Probes to the Moon or to Mars were capable of surviving and transmitting data for weeks or months (or, in some cases, years), but the surface conditions on Venus were much less hospitable. Venus’ atmosphere is so thick that the atmospheric pressure at the planet’s surface is almost 100 times that at the Earth’s surface. Venus’s thick clouds trap the Sun’s heat, resulting in a surface temperature of about 260 degrees Celsius. Because of this the space probes that managed to reach Venus’s surface lasted for only around an hour, so the amount of data they were able to return home was limited.

Ironically, although surface conditions on Venus are much harsher than on Mars, landing on the former is actually easier than landing on the latter. The thick atmosphere means that a series of parachutes can slow a spacecraft down from interplanetary speeds to a safe landing speed without having to rely on complex retrorockets to be fired before landing (like the ones needed on Mars due to the planet’s thin atmosphere).

Venus is a tremendous challenge for spacecraft. Many of the 19 Soviet missions failed, but Venera 4 was the first spaceship to enter the atmosphere of another planet; Venera 7 was the first to land on another planet; and Venera 9 transmitted the first photographs from the surface of another planet.

Models of Venus evolved from steamy jungles to dry deserts to global oceans of carbonated water. In 1967, Carl Sagan suggested that “float bladder macro-organisms” might live in the atmosphere. Radar studies of the surface in 1970 detected dark, circular regions that were interpreted as giant impact basins. All of these models would be found lacking before the end of the 1970s.

Although its main goal was to take measurements of the Venusian atmosphere with a variety of instruments, Venera 9, launched in 1975, wound up taking the first pictures returned from the surface of another planet. The spacecraft had two cameras, one on each of its sides (although one didn’t work because a lens cap failed to pop off correctly). The functioning camera used a periscope to peek through Venera 9’s hull so it could capture images without exposing itself to the extreme temperature and pressure conditions on the Venusian surface.

Later in 1975, Venera 10 was similarly successful in taking images of Venus’s surface, which revealed a smooth surface similar to a terrestrial lava flow. Complete lens cap failures on Venera 11 and 12 caused a slight delay in the flow of photos from Venus until 1981 when Venera 13 and 14 successfully returned color images of the planet’s surface.

Venera 13 and 14 used a more-sophisticated camera design that allowed them to send higher-resolution images back to Earth for a combined total of approximately three hours. The color images were made by taking versions of the same image using different color filters on the camera.

Unlike their predecessors, both cameras worked on Venera 13 and 14, allowing a complete view of the terrain surrounding the landers. Turns out the surfaces at both of these landing sites are covered with smooth, platelike rocks that are likely lava flows. The sky is yellowish, and the surface appears brownish-yellow.

Global surface maps are essential for understanding the geologic history of planets and how features at different parts of a planet’s surface relate to each other, when and how they were formed, and so on. Such maps can also be used to determine approximately how old the surface is, and how recently it was likely to have had active volcanoes or other activity.

Mariner 5 was originally intended as a backup Mars mission, but after the success of the Mars-bound Mariner 4, it was retrofitted and sent to Venus. It succeeded in taking measurements of Venus’s atmosphere, refining the measurements made by Mariner 2 and providing further clues about the planet’s hot surface temperature and thick atmosphere.

Magellan’s radar images have been combined with altimetry data to produce three-dimensional stereo views of Vensus’s surface, made by combining slightly different views of the surface taken on different orbits.

After Magellan accomplished its mapping goals, the spacecraft’s systems began to fail, and it was given one final task: to test a new technique called aerobraking (using a planet’s atmosphere to change the orbit of a spacecraft). Spacecraft controllers flew the spacecraft deep into Venus’s atmosphere until it burned up. The technique destroyed Magellan but helped engineers figure out how to use aerobraking to change the orbits of other spacecraft. Consequently, this technique has since been used successfully by Mars Global Surveyor and Mars Odyssey.

Magellan found no evidence of plate tectonics on Venus, meaning that volcanic activity on Venus operates very differently from Earth.

One surprise found on Venus was that the surface appeared to be uniformly younger than expected, as shown by the small number of impact craters, with an average surface age of about 500 million years. In geologic time, this is quite young. Scientists are still debating whether a single cataclysmic resurfacing event, such as a global volcanic eruption, could’ve occurred 500 million years ago, or whether the apparent surface age comes from ongoing geologic activity that slowly covers in parts of the planet’s surface.

The Clementine mission, a joint effort of NASA and the Ballistic Missile Defense Organization, took American space exploration back to the Moon when it launched on January 25, 1994. Although the primary purpose of Clementine was to test how spacecraft and sensors behave when in space for long periods of time, NASA was also able to sneak in some science from a variety of different instruments.

Clementine actively observed the Moon using ultraviolet and infrared imaging, laser altimetry, and other measurements. With the combined results of this various data, scientists pieced together a multispectral picture of the Moon’s surface, including a more in-depth study of lunar surface material and a modern digital global map of the Moon’s surface. These maps help scientists better understand the geologic history of the Moon and have been used as a basis for future lunar exploration.

Separately or with his wife Carolyn Spellman, Shoemaker discovered 32 comets and 1,125 asteroids, including Shoemaker–Levy 9 that impacted Jupiter in 1992. He was well known for his conviction that comets and asteroids pose a threat to life on Earth. He supported the theory of Luis and Walter Alvarez that an impact was responsible for the Cretaceous–Tertiary extinction of the dinosaurs 65 millions ago.

Eugene Shoemaker died in a violent car collision while studying impact craters in Australia in July 1997 at age 69.

NEAR sent back high-resolution images and other measurements, and it was a successful first launch of NASA’s Discovery Program of low-cost spacecraft. The asteroid’s extremely low gravity allowed the probe to survive the landing, creating a first in the grand scheme of space probe research: Something that wasn’t a lander landed.

NEAR orbited 433 Eros for nearly a year before landing and returned dozens of high-resolution photographs of the asteroid. Before it landed, the spacecraft was renamed NEAR Shoemaker in honor of the geologist Eugene M. Shoemaker.

The Stardust mission is historically significant because it marks the first time scientists had an opportunity to study an actual sample that came from primitive comet material. Because comets are thought to be primordial remnants from when the solar system was young, researchers hope to use this information to study the origin and evolution of the early solar system. In 2014 scientists announced the identification of possible interstellar dust from the Stardust capsule.

As the spaceship approached Earth, it released the sample-return capsule. On January 15, 2006, the capsule entered Earth’s atmosphere. Parachutes were deployed, and the capsule landed safely in the Utah desert.

The spacecraft used during the Deep Impact mission was designed in two Sections: a “Smart Impactor” was for crashing into the comet. It was armed with a camera so it could take pictures of the comet’s nucleus right up to the point when the impactor itself was destroyed. A flyby spacecraft was for photographing the crash and continuing on with a flyby. The flyby spacecraft was solar-powered and shielded from flying comet debris and boasted different types of cameras and other measuring devices.

The impact made a large crater and ejected a large plume of material, as NASA scientists had hoped it would. However, images and compositional measurements taken by the flyby spacecraft showed more dust, and less ice, than scientists had anticipated. Due to the composition and chemistry, scientists were able to better pinpoint where the comet may have originated — the far reaches of the Kuiper Belt.

MESSENGER's original mission at Mercury was supposed to last just one year, but NASA extended operations twice so the probe could continue its observations, which team members say have revolutionized our understanding of the planet.

MESSENGER mapped the planet in unprecedented detail, discovered that Mercury hosts a strangely offset magnetic field and confirmed that permanently shadowed craters near Mercury's poles harbor deposits of water ice. During the course of its four years at Mercury, MESSENGER captured more than 250,000 images and used its seven scientific instruments to gather extensive data sets that will keep scientists busy for years to come.