Our solar system continues to deliver surprises 400 years after Galileo Galilei used his telescope to get the basics sorted out.
Our solar system seemed a very different place when Galileo Galilei first turned his rudimentary telescope to the night sky 400 years ago.
Most people at the time believed Earth was the center of the universe. Five known planets - Mercury, Venus, Mars, Jupiter and Saturn - one Moon and the Sun orbited Earth.
Galileo was among a group of scholars who suspected the planets - including Earth - revolved around the Sun. This heliocentric theory was developed by Nicholas Copernicus half a century earlier, but had not gained widespread acceptance.
The telescope gave Galileo the perfect tool to put the theory to the test.
His earth-shaking discovery of four moons orbiting Jupiter - Io, Europa, Ganymede and Callisto - and his observations that Venus appeared in the sky in phases like our own Moon helped confirm the heliocentric theory - and eventually changed the view of our solar system forever.
Four hundred years later, we are still adjusting our definitions of the solar system.
Pluto, for example, was recently reclassified from planet to dwarf planet to plutoid after new discoveries suggested the variety of unique objects in our solar system didn't fit into the traditional definitions.
Of course, our view today is far better than Galileo's. Advanced telescopes and a fleet of robotic explorers are extending our vision deep into space and returning a flood of new information. Armchair astronomers can call up the latest images and data on their home computers.
A few highlights:
This NASA robotic explorer makes its third pass by Mercury in September 2009 as it moves into position to become the first spacecraft to orbit our solar system's innermost planet.
MESSENGER already has improved our view of Mercury, which previously had been visited only by Mariner 10 in the mid-1970s. MESSENGER has revealed a large portion of the planet's surface that had never been seen before.
Three Mercury flybys allow MESSENGER time to map nearly the entire planet in color, image most of the areas not seen by Mariner 10, the only other spacecraft to visit Mercury, and measure the composition of the surface, atmosphere, and magnetosphere.
"The region of Mercury's surface that we viewed at close range for the first time [in October] is bigger than the land area of South America," said Sean Solomon, MESSENGER's principal investigator. "When combined with data from our first flyby and from Mariner 10, our latest coverage means that we have now seen about 95 percent of the planet."
Scientists are eager for the chance to study Mercury from orbit and further their understanding of Mercury's magnetic field, geology and thin exosphere (it does not have an atmosphere). The study of Mercury will help in the understanding of how terrestrial planets formed and evolved.
The spacecraft - protected from the Sun's power by a sturdy sunshade - settles into a one-year orbital mission on March 18, 2011. For one year MESSENGER will operate in a highly elliptical (egg-shaped) orbit around Mercury about 200 kilometers (124 miles) above the surface at the closest point and 15,193 kilometers (9,440 miles) at the farthest. MESSENGER will orbit Mercury twice every 24 hours.
The Mars Fleet
Mars has been under near constant study from a variety of orbiters and landers since Mars Pathfinder delivered the first robotic rover to the Red Planet in July 1997. The spacecraft, including the record-setting twin Mars Exploration Rovers, are revealing the Red Planet in unprecedented detail.
Over the past three decades, spacecraft have shown us that Mars is rocky, cold, and sterile beneath its hazy, pink sky. The Martian wasteland we see today hints at a formerly volatile world where volcanoes once raged, meteors plowed deep craters and flash floods rushed over the land. Mars continues to intrigue with each landing or orbital pass.
All contemporary Mars spacecraft - in orbit and on the ground - follow a unifying theme: Follow the water. Many scientists believe Mars might have one had much more water, but somewhere in the planet's evolution it vanished from the surface. Mars spacecraft have sent back evidence of dry riverbeds, flood plains, gullies and sedimentary deposits.
Most recently, the Mars Phoenix lander confirmed the presence of water-ice in the Martian subsurface during a five-month mission. Scientists are now studying a wealth of data from Phoenix - combined with observations from orbit - that continue to unravel the mystery of Mars' past.
"Phoenix provided an important step to spur the hope that we can show Mars was once habitable and possibly supported life," said Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington. "Phoenix was supported by orbiting NASA spacecraft providing communications relay while producing their own fascinating science. With the upcoming launch of the Mars Science Laboratory, the Mars Program never sleeps."
Mars Science Laboratory is a dune buggy-sized rover that will carry a powerful suite of instruments to the Martian surface. Planned for launch in 2011, the rover will assess Mars habitability - whether it ever was, or still is, a world that can support life.
The findings of this and future missions will determine if humans will ever safely visit Mars.
Cassini has returned a wealth of stunning information and new details from Saturn since it began in orbital tour in 2004. The orbiter delivered the European Space Agency's Huygens probe to the surface of Titan, the furthest robotic landing from Earth to date.
Cassini has found evidence of the origins of Saturn's beautiful and mysterious rings. It also uncovered evidence of seas of liquid methane and ethane on Titan.
Another big surprise was the presence of ice geysers erupting from the moon Enceladus, a sign the small moon may have liquid water just below the surface.
Enceladus now joins three of the moons which Galileo discovered - Europa, Ganymede and Callisto - as places where scientists look for evidence of possible oceans (the first beyond the Earth!). Liquid water also has the potential to harbor life.
Cassini is now on an extended mission to continue exploring Saturn and its rings and moons. Enceladus and Titan are primary targets in the two-year extended mission, dubbed the Cassini Equinox Mission.
The spacecraft also will monitor seasonal effects on Titan and Saturn, explore new places within Saturn's magnetosphere, and observe the unique ring geometry of the Saturn equinox in August of 2009 when sunlight will pass directly through the plane of the rings.
NASA's New Horizons spacecraft is currently making the long trek across our solar system to make the first close-up observations of Pluto. The spacecraft will fly by Pluto and its moons in July 2015 and then head deep into the Kuiper Belt to examine more frozen worlds left over from the formation of our solar system more than 4 billion years ago.
The spacecraft will return the first close-up images of Pluto and Charon and the recently-discovered moons Hydra and Nix. It also will be the first spacecraft to explore the icy third zone of our solar system. Earth is among the rocky worlds of the terrestrial zone closest to the Sun. Jupiter is the largest of the four planets in the gas giant zone.
Pluto and its companions are unlike the terrestrial and gas giant planets. Scientists believe Pluto and its companions in the Kuiper Belt are the planetary embryos from which the larger planets developed. The mission is a unique chance to study the early stages of planetary evolution and get a closer look at the region of space where many comets originate.
New Horizons is the first launch to a new planetary destination since NASA's twin Voyagers three decades ago. The mission completes the initial robotic reconnaissance of our solar system.
Voyager 1 and Voyager 2
Still going more than 30 years after launch, NASA's twin Voyager spacecraft are sending back information billions of miles from Earth. The spacecraft are famous for their outer planets tour that gave us our first close look at Jupiter, Saturn, Uranus and Neptune.
Both spacecraft have entered the heliosheath, a vast region at the edge of our solar system where the solar wind runs up against the thin gas between the stars. Their trajectory will carry them into interstellar space.
A comparison of data from the Voyagers, which entered the heliosheath billions of kilometers from each other, revealed out the giant bubble of the Sun's influence is squashed or dented and not perfectly round. Voyager 2 found the termination shock - the heliosheath boundary - sloshes back and forth like surf on a beach.
In January 2009, Voyager 1 was about 16 billion km from Earth - the farthest manmade object from Earth. Voyager 2 was about 13 billion km from our home world.
The spacecraft are expected to have enough power to continue sending back signals well into the next decade - continuing the era of exploration Galileo kicked off four centuries ago.
Last Updated: 2 February 2011