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50 Years of Robotic Planetary Exploration: William Borucki (Science Principal Investigator for the Kepler Mission at NASA Ames Research Center)
23 Apr 2012
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What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?

I find significant the missions and discoveries that have helped, and continue to help us understand the extent of life in our Universe and its formation.

Although not strictly a mission for planetary exploration, the results of the COBE mission provided a critical validation for the Big Bang theory which is the basis for the understanding of the formation of the Universe, galaxies, stars, planets, and ultimately: life. John Mather and George Smoot were justly awarded the 1996 Nobel Prize in Physics for this achievement.

The robotic exploration of Mars is of tremendous importance. Here there are many great missions: from the orbiters and surveyors, to the Viking landers, to the rovers rolling around on the surface of the Red Planet. The Mars missions have been and continue to be extremely important in the search for life.

As mankind continues to grow and mature, it makes sense to expand our exploration of our environment. As our robotic systems explore beyond the Earth, we are finding strange and exotic scenes -- yet these scenes remind us of the Earth. The pictures from the rovers and the Mars orbiters are really stunning. The pictures show us craters, sand dunes, mountains, and great canyons -- features we've seen on Earth, but in a completely different setting. One of my favorite pictures from a Mars mission is a picture from the Mars Reconnaissance Orbiter. In this picture we see a giant crater with sand dunes in the center, and at the edge of this crater is our little rover Opportunity. This and other pictures from the Mars missions are the most exciting that I have ever seen from robotic spacecraft.

Another important aspect of the Mars missions is the opportunity for public participation. The public can contemplate the images and examine the data: they can see the endless deserts strewn with rocks, mountains higher than our own and canyons longer than any on Earth. They can also see cliffs where salty water might be coming out during the summer time. These images allow the public to participate in the exploration of Mars. This is important, because it is the public who supports this exploration. If they can't share our excitement, then exploration will cease.

Secondly, I consider the flybys of the outer planets and their moons to be very exciting because they have greatly expanded the locations where we might find alien life.

When the Cassini spacecraft flew by Saturn's icy moon Enceladus, we discovered jets of water spurting out of the moon from its ice-covered ocean. Water is the critical component for the development and existence of life. Thus, its discovery on Enceladus means that we now have identified another location where life might exist and in a location that is fairly close to us. In that ocean there could be complex forms of life. However, without sunlight, that life is likely to be quite different from terrestrial organisms, even if both are based on DNA.

Further, the Galileo mission provided evidence for an ocean on Jupiter's moon Europa. This ocean on Europa has cracks in the surface ice, which indicate that water could be coming up to the surface. The colored deposits along the cracks could be evidence of life brought up by the water. Future missions could scrape these deposits from the surface and return them to the Earth for study.

In addition, the Cassini orbiter and the Huygens entry probe showed that Titan has a fluid cycle of methane and ethane. There are clouds and rain, and the images show a topography of rivers, mountains and lakes. Although strangely reminiscent of that of Earth, any life there would be like nothing we have ever imagined because it could not be based on DNA (DNA there would be frozen solid).

In science fiction, we read about people mining asteroids and the Moon. I suspect that in a coming decade these things will become a reality as mankind's capability increases and we expand our exploration of the solar system. I doubt that people will bring the materials back to Earth, but instead they will use the material to build colonies and continue our exploration of the solar system.

Mars and the moons of the outer planets capture our imagination -- they are our first glimpses of what lies beyond the Earth.

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?

The Kepler mission is a giant step in the search for life in our galaxy because it is answering the question "What fraction of stars in our galaxy have Earth-size planets in the habitable zone where liquid water could exist?" If that fraction is large, then there are many places where life could evolve and thus life could be ubiquitous. If there are only a few or none, then we might be alone.

The Kepler instrument is a telescope/photometer that simultaneously measures the brightness of 150,000 stars to detect the reduction in light that occurs when a planet passes in front of (i.e., transits) a star. These measurements determine the planet size and orbital period. Combined with the properties of the stars they orbit, they indicate whether or not the planet is in the habitable zone of its star.

For a planet to be in a habitable zone it must be fairly far away from its parent star. If a planet is too close to its parent star, the planet becomes too hot and any water there boils off -- you end up with a planet with a steam atmosphere, or a terribly dry atmosphere (similar to conditions found on the planet Venus). If the planet is too far away from its parent star, then the water is frozen; again not a promising place for the evolution of life.

The Kepler mission has already discovered over 2,300 planetary candidates among the 150,000 stars examined and after correction for the probability of orbital alignment, this result proves that there must be billions of planets in our galaxy. Thus, even if we don't find alien life in our own solar system, there might still be many opportunities for life to exist throughout our galaxy. However, only a small fraction of the discovered planets orbit stars like the sun and have sizes similar to Earth; most are giant planets like Neptune or Jupiter or orbit stars much cooler than the sun.

Although Kepler has found many planetary candidates, we must first confirm that they are really planets so that the science community and the public will have an accurate appraisal upon which to base future exploration. Unfortunately, the confirmation process is very lengthy because we must wait until at least three transits are detected and then we must use ground-based telescopes to check that the signal did not come from some other astrophysical process. A false-positive event can be the result of a small star crossing a larger one or the mixing of light from a dim binary star with that of the target star. We can get a preliminary estimate of the frequency of planets in the habitable zone by examining the results for stars smaller and cooler than the sun and for planets larger than Earth. Planets in the habitable zone of cooler stars have shorter orbital periods making it possible to get the necessary three transits quickly. Larger planets make it easier to spot the signal in the noise caused by the variability of the star.

Kepler's first discovery and confirmation of planet in the habitable zone is Kepler-22b. It is a little less than two and a half times the size of the Earth and it orbits a stars just 250 degrees cooler than our sun. This discovery is a big step forward, but we still need many more discoveries and these must be of planets closer in size and composition to the Earth. In particular we are looking for rocky planets, not those that have massive hydrogen-helium atmospheres. Only the smallest planets are likely to qualify. Kepler-10b and two newly-discovered Earth-sized planetary candidates have been found, but they are too close to their stars -- too hot to be in the habitable zone.

Although the giant planets in the habitable zone probably don't have life, the moons of such planets are also in that habitable zone and these might be massive enough to have atmospheres. Thus life is a possibility on these moons and therefore such planets should not be ignored. Imagine the situation where there are several Earth-like moons orbiting a giant planet in the habitable zone. People living there might travel from one moon to another to see their relatives, take vacations or go shopping!

Kepler discoveries are not just finding planets that orbit a single star, but are also finding planets that orbit pairs of stars. Remember the planet Tatooine depicted as the home planet of Luke Skywalker in the "Star Wars" movie? Kepler has already found over a dozen such systems. After discovering the first one, we contacted George Lucas and jokingly told him that in the sky of Tatooine, the blue star should be large and the red star small rather than the reverse. He sent one of his crew to participate in the press release for that discovery.

Most of the stars in the galaxy are not single stars like the sun. Most are part of double-, triple-, or even quadruple-star systems. To understand the distribution of life in our galaxy, these too must be investigated.

Kepler also has a team of 500 scientists, many in Europe, who are using the Kepler data for a different purpose. They use the measurements of the brightness fluctuations of the stars to deduce what is occurring inside stars. A star like the sun will consume its hydrogen, expand over time and evolve to become a red giant that consumes its inner planets. During the process of becoming a giant star, its core compresses, collapses and then the star begins to burn its helium. With the Kepler data, scientists can follow the evolution of stars as they age and better understand the future of our sun.

In summary, robotic exploration of space in the last 50 years has allowed us to begin the search for life on the planets in our solar system, find and characterize thousands of planets orbiting other stars to determine whether Earth-size planets in the habitable zone of stars like the sun are common or rare in our galaxy and to explore the interior of stars to understand their evolution.

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