Space: Oh, The Places We'll Go!

Despite the recent loss of two Mars probes, NASA has a full slate of missions proposed to explore virtually every corner of our solar system in the coming century. Over the next 15 years, efforts will focus on understanding how the solar system and planets first formed, and how conditions for life arose on Earth. Other probes will search for signs of past or present life on Mars and the moons of "gas giant" planets like Jupiter and Saturn. Further into the century, we may create a series of space outposts, perhaps on the moon, Mars and Ceres, an asteroid half the size of the moon. Vast distances and harsh conditions will preclude human travel to most of the solar system, so scientists will rely on robotic probes. "We can foresee [humans'] reaching Mars and near-Earth asteroids in the next several decades," says John Mankins, head of Advanced Concepts Studies at NASA. If radically faster propulsion technology is developed, humans may make it as far as the moons of Saturn in the next century, but visits to other solar systems will almost certainly have to wait. Alpha Centauri, the nearest star system, is some 4.3 light-years away. A spaceship with current technology wouldn't get there for at least 200,000 years. In the meantime, there'll be plenty to keep us busy a lot closer to home.

Pluto Distance from Earth: 3.57 billion miles Mission: Pluto Express (launch 2001) will make the first close-up observations of the planet and its water-ice-covered moon Charon when it arrives sometime after 2010. It may then head beyond Pluto's orbit into the Kuiper Belt of asteroids.

Neptune Distance from Earth: 2.7 billion miles Mission: Its moon Triton, with surface temperatures of minus 391 degrees, is the coldest object in the solar system. Neptune Orbiter, probably after 2015, will study that planet's atmosphere and rings, and map the surface of Triton.

Beyond Mission: At 3 billion and 3 trillion miles from Earth, the Kuiper Belt and Oort Cloud are little-understood rings of debris orbiting outside the planets. Probes won't likely get any farther than the Kuiper Belt before 2100. Some scientists say we should start probes on the long journey to neighboring stars now.

Saturn Distance from Earth: 790 million miles Mission: The Cassini probe will orbit in 2004 to map the planet's surface and rings. It may be followed by the balloonlike Titan Biologic Aerobot, which will analyze that giant moon's climate and search for prebiotic chemicals.

Uranus Distance from Earth: 1.7 billion miles Mission: None currently planned. The gas giants- Saturn, Jupiter, Uranus and Neptune-have no solid surface for probes to land on. High levels of radiation make the moons of gas giants inhospitable, but humans may arrive by 2100.

Asteroid belt Distance from Earth: About 190 million miles. It's a zone of small bodies between Mars and Jupiter; may be debris from solar-system formation. Mission: A joint U.S.-Japan mission, MUSES, will land on one of the 10,000 asteroids in 2003

Mars Distance from Earth: 49 million miles Mission: Multiple missions are planned over the next 15 years, including the Mars Sample Return Missions, which will bring the first Martian soil and rock samples back to Earth starting in 2008. Manned missions could follow, with a base being established by midcentury.

Jupiter Distance from Earth: 390 million miles Mission: Many of its 16 moons, including Europa and Io, make tempting targets. Europa Ocean Observer will seek a suspected ocean of liquid water under a three-mile crust of ice in 2007; later missions may attempt to drill through.

Venus & Mercury Distance from Earth: 57 mil. and 26 mil. miles Mission: Venus is scorched by a runaway greenhouse effect; Venus Surface Mission (launch after 2007) will have to resist 900-degree temperatures to work out why Earth is so different. Mercury, with no atmosphere to help brake a probe, is an unlikely target.

Earth & moon Distance: Moon is 240,000 miles from Earth Mission: The International Space Station, to be completed in 2002, will orbit Earth as a base for space experiments. Lunar Giant Basin Sample Return (launch after 2003) will collect moon samples for analysis back on Earth.

Sun Distance from Earth: 94 million miles Mission: Since above-surface temperatures reach several million degrees, it's unlikely we'll ever be able to even send a probe. Instead, space-based instruments will study the sun's activity and composition from orbit.