The first obvious place to send a space probe was the Moon. It’s only 384,400 km away, so both NASA and the former Soviet Union launched unmanned lunar missions in the 1960s. Luna 3 was first to send photographs of the back side of the moon, which we cannot see with telescopes from Earth. Probes from the United States named Ranger 6, Ranger 7, and Ranger 8 took a total of 17,267 pictures before they each smashed on the Moon. These pictures confirmed that there were thousands of craters too small to be seen clearly from Earth.
The Soviet Union may’ve been unlucky when it came to exploring Mars but it experienced dramatic success with missions to Earth’s other neighbor: Venus. The Venera program ran from 1961 through 1984 and boasted 10 landing probes that gathered data on Venus’s surface, as well as 13 flyby or orbital probes that sent data home from the atmosphere surrounding Venus. The landers, in particular, were technological marvels that were able to withstand the extreme conditions at Venus’s surface: high pressure, temperatures hot enough to melt lead, and a corrosive atmosphere.
The four Venera spacecraft covered in this section all succeeded in capturing images of Venus’s surface. These photos were valuable because they were mankind’s first views of the surface of a mysterious world whose surface can’t be seen from space (unless you have radar eyes!). Though science-fiction-style steamy jungles had mostly been ruled out due to Venus’s hot surface temperatures, scientists were surprised to see a world that was more Earthlike than they’d originally imagined.
Venera 15 and 16, launched just five days apart in 1983, achieved what previous Venera missions couldn’t: They mapped Venus’s surface. Although the images sent back from previous Venera missions were both novel and useful, Venus’s thick atmosphere prevented traditional cameras from seeing through to the surface. Thus, a global study of the planet’s surface was impossible until radar instrumentation was added to the Venera spacecraft.
Even though its focus was on putting a human on the Moon before the Soviets could, NASA also set its sights on exploring the rest of the solar system in the 1960s. The Moon was close enough to make human space travel there feasible, but the initial forays to other planets had to be done by robotic spacecraft. Thus Mariner 2 became the first spacecraft to fly past another planet (Venus). Later, Mariner 5 returned to Venus.
Little was known about the surface of Venus throughout most of the 20th century because previous exploration of the planet in the 1970s had been foiled by thick clouds that prevented astronomers from seeing through to the surface. The United States’ Magellan mission, launched in 1989 by the Space Shuttle Atlantis, used a sophisticated radar system that was specially designed to be able to see through the clouds of Venus. Magellan was in orbit over the poles of Venus, which allowed it to map more than 98 percent of the planet’s surface.
The Magellan images revealed a Venusian surface that’s surprisingly Earth-like in many ways but lacking common geologic features found on the Earth. Most of Venus’s surface features are volcanic in nature, including huge lava plains, large shield volcanoes, and smaller lava domes. Lava channels that can be up to 6,000 kilometers long suggest that the lava was very fluid and likely erupted at a high rate. However, because Venus is a very hot planet, with a surface temperature as high as 454 degrees Celsius, water isn’t stable at its surface, meaning the familiar geologic features carved by water on Earth are missing on Venus.
After the heyday of American lunar exploration during the Project Apollo years, little further study of the Moon was done until the Clementine mission of the 1990s. This mission produced global digital maps of the Moon and was followed by the Lunar Prospector mission, which studied the Moon’s composition from orbit.
The crash of Lunar Prospector in 1999 resulted in the first burial of human remains on the Moon. A lipstick-sized canister of ashes of geologist Eugene Shoemaker was strapped inside the spacecraft. As the first head of the United States Geological Survey’s Center of Astrogeology, Eugene Shoemaker is considered the father of astrogeology, the geologic study of celestial bodies such as the Moon, asteroids, and Mars. He headed the Apollo lunar geology efforts and trained the astronauts.
In 1995 the Solar and Heliospheric Observatory (SOHO) was launched, a collaborative project between NASA and the European Space Agency (ESA). SOHO’s primary mission was to study and discover more about the Sun from close range. Specifically, its mission goals were to understand the core structure of the Sun, study the solar corona, and demystify the details behind solar winds. The observatory consists of two separate modules, a Service Module and a Payload Module.
NEAR (Near-Earth Asteroid Rendezvous) made history as the first spacecraft to orbit an asteroid. Launched in February 1996, NEAR was destined to rendezvous with the near-Earth asteroid 433 Eros to help provide more information about the composition, magnetic field, mass, and other attributes of the asteroid. Even though NEAR was never built as a lander, in February 2001, at the end of its mission, the probe was commanded to go into a lower and lower orbit until it eventually touched 433 Eros’s surface.
The Stardust mission was the first sample return of particles from a comet. The spacecraft launched in February 1999 and reached Comet Wild 2 in January 2004. As the spacecraft performed a close flyby of the comet, it collected samples from the comet’s coma (essentially the ice and dust particles that make up the comet’s “tail”). The comet particles were captured in a very low-density collector called an aerogel. The capsule containing the sample returned to Earth in 2006, landing with a parachute in the Utah desert. The main Stardust spacecraft continued past Earth and flyed to Comet Tempel 1 in 2011. Stardust ceased operations in March 2011.
Deep Impact wasn’t just the name of a NASA mission — it was what scientists wanted their spacecraft to leave on Comet Tempel 1 so they could study the comet’s insides. The plan was to send part of the spacecraft on a collision course with the comet and use the impact to eject material from inside the comet into outer space, where it could be studied. Impact occurred on July 4, 2005. Afterwards Deep Impact was placed on an extended mission to study other comets until contact was lost in 2013.
The MESSENGER spacecraft, whose name is short for MErcury Surface, Space ENvironment, GEochemistry and Ranging, launched on August 3, 2004. The probe took a long and looping trip through the inner solar system, relying on flybys of Earth, Venus and Mercury to slow down enough to be captured by Mercury's gravity. MESSENGER finally arrived at Mercury on March 17, 2011, becoming the first probe ever to orbit the heat-blasted world, and just the second spacecraft ever to study it up close.