NASA's Voyager 1 spacecraft was launched in 1977. More than three decades later, in 2012, it became the first human-made object to enter interstellar space by crossing the heliopause, or the edge of the heliosphere. That's the boundary beyond which most of the sun's ejected particles and magnetic fields dissipate.

Depending on how carefully you do the calculations and how you arrange them, all of the planets in the solar system could fit in between Earth and its moon. The distance between the Earth and the moon varies, as does the diameter of each of the planets — they're wider at their equators, so Saturn or Jupiter or both would have to be tilted sideways for this to work, according to news site Slate. But imagine lining them all up, pole to pole. They'd just barely squeeze in between us and our closest companion in space, blocking out the sky with their rings and gas giant bulk as they did so. The moon is the farthest from Earth that we've ever sent humans, and it's both mind-bogglingly distant and incredibly close depending on how you think about it. Eight enormous planets could fit between here and there, and yet according to NOAA, the distance from Earth to the sun is more than 390 times the distance from the Earth to the moon.

Uranus usually appears in classroom solar system models as a featureless blue ball, but this gas giant of the outer solar system is pretty weird on closer inspection. First, the planet rotates on its side, appearing to roll around the sun like a ball, according to NASA's Uranus guide. The most likely explanation for the planet's unusual orientation (about 90 degrees sideways compared to the other planets) is that it underwent some sort of titanic collision in the ancient past.

Io's eruptive nature is caused by the immense forces the moon is exposed to, nestled in Jupiter's gravitational well and its magnetic field. The moon's insides tense up and relax as it orbits closer to, and farther from, the planet, generating enough energy for volcanic activity.

While Mars seems quiet now, gigantic volcanoes once dominated the surface of the planet. This includes Olympus Mons, the biggest volcano ever discovered in the solar system. At 374 miles (602 km) across, the volcano is comparable to the size of Arizona. It's 16 miles (25 kilometers) high, or triple the height of Mount Everest, the tallest mountain on Earth.

In addition, while Earth's crust constantly moves, the Martian crust likely does not. The Hawaiian islands were formed as a hot spot in the mantle created a chain of volcanoes in the crust cruising by above it, so if the surface of Mars isn't moving, a volcano could build-up for longer in one spot.

At 2,500 miles (4,000 km) long, the immense system of Martian canyons known as Valles Marineris is more than 10 times as long as the Grand Canyon on Earth. Valles Marineris escaped the notice of early Mars spacecraft and was finally spotted by the global mapping mission Mariner 9 in 1971. Valles Marineris could stretch from coast to coast of the entire United States!

Venus is a hellish planet with a high-temperature, high-pressure environment on its surface. Bone-dry and hot enough to melt lead, it's not exactly a welcoming environment (and has probably always been inhospitable to life). When heavily shielded Venera spacecraft from the Soviet Union landed there in the 1970s, according to NASA each lasted a few minutes or, at most, a few hours before melting or being crushed beyond their ability to function. But even above its surface, the planet has a bizarre environment. Scientists have found that its upper winds flow 50 times faster than the planet's rotation. The European Venus Express spacecraft tracked the winds over long periods and detected periodic variations. It also found that the hurricane-force winds appeared to be getting stronger over time.

Water can be found as ice in permanently shadowed craters on Mercury and the moon, although we don't know if there's enough to support prospective human colonies in those places. Mars also has ice at its poles, in frost and likely below the surface dust. Even smaller bodies in the solar system have ice: Saturn's moon Enceladus, and the dwarf planet Ceres, among others. NASA scientists suspect Jupiter's moon Europa may be the most likely known candidate for extraterrestrial life because, against all expectations, there is likely liquid water below its cracked and frozen surface. Europa, much smaller than Earth, may host a deep ocean that researchers suggest could contain twice as much water as all of Earth's oceans combined.

We've been exploring space for more than 60 years, and have been lucky enough to get close-up pictures of dozens of celestial objects. Most notably, we've sent spacecraft to all of the planets in our solar system — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — as well as two dwarf planets, Pluto and Ceres.

So far, scientists have found no evidence that life exists elsewhere in the solar system. But as we learn more about how "extreme" microbes live in underwater volcanic vents or frozen environments, more possibilities open up for where they could live on other planets.

or many years, scientists believed that Earth was the only tectonically active planet in the solar system. That changed after the Mercury Surface, Space Environment, Geochemistry and Ranging spacecraft did the first orbital mission at Mercury, mapping the entire planet in high definition and getting a look at the features on its surface.

1. Along with being the solar system's largest planet, Jupiter also hosts the solar system's largest storm. Known as the Great Red Spot, it's been observed in telescopes since the 1600s and studied from modern instruments like NASA's Juno, which recently provided evidence that the storm is hundreds of miles tall (and likely fed by winds from thousands of miles below, too). The storm has been a raging conundrum for centuries, but in recent decades another mystery emerged: the spot is getting smaller. In 2014, the storm was only 10,250 miles (16,500 km) across, about half of its historic size. The shrinkage is being monitored in professional telescopes and also by amateurs. Amateurs are often able to make more consistent measurements of Jupiter because viewing time on larger, professional telescopes is limited and often split between different objects. 2. In January 2015, California Institute of Technology astronomers Konstantin Batygin and Mike Brown announced — based on mathematical calculations and simulations — that there could be a giant planet lurking far beyond Neptune. Several teams are now on the search for this theoretical "Planet Nine," and research suggests it could be located within the decade. This large object, if it exists, could help explain the movements of some objects in the Kuiper Belt, an icy collection of objects beyond Neptune's orbit. Brown has already discovered several large objects in that area that in some cases rivaled or exceeded the size of Pluto. (His discoveries were one of the catalysts for changing Pluto's status from planet to dwarf planet in 2006.) But scientists are pursuing another theory, too: that "Planet Nine" could in fact be a grapefruit-sized black hole, warping space similarly to the way a gigantic planet would. And yet another team suggests that the weird movements of the far-flung Kuiper Belt occupants could be the collective influence of several small objects, not an undiscovered planet or black hole at all. 3. Neptune is roughly 30 times as far from the sun as Earth, and it gets correspondingly less heat and light. But it radiates far more heat than it's taking in and has far more activity in its atmosphere than planetary scientists would suspect, especially compared to nearby Uranus. Uranus is closer to the sun and yet radiates about the same amount of heat as Neptune, and scientists aren't sure why. Winds on Neptune can blow up to 1,500 miles per hour (2,400 km/h). Is all that energy coming from the sun, from the planet's core, or gravitational contraction? Researchers are working to find out. 4. Earth has several bands of magnetically trapped, highly energetic charged particles surrounding our planet, known as the Van Allen belts (named after the discoverer of the phenomenon.) While we've known about the belts since the dawn of the space age, the Van Allen Probes (launched in 2012) have provided our best-ever view of them. They've uncovered quite a few surprises along the way. We now know that the belts expand and contract according to solar activity. Sometimes the belts are very distinct from one another, and sometimes they swell into one massive unit. An extra radiation belt (beyond the known two) was spotted in 2013. Understanding these belts helps scientists make better predictions about space weather or solar storms.