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.

Since ancient times, astronomers have looked up at the night sky and seen the Andromeda galaxy. As the closest galaxy to our own, scientists have been able to observe and scrutinize this giant spiral galaxy for millennia. By the 20th century, astronomers realized that Andromeda was the Milky Way’s sister galaxy and was moving towards us. In 4.5 billion years, it will even merge with our own to form a supergalaxy.

Just sending a rocket into orbit is expensive. Getting to Mars would cost even more. That's why NASA is looking into cheaper options like atomic rockets.