Seven Surprises From the First Flybys of Each Planet in the Solar System

As the closest planet to Earth, Venus was the logical first target for a flyby mission. Launched on August 27, 1962, the Mariner 2 mission to Venus was the first spacecraft to successfully conduct a planetary encounter. It flew within 21,600 miles of Venus on December 14. Mariner 2 didn’t have a camera, but it did carry microwave and infrared instruments that revealed the surface of Venus is about 930 degrees Fahrenheit, hot enough to melt lead, and its atmosphere contains mostly carbon dioxide. The findings quashed any hopes that Venus might harbor surface life, but they did pave the way for more detailed planetary studies. In the late 1970s and early '80s, the Soviet Union sent a series of landers to Venus designed to function for a short time before succumbing to the incredibly hostile environment. The Venera probes completed the first-ever soft landing on another planet, made the first broadcast from the surface of another world, and took the first color pictures of the Venusian surface.

After a failed attempt with Mariner 3, NASA successfully launched Mariner 4 toward the Red Planet on November 28, 1964. On July 14 and 15 of the following year, Mariner 4 flew by Mars and took the first deep-space photos of another world. The pictures, covering about one percent of the planet’s surface, revealed desolate, crater-ridden terrain that made the scientific community reconsider long-held theories about life existing on modern Mars. Mariner 4 also returned an estimated daytime temperature of -148 degrees Fahrenheit and determined that Mars has no magnetic field, leaving it exposed to solar and cosmic radiation. Still, Mars has been one of the most popular targets for planetary studies, in part because of hints that it may once have supported life. NASA’s MAVEN orbiter is currently studying the Martian atmosphere to determine when the planet dried out, while the Curiosity rover has been finding evidence for ancient lakes and rivers on the surface. NASA plans a sample-return mission with the European Space Agency in the 2020s, which would bring pieces of Mars back to Earth for further study.

Pioneer 10 was the first spacecraft to travel beyond Mars’s orbit and through the main asteroid belt, showing that this band of space rocks is actually less of a danger to spacecraft than thought. Launched on March 2, 1972, Pioneer 10 achieved closest approach to Jupiter on December 3, 1973. The craft transmitted more than 500 images of the gas giant and some of its moons back to Earth, including close-up pictures of Jupiter’s Great Red Spot, a constantly raging anticyclonic storm two to three times the size of Earth. The probe continued taking data even after it left Jupiter and sped past Saturn, revealing that Jupiter has an enormous magnetic “tail” that reaches all the way to the ringed planet’s orbit. It will be more than two million years before Pioneer 10 travels past the next-nearest star on its trajectory, Aldebaran, in the constellation Taurus. On the off chance of an alien intercept, the craft carries a plaque with images of a man and woman and symbols intended to provide information about the location of Earth in the galaxy.

The last of the Mariner missions was the first spacecraft to use what’s known as a gravitational slingshot: using the pull of a planet (in this case, Venus) to alter the probe’s speed and trajectory toward its primary target. The technique has since proven crucial to reaching planets in the distant solar system—New Horizons got a gravitational boost from Jupiter on its way toward Pluto. Launched on November 3, 1973, Mariner 10 performed three flybys of Mercury over the course of a year, the first on March 29, 1974. Mariner 10 helped confirm that Mercury has a cratered Moon-like surface, no atmosphere, a small magnetic field and a large iron-rich core. But the probe only managed to capture about 40 percent of Mercury’s surface, and that was the best imagery we had to work with until NASA’s MESSENGER orbiter arrived at Mercury more than 30 years later.

Launched on April 5, 1973, Pioneer 11’s closest approach to Saturn happened more than six years later, on September 1, 1979. The craft discovered a new ring of Saturn, the narrow F ring, and found a new moon, a 124-mile-wide body that it almost collided with. Pioneer 11 also determined that Saturn is primarily made of liquid hydrogen and has an overall temperature of about -292 degrees Fahrenheit. Data from Pioneer 11 helped set the stage for the Cassini spacecraft, which currently orbits Saturn and has made numerous discoveries about the planet and its moons. For instance, Cassini deployed the ESA lander Huygens to the hazy moon Titan in January 2005. Combined data from the lander and orbiter revealed that chilly Titan is the only known object in the solar system other than Earth with stable bodies of surface liquid—not water, but liquid methane and ethane.

While Voyager 1 is famously the first spacecraft to enter interstellar space, its twin, Voyager 2, is the only spacecraft to have visited the ice giants Uranus and Neptune. Both probes were designed to take advantage of a rare planetary alignment that occurs every 175 years. The spacecraft went on a “Grand Tour” of the solar system, harnessing the gravity of the aligned giant planets to slingshot from one to the next with minimal fuel. Voyager 2 launched on August 20, 1977. Its closest approach to Uranus (above left) was on January 24, 1986, and it visited Neptune (right) on August 25, 1989. Voyager 2 discovered 11 of Uranus’s 27 known moons and measured the bizarre, corkscrew-shaped magnetic field that is a result of the planet’s dramatically tilted axis. At Neptune, Voyager 2 discovered a Great Dark Spot, similar to Jupiter’s Great Red Spot, though Neptune’s storms appear to have significantly shorter lifetimes. Voyager 2 also discovered six moons orbiting Neptune and performed a flyby of the large moon Triton that revealed active geysers and polar caps. Both Voyager probes are still in contact with Earth, currently taking measurements in the farthest reaches of the solar system.

Originally considered a planet when it was discovered in 1801, Ceres was reclassified as an asteroid and most recently as a dwarf planet. It is the largest object in the main asteroid belt between Mars and Jupiter, and it is considered a protoplanet. Launched on September 27, 2007, Dawn has conducted flybys and orbital studies of Ceres and Vesta, another large body in the asteroid belt. After leaving Vesta in 2011, Dawn entered orbit around Ceres on March 6, 2015, making it the first spacecraft to orbit two celestial bodies and the first to study a dwarf planet at close range. Dawn recently took photos of mysterious bright spots on the surface of Ceres thought to be ice or another highly reflective material. Dawn will continue to orbit Ceres at successively lower altitudes, mapping the surface with visible and infrared spectrometers.