Step outside on a clear night, and you can be sure of something our ancestors could only imagine: Every star you see likely plays host to at least one planet.

The Moon and Mars will probably be the first places in the Solar System that humanity will try to live after leaving the safety and security of Earth. But those worlds are still incredibly harsh environments, with no protection from radiation, little to no atmosphere, and extreme temperatures.

Volcanic activity on Io was discovered by Voyager 1 imaging scientist Linda Morabito. She spotted a little bump on Io’s limb while analyzing a Voyager image and thought at first it was an undiscovered moon. Moments later she realized that wasn’t possible — it would have been seen by earthbound telescopes long ago. Morabito and the Voyager team soon came to realize they were seeing a volcanic plume rising 190 miles (300 km) off the surface of Io. It was the first time in history that an active volcano had been detected beyond the Earth.

Forty-five years have passed since humans last set foot on an extraterrestrial body. Now, the moon is back at the center of efforts not only to explore space, but to create a permanent, independent space-faring society.

When searching for extra-solar planets, astronomers most often rely on a number of indirect techniques. Of these, the Transit Method (aka. Transit Photometry) and the Radial Velocity Method(aka. Doppler Spectroscopy) are the two most effective and reliable (especially when used in combination). Unfortunately, direct imaging is rare since it is very difficult to spot a faint exoplanet amidst the glare of its host star.

I saw my first artificial satellite with my naked eyes during the summer of 1994. I was watching pieces of comet Shoemaker-Levy 9 impact Jupiter from a small observatory with a college astronomy club when someone pointed up – away from the telescope – and said, “Look, it’s a satellite!”

Physicists trying to understand the fundamental structure of nature rely on consistent theoretical frameworks that can explain what we see and simultaneously make predictions that we can test. On the smallest scale of elementary particles, the standard model of particle physics provides the basis of our understanding.

When it comes time for NASA to send astronauts back to the Moon and on to Mars, a number of new spacecraft systems will come into play. These include the Space Launch System (SLS), the most powerful rocket ever built, and the Orion Multi-Purpose Crew Vehicle (MPCV) – a next-generation spacecraft that will carry crews beyond Low Earth Orbit (LEO).

Shortly after 4 a.m. on a crisp, cloudless September morning in 1859, the sky above what is currently Colorado erupted in bright red and green colors. Fooled by the brightness into thinking it was an early dawn, gold-rush miners in the mountainous region of what was then called the Kansas Territory woke up and started making breakfast. What happened in more developed regions was even more disorienting, and carries a warning for the wired high-tech world of the 21st century.

In 1961, famed astrophysics Frank Drake proposed a formula that came to be known as the Drake Equation. Based a series of factors, this equation sought to estimate the number of extra-terrestrial intelligences (ETIs) that would exist within our galaxy at any given time. Since that time, multiple efforts have been launched to find evidence of alien civilizations, which are collectively known as the search for extra-terrestial intelligence (SETI).

70,000 years ago, our keen-eyed ancestors may have noticed something in the sky: a red dwarf star that came as close as 1 light year to our Sun. They would’ve missed the red dwarf’s small, dim companion—a brown dwarf—and in any case they would’ve quickly returned to their hunting and gathering. But that star’s visit to our Solar System had an impact astronomers can still see today.

For decades, scientists have been speculating that life could exist in beneath the icy surface of Jupiter’s moon Europa. Thanks to more recent missions (like the Cassini spacecraft), other moons and bodies have been added to this list as well – including Titan, Enceladus, Dione, Triton, Ceres and Pluto. In all cases, it is believed that this life would exist in interior oceans, most likely around hydrorthermal vents located at the core-mantle boundary.

For centuries, astronomers have been observing Jupiter swirling surface and been awed and mystified by its appearance. The mystery only deepened when, in 1995, the Galileo spacecraftreached Jupiter and began studying its atmosphere in depth. Since that time, astronomers have puzzled over its colored bands and wondered if they are just surface phenomenon, or something that goes deeper.

Mathematical physicist and cosmologist Stephen Hawking was best known for his work exploring the relationship between black holes and quantum physics. A black hole is the remnant of a dying supermassive star that’s fallen into itself; these remnants contract to such a small size that gravity is so strong even light cannot escape from them. Black holes loom large in the popular imagination – schoolchildren ponder why the whole universe doesn’t collapse into one. But Hawking’s careful theoretical work filled in some of the holes in physicists’ knowledge about black holes.

Supermassive black holes, which lurk at the heart of most galaxies, are often described as “beasts” or “monsters”. But despite this, they are pretty much invisible. To show that they are there at all, astronomers typically have to measure the speed of the clouds of gas orbiting those regions.

Despite thousands of years of research and observation, there is much that astronomers still don’t know about the Milky Way Galaxy. At present, astronomers estimate that it spans 100,000 to 180,000 light-years in diameter and consisting of 100 to 400 billion stars. In addition, for decades, there have been unresolved questions about how the structure of our galaxy evolved over the course of billions of years.