on Tempel 1," said JPL science team member Don Yeomans. "We are
currently analyzing natural outbursts we've observed." Tempel 1s morphology is still
under analysis, however preliminary chemical composition observations have been made.
Tempel 1 has made many passages through the inner solar system orbiting the Sun every
5.5 years. This made Tempel 1 a good target to study evolutionary change in the mantle,
or upper crust. Tempel 1 has an approximate radius of 3 km (1.9 miles). Based on a
variety of observations, the nucleus is believed to be roughly 6 km (3.73 miles) in
diameter and that it is somewhat more elongated rather than a sphere.
In fact, only three comet nuclei have been observed - Halley, Borrelly, and Wild 2, with
Wild 2's shape that of a flattened sphere. Stardust scientists have now accounted for 22
jets (out gassing plumes) on Wild 2. Originally it was believed to have only two.
Going their Separate Ways
Since both visits by the JPL spacecrafts, Comets Wild 2 and Tempel 1 continue their
journeys in our solar system. Wild 2 is approaching Jupiter's orbit and Tempel 1 is
approximately in line with Mars' orbit.
In late February 2009, Stardust team members will once again look to the sky in search of
the "Wild" celestial traveler, where it will be most readily observable since
Stardust's 2004 encounter.
Deep Impact team members will have to wait exactly two years to observe Tempel 1, as it
will be the most readily observable in February 2011.
Although none of the team members will be able to see either comet without the aid of
some kind of optical enhancement, no matter what they use to see it with they will
have a twinkle in their eyes - a twinkle of a fond memory and a familiar glow.
|Comet Wild 2 versus Comet Tempel 1
||January 6, 1978
||April 3, 1867
||Ernst Wilhelm Leberecht Tempel
||6.40 Years (Short-Period Comet)
||5.52 Years (Short-Period Comet)
||Between the orbits of Mars and Jupiter
||Between the orbits of Mars and Jupiter
||Steep, near vertical cliffs; house-size boulders; pinnacles; flat-floored craters with near vertical walls; haloed pit craters; overhangs; materials with varying albedo (reflectivity)
||Still under study
||Approximate Triaxial Ellipsoid: 1.65 x 2.00 x 2.75 km
||Approximate Mean Radius: 3km
|Next Best Readily Observable Date:
||Late February 2009
||Late February 2011
||Flyby and Sample Return
||Flyby and Impact
||January 2, 2004
||July 4, 2005
|Relative Encounter Velocity:
- Collect at least 500 comet particles greater than 15 mm in size and return them to Earth
- Collect Interstellar dust particles and return them to Earth
- Collect at least 30 images of the Comet Wild 2 nucleus within 2000 km
- Collect at least 75% of the science telemetry data recorded from cruise and comet encounter
- Produce a plan for initial analysis and the long-term curation of the returned samples
- Observe how the crater forms
- Measure the crater's depth and diameter
- Measure the composition of the interior of the crater and its ejecta
- Determine the changes in natural outgassing produced by the impact
Last updated August 19, 2005