JUDY WOODRUFF: Finally tonight, a journey long in the making, as a NASA spacecraft nears the planet Mercury's orbit, six years after it was launched.
NewsHour science correspondent Miles O'Brien reports on how this mission fits into a major era of planetary exploration.
MILES O'BRIEN: I took a stroll through our solar system the other day. It is not the same place I learned about in grade school.
JIM ZIMBELMAN, National Air and Space Museum: The last several decades have just been an explosion of information about the solar system.
MILES O'BRIEN: My guide was planetary geologist Jim Zimbelman with the National Air and Space Museum. The gallery is called "Beyond," based on the book by Michael Benson.
JIM ZIMBELMAN: That, to me, is remarkable, that we have learned so much so quickly, and it's ongoing. That's the fun part.
MILES O'BRIEN: Ongoing, indeed. We may take it for granted, but planetary scientists don't. They are quick to remind us we are living in a golden age of astronomical discovery.
SEAN SOLOMON, Carnegie Institution of Washington: It's fantastic. It's like being alive during the first exploration of North America.
MILES O'BRIEN: Scientist Sean Solomon hopes to write the next chapter at the planet closest to the sun, Mercury.
SEAN SOLOMON: So, we're finishing our family notebook by going to Mercury. Mercury is going to tell us about how the smallest and in many ways the most extreme of the siblings of Earth was born and evolved.
MILES O'BRIEN: Solomon is principal investigator for NASA's Messenger spacecraft, the first designed to enter the orbit of Mercury. If all goes well, it will see the planet as never before, measure its weak atmosphere and magnetic field. Once volcanically active, Mercury is the densest planet, a place where a day lasts two years.
The temperature range is 1,100 degrees. Designing a spacecraft to withstand this extreme environment has been a huge challenge.
Eric Finnegan is Messenger's chief engineer at the Applied Physics Laboratory in Maryland. The heat shield is made of several layers of ceramic fabric and Mylar. It is thin, but amazingly protective. On the sunny side, temperatures will reach 650 degrees Fahrenheit.
On this side, it would be what?
ERIC FINNEGAN, The Johns Hopkins University Applied Physics Laboratory: So, about -- about a foot away, because this is mounted on a titanium frame.
MILES O'BRIEN: Right.
ERIC FINNEGAN: So about a foot away would be like room temperature, like we are today.
MILES O'BRIEN: That's amazing.
The sun is 11 times stronger at Mercury than here on Earth, so only one-third of the solar panels are populated with power-generating cells. The rest are mirrors. And the panels will be tilted away, so they don't melt. If all goes well, Messenger will orbit Mercury for a year, with 730 revolutions of the planet.
SEAN SOLOMON: Every time we go to a new planet or a new planetary system, we are surprised. We are surprised by the diversity of phenomena and the richness of -- of the processes that we can document.
MILES O'BRIEN: That certainly has been the case on Mars.
When you look at that picture, you can imagine yourself -- you could be hiking in Arizona, right?
Jim Zimbelman and I were marveling at some images captured by the seemingly indestructible rovers Spirit and Opportunity, now seven years into their 90-day missions. They were sent to the red planet to find proof it was once warm and wet, and now they are knee-deep in data.
(CHEERING AND APPLAUSE)
MILES O'BRIEN: Steve Squyres is the principal investigator for the Mars Exploration Rovers, which landed successfully on Mars in 2004. Seven years later, it seems the euphoria is sustained.
STEVE SQUYRES, Cornell University: And now we have occupied the planet. We got rovers. We got orbiters. We have been to the farthest reaches of the outer solar system with Voyager. We're about to go into orbit around Mercury. We understand the solar system in a way that would have just been unimaginable when I was young.
MILES O'BRIEN: NASA's next craft headed for Mars is taking shape inside a clean room at the Jet Propulsion Lab in California. The Mars Science Laboratory, called Curiosity, is as big as a Mini Cooper, nuclear-powered, and designed to hunt for another essential ingredient of life: organic molecules.
Ashwin Vasavada is the deputy project scientist for Curiosity.
ASHWIN VASAVADA, Jet Propulsion Laboratory: This is about determining whether the environment on Mars, particularly early Mars, could have supported life, if life was there.
MILES O'BRIEN: Curiosity is slated to land on Mars -- actually, be lowered in an unprecedented nail-biting sky crane fashion -- in 2012. It is designed to travel 660 feet per day for just under two years.
NASA says Curiosity will not be able to settle once and for all the question of life on Mars. Planetary scientists believe the best way to do that is to bring some rocks back from Mars to Earth. A robotic sample return mission remains at the top of their wish list in the newly released Decadal Survey chaired by Steve Squyres.
STEVE SQUYRES: What we need is rocks that we can put in terrestrial laboratories, but that we have selected, not that just fell from the sky, but that we have chosen for the maximum potential of preserving evidence of ancient Martian life.
MILES O'BRIEN: Number two on the list? A visit to this fascinating place.
You can't help but wonder what it would be like to be standing on that icy surface.
Welcome to Jupiter's icy moon, Europa.
JIM ZIMBELMAN: And we have explored the polar areas on Earth, which is similar, but this entire moon is coated with that ice.
MILES O'BRIEN: The Galileo spacecraft to study Jupiter flew by Europa several times in the late '90s and captured these stunning images. Scientists are all but certain there is a liquid ocean beneath the ice. In fact, they believe Europa, the size of our moon, has three times the amount of water in our oceans.
BOB PAPPALARDO, Jet Propulsion Laboratory: I think Europa has probably the best chance for life in our solar system beyond Earth.
MILES O'BRIEN: Bob Pappalardo is a senior scientist at the Jet Propulsion Lab. He is leading efforts to design a mission to Europa which would use radar to measure the thickness of the ice, and maybe lay the groundwork for a mission to see if anything is living in a Europa ocean.
BOB PAPPALARDO: Now, we're not talking fish. We're not talking macrofauna. We're talking probably single-cell organisms, if there's life at all.
MILES O'BRIEN: Europa isn't the only icy moon in our solar system that has scientists intrigued. There Saturn's moon Enceladus.
JIM ZIMBELMAN: So, the ice can't be tens of kilometers thick to have that kind of fracturing going on and -- and preserved as clearly as it is.
MILES O'BRIEN: These are water geysers on Enceladus, the images captured by the Cassini spacecraft in 2006.
LINDA SPILKER, Jet Propulsion Laboratory: You have liquid water. You have some of the right ingredients for life.
MILES O'BRIEN: Linda Spilker is Cassini's project scientist.
LINDA SPILKER: We just don't have the instruments on Cassini to be able to definitively answer that question. But whenever you have liquid water, there's always that possibility, probably very remote, that you might have life with that world.
MILES O'BRIEN: The hunt for life also means understanding more about the evolution of planets. A craft called Dawn will arrive at the asteroid Vesta in July. Asteroids are like planets with arrested development, and thus serve as a frozen time capsule of the very beginning of our solar system.
And while Messenger homes in on Mercury, a craft called New Horizons is aiming for an arrival at Pluto in 2014. When New Horizons launched, Pluto was still considered a planet. It has since been demoted. Like I said, the solar system sure looks different these days.
Ed Weiler is NASA's chief scientist.
ED WEILER, NASA: I would predict, in this century, humans will prove that life exists somewhere else in the universe, whether it's past, present, fossils, bugs, whatever.
MILES O'BRIEN: But this gallery did not come cheap, and just the trio of missions envisioned to bring back Mars rocks would cost at least $10 billion. Has the science outgrown the budget?
JIM ZIMBELMAN: Maybe in a national sense. Maybe it is a global mission now -- or it should be -- that the kinds of science we're talking about may require not just one nation, but many nations deciding this is the important science to do.
MILES O'BRIEN: Out here, I suppose we are Earthlings first. Maybe that is best way to search this limitless frontier for something that might very well be looking for us.