7 Space Missions to Remember

This is the International Year of Astronomy. The U.N. and International Astronomical Union have declared it so, but with a slogan like "The Universe -- yours to discover," it could be sponsored by the auto club.

Still, if you've lost track of those wonderful spacecrafts NASA has been launching for a half-century, here is an opportunity to catch up with a few of them.

1. Pioneer 3 & 4 (1958, 1959)

The space race was running its first lap when NASA aimed for the moon and launched Pioneer 3 in late 1958 and Pioneer 4 in early 1959. Pioneer 4 successfully passed within 30,000 miles of the moon, and traveled 407,000 miles from Earth before the ground station could no longer track it. Pioneer 4 became the first U.S. spacecraft to orbit the sun, which it is still doing. Pioneer 3, however, wasn't as successful: a glitch sent it 63,000 miles into space, after which gravity brought it back to Earth. In the meantime, Pioneer 3's Geiger counter discovered a second radiation belt around Earth.

2. Viking 1 & 2 (1975)

Launched within weeks of each other in the summer of 1975, the Vikings were the first spacecrafts to reach the surface of another planet safely. Then, as now, the big curiosity was about life on Mars, and the Vikings were sent to search for microorganisms in the Martian soil. The Viking ships were composed of two parts: orbiters, which circled the planet, and landers, which were directed to land on the surface of the planet itself. Viking Lander 1 shot photographs that revealed the Martian sky is pink; Viking Lander 2 recorded a "Marsquake." They continued to send data to Earth until the early 1980s.

3. Voyager 1 & 2 (1977)

The Voyagers, which made a tour of the outer solar system before heading toward interstellar space, are the oldest functioning spacecrafts. There were launched in 1977, because that year the planets literally aligned: Jupiter, Saturn, Uranus and Neptune were all collinear at that time. The crafts were able to use the gravity of each planet to slingshot them into the orbit of the next. The Voyagers passed by Jupiter in 1979 and viewed for the first time Jupiter's rings and volcanic activity on its moon Io, and Saturn in 1981. Voyager 1 then turned and headed away from the ecliptic, the plane in which the planets orbit.

Voyager 2 reached Uranus in 1986 and Neptune in 1989. Last year, it reached the transition zone between the solar system and interstellar space. From information radioed to Earth, scientists determined "the bubble of solar wind surrounding the solar system is not round, but has a squashed shape," according to Science Daily. NASA believes the two ships will continue functioning until at least 2020.

4. Galileo (1989)

5. Cassini-Huygens (1997)

6. Mars Spirit & Opportunity (2003)

Spirit and Opportunity are two rovers that have been poking around opposite sides of Mars since 2004, searching for the arid planet's watery past. According to NASA, "each has found evidence of long-ago Martian environments where water was active and conditions may have been suitable for life." Both descended using a parachute, then a thruster shot, with airbags cushioning the landing. Although they were designed to operate for only three months, the two are still rolling over the Martian surface today. Opportunity has driven more than seven miles; Spirit more than four. Spirit lost the use of its right-front wheel in 2006, and now drives backward. Some months ago it failed to activate when the morning light hit its solar panels, although it later responded to commands from the Jet Propulsion Laboratory. NASA has since referred to these hiccups as "amnesia."

7. Messenger (2004)

David Holzel would like to tour the solar system. Until then, he blogs at David Wrote This.