Visiting many worlds.

Byline: The Register-Guard

The Messenger spacecraft blasted off toward Mercury last week, and by the time it reaches its destination, the planets in our solar system without some piece of human hardware on or near them will have slipped into the minority. The world, led by NASA, is in a golden age of space exploration - and it's occurring while the United States' manned orbiter fleet is grounded. This helps explain why President Bush has said little lately about a manned mission to Mars.

The Mercury Surface, Space Environment, Geochemistry and Ranging Mission will make three flybys of the planet closest to the sun, then enter orbit in 2011. It will map and measure the least-explored of the solar system's rocky, or terrestrial, planets. Only 45 percent of Mercury's surface has been photographed close-up; no spacecraft has visited the planet since the Mariner mission in 1974-75.

Meanwhile, two rovers, Spirit and Opportunity, are still trundling over the surface of Mars. The rovers completed their missions in April, but they've exceeded designers' expectations and continue to beam reports to mission control. Spirit has 2.1 miles on its odometer, six times the distance it was built to cover. More Mars missions are planned, and by 2014 NASA hopes to send a probe that will return to Earth with samples of martian soil.

The Cassini spacecraft and its tag-along Huygens probe, launched in 1997 by a consortium of 17 nations, entered orbit around Saturn in July and is exploring the planet's moons. Among them is Titan, the only moon in the solar system with an atmosphere. The Huygens probe will detach from Cassini and attempt to land on Titan early next year.

In the next few years NASA, the European Space Agency, Japan and various international parnerships plan missions to the moon, Venus, two different asteroids and two different comets. A probe called Genesis is collecting samples of solar wind and will return to Earth in September. A mission to Pluto, the last unvisited planet, and the Kuyper belt beyond it is scheduled for launch in 2006.

And then there's NASA's Great Observatories Program. The program's star is the Hubble Space Telescope, regarded as the most productive scientific instrument ever built. Its successor may be in orbit within a decade. Other space telescopes include the Chanda X-Ray Observatory, the Compton Gamma Ray Observatory and the infrared Spitzer Space Telescope. In October NASA will launch the Swift Gamma Ray Burst detector.

The cost of unmanned deep-space missions has come down, because engineers have learned to use or modify existing designs for multiple missions. Most probes cost in the $100 million to $200 million range. Meanwhile, the price tag of the International Space Station is $35 billion and climbing, and on Tuesday NASA announced that getting the shuttle fleet back into orbit will cost $900 million more than projected.

Manned space exploration retains a powerful emotional appeal; no one holds ticker-tape parades for machines.

Yet robots go billions of miles into space at a cost of millions, while astronauts go hundreds of miles into space at a cost of billions. The cost factors so strongly favor unmanned missions that they've come to dominate current efforts in space. Even while NASA struggles to keep its manned program alive, remote-control probes are exploring every corner of the solar system, with spectacular success.