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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Fundamental Questions
Yvonne Pendleton
Director, NASA's Lunar Science Institute
One of the most surprising discoveries of the Voyager 1 mission was the violent volcanoes of Jupiter's moon Io.

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

In contemplating the many extraordinary accomplishments in planetary science over the past fifty years, three missions spring to the top of my list: Voyager, Cassini and Kepler.

The Voyager encounters took place during a "coming of age time" for me. It was the summer of 1979 (I had just finished my undergraduate degree) when I first began work at the NASA Ames Research Center Space Sciences Division. Starting with my very first day on the job at NASA Ames (and continuing through the eighties while I was still in graduate school) I experienced the thrill of hearing scientists discuss and often confirm theoretical predictions (such as the volcanoes on Io), and not to mention the excitement of press conferences held in our Space Sciences Auditorium.

This dramatic view of Jupiter's satellite Io was taken by Voyager 1 and shows two simultaneously occurring volcanic eruptions.

Particularly, I was intrigued by the volcanism on Io. The reason that this was so striking for me was because there had been a paper from the researchers at Ames (Reynolds, R. and Cassen, P., Geophysical Research Letters, vol. 6, Feb. 1979, p. 121-124), which predicted the volcanoes on Io and then Voyager saw them. How transformative for a graduate student to see, first hand, that theory and observation can work together like that! This event highly influenced my own career in astrophysics; I became determined to intertwine the predictive powers of theory, laboratory studies and observational astronomy in my own work.

The next suite of solar system discoveries that really grabbed my imagination (and still has not let go!) concern the Cassini mission results on Saturn's moons, especially Titan, but also the primary outer satellites: Hyperion, Iapetus and Phoebe. Several of the earlier missions revealed the potential for Saturn and its moons to provide endless reasons to further explore this system. However, it is the Cassini mission that has unquestionably demonstrated that the Saturn system is one of the most interesting places in the solar system. From the methane lakes on Titan and the plumes of material on Enceladus, to the intriguing dark material on half of Iapetus and its neighbors, Hyperion and Phoebe -- the Cassini mission continues to reveal clues to puzzles that fascinate me. Once belonging to the realm of science fiction, Titan's liquid oceans and hydrocarbon sand dunes are spectacular, and they are real! Phoebe, a likely captured object from the Kuiper Belt region, could have material with significant interstellar and solar nebula heritage too.

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.

Finally, I have to list the Kepler mission as a planetary mission, even though it is viewing space beyond our own solar system.

It seems there are two fundamental questions that capture the inquisitive nature of most humans: One, "How did life originate in the Universe?" And two, "Are we alone?" In planetary and space science there is an interesting crossover in these areas. We are studying the origin and evolution of the basic building blocks of life and questioning the processing and delivery of those materials to solar system bodies where life might have gotten a foothold. Simultaneously, we are finding thousands of planet candidates with at least one confirmed, almost Earth-sized planet in the habitable zone of a sun-like star! So, thanks to the Kepler mission, we are on the brink of understanding how common other Earths are in our galaxy, as well as how other solar systems have developed in general.

This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star.

Combining knowledge of how cosmic materials necessary for life on Earth formed and were delivered to our planet, while at the same time we are probing the existence of other solar systems puts us at the nexus of a quantum leap in human understanding. For that reason I have to place the Kepler mission, and its outstanding success, at the top of my list of accomplishments in the past fifty years. I see it as a success story that spans planetary science and astrophysics, in a most satisfying way.

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?

Pictured here is an artist's vision of NASA's Stardust spacecraft closing in on Comet Wild 2. The spacecraft collected samples from the comet and returned them to Earth.

I study organic material (gas and dust) in the interstellar medium. I am interested in how basic hydrocarbons and interstellar ices formed, were processed and survived the formation of our solar nebula 4.5 billion years ago. I am fascinated by the ways in which these materials were incorporated into primitive solar system bodies and by the possibility that some were delivered to the Earth at just the right time to play a role in the origin of life as we know it.

In terms of addressing the fundamental questions astronomers have from the early days of our solar system, the Stardust mission is the most profound in this area, having successfully collected cometary and interstellar dust samples in the same mission. Work on the Stardust samples is ongoing, and the wealth of information to be gleaned will continue for years to come as new tools and enhanced techniques refine our ability to probe these phenomenal samples.

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 am the director of the NASA Lunar Science Institute, a virtual institute that joins together researchers across the country and around the world to investigate the formation, composition, conditions, and potential of our Moon. The plethora of international lunar missions in recent years has provided a look at the Moon that fills us with renewed interest and awe.

It is really amazing to have so many assets focused on the Moon. To me, there have been several surprises.

We now speak of a "wet Moon," although that should be kept in perspective. Comparable in amount to water found in the Sahara Desert on Earth, the Moon is not wet in the typical sense. Still, it contains far more water than anyone would have suspected to be there. There is the water that is coming from inside the Moon to the surface, which was collected in the Apollo lunar samples and studied anew with technologies formerly unavailable. Results show astonishing quantities of water sheltered inside protective crystal matrices.

Another indicator of the components of water on the Moon comes from the Indian mission, Chandrayaan, whose Moon Mineralogy Mapper (the M3 instrument) found the hydroxyl radical, OH, over much of the lunar surface.

Then of course there is the LCROSS finding and the LRO data that suggests that there is water preserved in the heavily shadowed craters on the Moon.

Earth is captured here in a natural color portrait made possible by the passing of Saturn directly in front of the sun from Cassini's point of view.

Some may argue about the amount of water or the interpretation of some of the data, but no one can argue that these missions provided us a new look at our nearest neighbor.

Also, a recent LRO result found the coldest spot in our solar system is actually on the Moon. This finding wraps right back to my own interest in the delivery of organic material to the early Earth. Comets and asteroids hit the Moon as well as the Earth, but the preservation record in certain locations on the Moon is likely to be as pristine as any place we will ever visit. If the ices preserved on the Moon are primitive solar system materials; I am excited to think about the organics that might have hitched a ride.

A few images of the solar system that are among my favorites include:

The image to the right is an image of the Earth from the vicinity of Saturn. It is just so special -- it is a perspective piece of our place in the solar system.

There are also two images I simply love from the Hubble Space Telescope, not so much for the science that they reveal, but for the inspiration they provide.

NASA's Hubble Space Telescope took this iconic image of the Eagle nebula, dubbed the "Pillars of Creation," highlighting its finger-like pillars where new stars are thought to be forming.

The Hubble Deep Field reveals that even seemingly empty regions of space are teaming with galaxies, each filled with stars that are undoubtedly surrounded by planets.

Several hundred never before seen galaxies are visible in this "deepest-ever" view of the universe, called the Hubble Deep Field (HDF), made with NASA's Hubble Space Telescope.

I am also drawn to the Apollo site images and the Lunokhod rover image by the Lunar Reconnaissance Orbiter (LRO). That we now have images with the clarity and precision to be able to see our early exploration steps is just amazing. Thanks to LRO, Moon maps will now be precise not only in imagery, but also in registered coordinates. This allows us to pinpoint exactly where items like the Lunokhod rover reside on the surface of the Moon.

Lunokhod 1 Rover in its final parking spot. Credit: NASA/GSFC/Arizona State University
Pinpointed here is the Apollo 11 lunar module.

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