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.

Cosmic Discoveries

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.

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.

Encountering a "Wild" Comet

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 hamburger shaped."

This spectacular image of comet Tempel 1 was taken 67 seconds after it obliterated Deep Impact's impactor spacecraft.

Making an Impact

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.

Forming Conclusions

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.