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50 Years of Robotic Planetary Exploration: Anita Cochran (Assistant Director of the McDonald Observatory and a Senior Research Scientist at the University of Texas at Austin)
Kepler-22b
This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star.

What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?

The most significant mission has been the Kepler mission because it has finally quantified how many planetary systems exist in the galaxy. 16 years ago, we knew of one solar system -- ours. Now we know of more than 1,000 planetary candidates that reside in other planetary systems throughout our galaxy. Characterizing these systems can tell us a great deal about planet formation.

Titan Lakes
The existence of oceans or lakes of liquid methane on Saturn's moon Titan was predicted more than 20 years ago. But with a dense haze preventing a closer look it was not possible to confirm their presence; that is until the Cassini flyby on 22 July 2006.

Of course, the first time we look at any planetary body, we learn a great deal. For example, the Huygens probe on the Cassini mission was extremely important for our knowledge of Titan. Titan, the only satellite with a thick atmosphere, is critical to our understanding of nitrogen-based atmospheres.

Saturn's moons Titan and Tethys
Saturn's moon Tethys with its prominent Odysseus Crater silently slips behind Saturn's largest moon Titan.

Tethys is not actually enshrouded in Titan's atmosphere. Tethys (1,062 km, or 660 miles across) is more than twice as far from Cassini than Titan (5,150 km, or 3,200 miles across) in this sequence. Tethys is 2.2 million km (1.4 million miles) from Cassini. Titan is about 1 million km (621,000 miles) away.

In your field of work, what are some examples of the great achievements and discoveries in planetary science and robotic exploration throughout the past 50 years?

Comet Particles
This image shows the tracks left by two comet particles after they impacted the Stardust spacecraft's comet dust collector. The collector is made up of a low-density glass material called aerogel.

I work in the field of comets. The Stardust mission was a great achievement. Stardust brought back a sample of comet dust from comet Wild 2 and the analysis of this dust has led us to understand that the solar nebula was much more mixed than formerly thought possible. This means that material that was near the sun and material that was far from the sun was all mixed together very early in the history of the solar nebula.

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

The Deep Impact mission has led us to better understand the structure of comets, helping to confirm the dirty snowball model that Fred Whipple first proposed in 1950. However, Deep Impact also told us that comets are more homogeneous than previously thought possible.

Comet
In 1986, Giotto's encounter with Comet Halley provided the first ever opportunity to take images of a comet nucleus. Credits: Halley Multicolor Camera Team, Giotto Project, ESA

Before Deep Impact, we thought that the outer layers of the comet might be highly changed from when the comet was formed due to heating and the loss of the most volatile (easily changed) gases. However, Deep Impact showed us that the outer layers look chemically very similar to the inner layers.

Comet McNaught
This image of Comet McNaught was captured by the European Southern Observatory in Chile as both the comet and the sun were setting over the Pacific Ocean. The ocean surface appears nearly flat at this distance.

From the ground, we have studied the chemical compositions of more than 100 comets and we find evidence for at least two distinctly different reservoirs for comets.

Comet Neat
This image of Comet C/2001 Q4 (NEAT) was taken at the WIYN 0.9-meter telescope at Kitt Peak National Observatory near Tucson, Ariz. on 7 May 2004.

We discover these differences in the comet's chemistry. About 70% of comets seem to have very similar compositions for their ices. The other 30% seem to be depleted in what we call "long chain carbon molecules." Long chain carbon molecules are molecules made up of many carbons (plus other things like hydrogen) strung together.

The "depleted" comets are not deficient in carbon, per se. They are deficient in carbon molecules of a particular structure. We believe this is due to where these comets formed, not how they evolved. We are studying other molecules to try to better understand what other chemical differences might exist. These chemical differences point to the formation of some comets having occurred in different pressure and temperature regions than the others. If we can figure out these differences, we will understand more about the early history of the solar system.

Comet Siding Spring
Comet Siding Spring appears to streak across the sky like a superhero in this new infrared image from NASA's Wide-field Infrared Survey Explorer, or WISE. The comet, also known as C/2007 Q3, was discovered in 2007 by observers in Australia.

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