January 14

The Huygens probe, a part of the Cassini mission to Saturn, parachuted into the cold, dense atmosphere of Titan, Saturn's largest moon. It took about two and a half hours to drop through the atmosphere, and scientists received data for an additional 70 minutes after touchdown. (Huygens was designed a probe, not as a lander, so it had only a few hours of battery power.) The European-built Huygens transmitted about 350 pictures during its descent and after touchdown; a communications failure prevented it from sending back 350 more. Early analysis of the results reveals that the Titan landscape is similar to Earth's, although molded by different processes, including rains and rivers of liquid methane and volcanoes of frozen water and ammonia. Cassini will continue to orbit Saturn, and will swing close to Titan and some of Saturn's other moons several times during the year.

July 4

The probe from the Deep Impact mission slammed into Comet Tempel-1. The probe gouged a hole in the comet, blasting many tons of vaporized gas and particles of rock into space. A flyby craft, which released the probe a day earlier, observed the impact and measured the composition of the material blasted into space. Scientists believe that material at the bottom of the crater changed little since Tempel-1 formed 4.5 billion years ago.

August 2

MESSENGER, which is en route to Mercury, flew past Earth. The encounter gave the craft the extra gravitational "kick" it needs to reach the planet closest to the Sun. MESSENGER will fly past Mercury three times in 2008 and 2009, then enter orbit around the planet on March 18, 2011.

August 9

Space shuttle Discovery returned to Earth after completing a two-week mission in orbit. The STS-114 mission delivered equipment and supplies to the International Space Station, and spacewalking astronauts installed new gyroscopes that provided greater control of the outpost. Astronauts also tested techniques for repairing the shuttle in orbit and conducted minor repairs to Discovery itself. Despite the successful mission, the loss of a chunk of form from the shuttle's external tank during launch has forced NASA to delay future launches while it studies and remedies the problem.

August 12

The next craft in NASA's Mars exploration program launched from Cape Canaveral. Called Mars Reconnaissance Orbiter, it will snap some of the sharpest pictures of Mars ever obtained from orbit. Like earlier craft, it is designed to look for evidence of water on the planet. MRO will arrive at Mars in March 2006.

September 12

A Japanese probe named Hayabusa ("falcon") arrived at a 600-meter asteroid named Iotkawa. Hayabusa will fly in formation with the asteroid for about three months. In November, it will drop a probe to the asteroid's surface, then fire small pellets at the surface to break off tiny bits of debris. It will capture some of these bits and return them to Earth in June 2007.

October 26

The first mission to Venus in more than a decade, Venus Express, is scheduled for launch from the Baikonur Cosmodrome in Kazakhstan. The European mission, which is built with spare parts and instruments from earlier planetary probes, will study the planet's atmosphere and the atmosphere's interaction with the surface and the space environment around Venus.

Continuing

Mars Global Surveyor, which arrived at Mars in 1997, will continue to study the Martian surface from orbit. Mars Odyssey, which entered Martian orbit in October 2001, gained a two-year extension to its mission at the end of 2004. Odyssey also serves as a radio relay station for the Mars rovers.

The European Space Agency's Mars Express, which arrived at Mars in 2003, will continue to snap high-resolution images from orbit. Scientists also hope to use the craft's radar to peer up to one meter below the surface in search of frozen water. Use of that instrument has been delayed by technical problems.

Scientists hope that Spirit and Opportunity, the twin Mars rovers, will scoot around the Martian surface well into 2005. Both landed on Mars in early 2004, and were scheduled to operate for 90 Mars days (about 92 Earth days). Spirit is exploring hills inside Gusev crater, while Opportunity studies Meridiani Planum halfway around the planet.

See also Explorations in 2004