As the 2010s come to a close, it's time to revisit how some of the biggest space science stories shaped the decade.

From the rise of TESS to flybys of Pluto and Cassini's dramatic demise, the past ten years have produced some incredible science. Here are some of our favorite discoveries from the decade.

2010 saw cosmic chases and cosmic rays

The Deep Impact spacecraft proved to be an overachiever in 2010, chasing down a second comet after it had already observed one. After Deep Impact visited Comet Tempel 1 in 2005, NASA realized the spacecraft still had enough fuel to visit another comet as well. 2.9 billion extra miles (4.6 billion kilometers) later, it met up with Comet Hartley 2.

To researchers' surprise, the peanut-size comet was actually quite active, spewing cyanide-laced gas from its surface. This extra flyby made Deep Impact the first spacecraft to visit two comets in one mission.

Also in 2010, the sun began to wake up, with some extremely powerful solar flares! The sun goes through 11-year cycles of solar activity, and a particularly weak cycle came to an end in 2010. The result was that the sun began to exhibit powerful solar activity, which continued into 2011. These solar flares and eruptions can create dazzling aurora displays.

Scientists aren't quite sure why the sun seemed to be deviating from its typical intensity patterns but suspect it might be because of something similar to what the Earth experiences during El Nino climate patterns.

Timely arrivals in 2011

After a 6.5-year journey, NASA announced in 2011 that the Messenger spacecraft had safely entered into orbit around the planet Mercury. The completion of this tricky maneuver made Messenger the first artificial satellite to orbit Mercury. From this perch it collected data on the planet's geology, composition and thin atmosphere.

In another milestone arrival, NASA's Dawn spacecraft reached the asteroid belt between Mars and Jupiter to observe one of its largest rocks, Vesta. The asteroid was one of two stops for the spacecraft, which launched in 2007.

From some of Dawn's initial observations in 2011 scientists learned that this huge space rock had a battered surface, sporting mountain ranges and deep craters. The overall aim of the visit was to gather near-global visible and infrared images of Vesta in order to learn more about asteroid topology and composition.

We said goodbye to Voyager 1 in 2012

So long, Voyager 1! The spacecraft, which originally launched in 1977, finally passed beyond the influence of our sun and entered interstellar space in 2012. During its decades-long mission, the spacecraft sent back breathtaking photos of our solar system, including the famous "Pale Blue Dot" photograph it took in 1990. After it surpassed the Pioneer 1 spacecraft, Voyager 1 clocked the most distance traveled in space by any human-made object. (The spacecraft continues to relay data from its cosmic journey back to Earth even in 2019.)

Terrestrial explosions and discoveries in 2013

Originally dubbed the "comet of the century" due to its anticipated splendor, Comet ISON passed by the sun on November 28, 2013 (American Thanksgiving) and broke apart. Scientists had expected the comet to give off a dazzling light show as a result of what they'd believed to be a large nucleus, but the comet did little more than fade away.

The observations were a testament to just how hard comet detection and prediction can be. Upon further observation, scientists concluded that the comet's nucleus might have been much smaller than originally estimated. Nevertheless, the comet's slow approach still allowed scientists to study and learn more about comet behavior before it sizzled away.

Other drama came when it wasn't expected. A reminder of space's might, in February 2013 a 56-foot (17-meter) meteor exploded 930 miles above the city of Chelyabinsk in Russia, just east of Moscow. The explosion, which scientists say was equivalent to that of 470 kilotons of TNT, injured hundreds of people and damaged buildings across the area. Scientists say this impact may have been the most powerful terrestrial meteor impact since a 130-foot (40 m) object exploded over Siberia in 1908 and flattened 825 square miles (2,137 square km) of forest.

While not incredibly common, such impacts also aren't extremely rare either, and there's not much scientists can do about them. Instead, scientists continue to work on identifying and planning for larger asteroid impacts that could cause much more damage.

Also in 2013, scientists were able to identify evidence of cosmic rays on Earth. Because these rays are very difficult to detect, scientists instead relied on observing neutrinos left behind by the rays. Neutrinos themselves are also notoriously hard to detect because they almost never interact with matter, but in the case of these cosmic rays, the IceCube Observatory in Antarctica found that they did.

The neutrinos, named Bert and Ernie after the Sesame Street characters, significantly more energetic than those produced during an event detected in 1987, but still not powerful enough to provide scientists with definitive information about the origin of the rays. Astrophysicists concluded that top candidates might be a supernova, black hole or gamma ray burst.

Successful landings and new family photos in 2014

Touchdown on a comet! In a historic first, the European Space Agency (ESA) visited a comet's surface in 2014. The spacecraft, named the Philae lander, touched down and made brief observations. It was a challenging landing, since the comet was a very small, distant target and the Philae lander had to make a leap from the larger Rosetta spacecraft, in order to touch down.

Unfortunately, Philae bounced into a shadowy area on the comet where it was unable to use its solar panels. The lander soon slipped into hibernation mode. But before doing so, Philae was able to detect an icy surface on the comet and organic molecules like carbon.

Say cheese! In 2014, for the first time ever scientists were able to take an image of our universe's cosmic web. While galaxies seem like the epicenters of our universe, they're actually extremely small compared to all other matter in space. Using light from a quasar as a flashlight, scientists were able to get a better look at these cosmic dust tendrils.

This observation was fascinating unto itself, but also gave scientists a better model for how to track down elusive dark matter, because dark matter is believed to mirror the regular matter we are able to observe.

Important new 'hello's in 2015

Hello, Pluto! NASA's New Horizons spacecraft successfully flew past the icy dwarf planet in 2015. This is the first time we've been able to see Pluto and its moon Charon up close. Notably, scientists discovered that Pluto has an adorable heart on its surface and that it and Charon might actually be more geologically active than scientists first imagined. Where scientists expected to find a pock-marked surface, similar to that of our moon, they instead found a relatively young-looking surface. This smoothness led scientists to believe that the surface may have been reshaped more recently, perhaps by something like ice.

In September 2015 scientists confirmed that geysers observed on Saturn's moon Enceladus are evidence of a global ocean inside the moon, not an isolated lake. Scientists were able to determine this based on a slight wobble detected in Enceladus's orbit around Saturn. This discovery, as well as previous discoveries by Cassini of hydrothermal activity on the moon, make Enceladus a prime candidate for future, life-hunting missions in years to come.

2016 saw out-of-this-world discoveries

2016 was a historic year for physics. Scientists were able to observe for the first time ever evidence of gravitational waves, the wrinkling of space-time that occurs when objects collide. To do so, they relied on a massive laser interferometer called LIGO (Laser Interferometer Gravitational-Wave Observatory) with its twin detectors in Washington and Louisiana. Gravitational waves had been theorized by Einstein but had never been detected before.

Scientists detected two sets of gravitational waves in 2016, both triggered by the collisions of black holes millions of years prior that echoed across the universe. These discoveries would go on to win the Nobel Prize for physics in 2017.

In another strange discovery for the year, the Hubble Space Telescope spotted what appeared to be 125-mile-high (200 kilometers) geysers of water vapor erupting from the south pole of Jupiter's moon, Europa. Such a geyser was first spotted in 2012, but astronomers thought that observation was a fluke.

Scientists have known for a while now that an icy ocean lies beneath Europa's surface, but evidence of geyser eruptions means that it might be possible for probes to directly analyze Europa's water for signs of microbial life. NASA's Europa Clipper spacecraft, slated for launch next decade, will further investigate those possibilities.

Big discoveries and somber goodbyes in 2017

Aug. 17, 2017 was a life-changing day for astronomers. On that day, scientists observed the collision of two incredibly dense neutron stars, by detecting both gravitational waves and light created in the collision. While this was the fifth time scientists had observed gravitational waves, it was the first time they were able to observe such events through other measurements as well.

A massively international effort between observatories in Italy, Chile and a NASA space telescope allowed scientists to chase the gravitational-wave signal across the sky and locate the event using light observations. The team was able to confirm that the collision produced heavy elements like gold.

Just days later, people gathered at science museums and open spaces on Aug. 21 to observe a once-in-a-century event: the great American solar eclipse. The event crossed the country coast to coast, from Oregon to South Carolina, and ra 70-mile wide swath caught totality.

Farther from home, 2017 saw the demise of a beloved mission, Cassini. The spacecraft launched in 1997 to orbit and observe Saturn and its many moons — and boy, did it. During its 13-year dance with Saturn, the spacecraft discovered half a dozen moons, geysers on Enceladus and lakes on Titan, not to mention a treasure trove of beautiful images of the planet.

But all good things must eventually come to an end. After more than a decade of circling Saturn, Cassini was running out of fuel and its mission team decided to send it out with one last hurrah. The mission came to a fiery conclusion on Sept. 15, 2017, as Cassini purposefully dove into Saturn, burning up in its atmosphere like a meteor. The maneuver kept nearby moons safe from contamination and offered unprecedentedly close-up observations of the planet.

In October 2017 scientists detected the first known alien visitor passing through our solar system. A team of scientists modeled the path of the object, a space rock later named 'Oumuamua, and determined that it didn't originate in our solar system. While scientists have long theorized about how an alien object could come in contact with our solar system, this was the first observation of such an object.

The rise and fall of great missions in 2018

2018 was both an exhilarating and mournful year for space missions, ushering in exciting new prospects while also saying goodbye to some old favorites.

After nearly 15 years on the Martian surface, the Mars Opportunity rover finally lost contact with Earth on June 10, 2018, after a planet-wide dust storm caused the rover to retreat into low-power mode. Ater listening for months for a signal from the rover, the Opportunity team determined the mission complete at the end of January.

Opportunity and its twin rover, Spirit, landed on Mars in 2004 with a life-expectancy of 90 Martian days; both outlived that timeline with gusto. Spirit survived seven years on the Martian surface and Opportunity went on to last nearly 15, traveling a grand total of 26.5 miles (42.65 km) across the Red Planet and performing important geological analysis all the while. The conclusion of the mission touched the team involved with the mission and space enthusiasts alike.

The same year saw the end of another iconic mission, the exoplanet-hunting space telescope Kepler. The mission launched in March 2009 to see what kind of planets might be lurking beyond our solar system. While Kepler suffered an early failure in 2013 that effectively ended its initial mission, scientists were able to regain control of the telescope and transition it into a second phase, dubbed K2.

By the time that mission ended in November 2018, because the spacecraft didn't have enough fuel, Kepler had discovered a whopping 2,682 exoplanets between its two missions. And even now, there are still hundreds more exoplanet candidates identified by the mission but waiting to be confirmed by follow-up observations.

In a burst of good timing, Kepler's successor was already up and running after launching in April 2018. Like Kepler, the Transiting Exoplanet Survey Satellite (TESS) is designed to search for exoplanets. It is scanning both hemispheres of the sky during its first two years of operation, which continue into summer 2020. At the end of its first year, TESS had already found 28 confirmed exoplanets — several of which appear to be in the so-called habitable zone — and 993 potential planets.

With any luck, these swaths of exoplanet data collected by both TESS and Kepler will give the James Webb Space Telescope plenty to work with. The James Webb is scheduled to launch in 2021 and will, among other work, examine exoplanet atmospheres to learn more about these potentially habitable worlds.

2019

Scientists rang in the new year in 2019 with an incredibly distant flyby. New Horizons flew past a Kuiper Belt object dubbed 2014 MU69 just as the calendar turned over in North America. Now officially called Arrokoth, the object is a two-lobed spinning pancake in the cold, dark reaches of outer space. Now, scientists on the team are trying to determine whether the spacecraft has one more flyby left in it.

Other spacecraft made incredible space-rock visits this year as well. The Japanese spacecraft Hayabusa2 was orbiting an asteroid called Ryugu when the year began; over the course of 2019 it collected several samples, made an artificial crater on Ryugu's surface and turned back to Earth, where it will arrive late next year. A similar asteroid mission from NASA, OSIRIS-REx, spent all year studying its own space rock, Bennu, and strategizing how to collect samples from it.

In April, an international collaboration published the first ever image of a black hole, capturing imaginations around the world. The feat required turning all of Earth into a massive telescope of sorts, dubbed the Event Horizon Telescope, and processing truly mind-blowing amounts of data. The team released an image of the black hole at the center of a galaxy called M87; scientists have also been processing data about a similar structure at the heart of the Milky Way.

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