Planetary exploration is fraught with peril. Spacecraft—let alone humans—face a dramatic range of conditions, from airless worlds bombarded by tiny meteorites to environmental extremes. Even space itself poses risks. Spacecraft must sometimes endure intense radiation or extreme solar heat. They must be carefully engineered to survive.
The spinning vortex of Saturn’s north polar storm appears in this false-color image from the Cassini spacecraft. The images were taken with the Cassini spacecraft narrow-angle camera using a combination of spectral filters sensitive to wavelengths of near-infrared light. Red indicates low clouds and green indicates high ones. Image courtesy of NASA/JPL-Caltech/SSI.
Before space travel, scientists knew little about what lay beyond Earth’s atmosphere. The hazards of radiation and speeding particles could only be estimated from ground-based or high-altitude observations.
In 1962, Mariner 2 became the first spacecraft to travel to another planet. As it flew toward Venus, it recorded data showing that interplanetary space is relatively safe for uncrewed spacecraft. However, the regions close to the Sun and around giant Jupiter pose great perils. Missions with much longer exploration lifetimes than Mariner 2 require many more protective features.
Mariner 2 flew by Venus to measure its surface temperature. Contact with Mariner 2 was lost on January 2, 1963; it is now in orbit around the Sun. The model spacecraft in the Museum’s collection, seen here, was constructed from test components by engineers from NASA's Jet Propulsion Laboratory. Image courtesy of the Smithsonian Institution.
But why? Isn’t the space between each planet just that—space? We often picture space as empty, but there is in fact much more to look out for than meets the eye. Electric and magnetic fields are present nearly everywhere. Particles from the Sun and from outside our Solar System travel through space at enormous speeds.
Jupiter’s powerful magnetic field creates brilliant aurora displays, like this one captured by the Hubble Space Telescope. Image courtesy of NASA, ESA, and J. Nichols (University of Leicester).
For example, solar flares—or giant solar explosions—send streams of particles far from the Sun, which can disrupt radio communication on Earth and create hazards for astronauts in space. Temperature is also a challenge while traveling through space. For instance, the Magellan spacecraft “hid” its electronics behind its solar panels to limit heating by intense sunlight while it was orbiting Venus—the second planet from the Sun.
NASA’s Solar Dynamics Observatory recorded these solar flares in 2014. Image courtesy of NASA.
There are more than just electric and magnetic fields and drastic temperature changes to look out for in space. Dust grains leftover from when our solar system formed, together with roving rocks called asteroids, move through interplanetary space. Most burn up when they fall through a planet’s atmosphere, however, if one does land, it is known as a meteorite. On airless worlds like the Moon, countless impacts by tiny meteorites slowly break down rock into fine-grained dust. Less frequent impacts by bigger objects create large craters and vast basins.
Custom Image Caption
The small white patches on this lunar rock are called “zap pits.” Zap pits are where tiny meteorites have created tiny craters surrounded by circles of melted material. The smoother part in the center of the photo is where a piece of the rock broke off—showing the surface below with almost no zap pits. Image courtesy of NASA.
Custom Image Caption
Large craters on icy moons of the outer planets have vast rings of cracks around them, but they are shallower than craters on rocky worlds. Valhalla crater on Callisto (a moon of Jupiter) is almost 2,500 (4,000 kilometers) wide. Image courtesy of NASA.
Planets like Mercury have extreme temperatures spacecraft must contend with. The MESSENGER spacecraft made three flybys of Mercury before entering into orbit in 2011. It was the first spacecraft to explore the entire surface of Mercury. MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) carried cameras, a laser altimeter to measure landform heights, and other instruments to determine the chemistry of the rocky surface. Since Mercury is the closest planet to the Sun, MESSENGER had to carry a shield-like sunshade to keep the spacecraft and its instruments cool. Talk about hot real estate!
Probes that venture to the surface of a planet must be able to control their descent, sometimes withstand intense heat and pressure, and survive long enough to collect and send data—no easy feat! Early lunar probes simply crashed onto the Moon’s surface, destroying themselves in the process. Since then, scientists and engineers have developed a variety of techniques have been used for different planetary environments to make soft landings on other worlds.
Custom Image Caption
This photo is dated December 22, 1961 and is from a group of images called “Ranger 3 still shots at Cape Canaveral.” Given the date and the caption, it’s safe to say that this is a photo of technicians preparing the ill-fated Ranger 3 spacecraft for launch to the Moon—NASA's first attempt to land a spacecraft on the Moon. A series of malfunctions sent the spacecraft hurtling past the Moon, and Ranger 3 was unable to enter lunar orbit. Image courtesy of NASA/JPL-Caltech.
Custom Image Caption
Saturn’s moon Titan has a thick atmosphere. The European Space Agency’s Huygens probe deployed a parachute to land on the surface in 2005. The artist’s concept pictured here is based on photos taken by the lander. Image courtesy of NASA/ESA.
Custom Image Caption
The ingenious sky crane system lowered the large rover Curiosity to the surface of Mars in 2021. It used a combination of parachutes at high altitude and then rockets to slow the craft as it approached the surface. Image courtesy of NASA/JPL-Caltech.
Now that we know traveling in the space between and landing on other planets poses hazards, what happens when uncrewed spacecraft get to an actual planet? Unfortunately, conditions don’t get much better.
The temperature on a planet depends not only on its distance from its star, but also on the composition and density of its atmosphere. The worlds closest to the sun—Mercury, Venus, Earth, and Mars—vary widely. Airless Mercury can be hot enough to melt lead during the day, but cold enough to turn gasses into ice at night. On worlds beyond Mars, extreme cold helps create rugged landscapes of exotic ices.
This image from the Mars rover Opportunity shows clouds in the Martian sky. The thin atmosphere of Mars allows temperatures on the planet to swing from 68 degrees Fahrenheit (20 degrees Celsius) on the planet’s warmest days near the equator to –58 degrees Fahrenheit (-50 degrees Celsius) at night. Image courtesy of Mars Exploration Rover Mission, Cornell, JPL, NASA.
Let’s take a closer look at Venus. Blanketed by a thick atmosphere, Venus holds in heat. Its day and night temperatures are almost the same. Venus’s atmosphere presses down on its surface with 90 times the weight of the Earth’s atmosphere—that’s the same as being 3000 feet (900 meters) below the surface of the ocean—a place only specialized submarines can reach! Heat is trapped by dense clouds and haze. The average surface temperature of 864 degrees Fahrenheit (462 degrees Celsius) is among the highest in our solar system.
This infrared image from Japan’s Akatsuki spacecraft highlights Venus’s dense cloud structure.
Venus may be the most hostile of all the planets. Just reaching the surface is challenging. The dense atmosphere slows a probe’s descent, but heat and pressure increase as it approaches the ground. Once on the surface, the intense heat destroys spacecraft electronics. These challenging conditions require innovative technologies.
This is a photograph of the engineering model of the balloon descent module which rode atop the Russian Vega spacecraft. The balloon descent module, which stored its balloon in the upper sphere, was used for exploring Venus’s atmosphere. A lander, shown in this image, was also released to make compositional measurements of Venus’s surface. Image courtesy of the Smithsonian Institution.
Exploring other planets is an ongoing process. While we may not be able to visit other planets just yet—we are learning more all the time about other worlds.