2016 was a very busy year in space.
A number of high-profile missions lifted off, others reached their destinations after long journeys through deep space, and a few, sadly, crashed and burned.
Here's Space.com's look at the most important spaceflight stories of the year. [The 100 Best Space Photos of 2016]
1. Rocket landings galore
Some of the most exciting spaceflight action of 2016 involved rockets coming down rather than going up. California-based company SpaceX managed to land the first stage of five different Falcon 9 rockets during operational orbital launches this year; one of the boosters touched down back at the launch pad, whereas the other four landed on robotic "drone ships" stationed in the Atlantic Ocean.
And the Washington-based company Blue Origin launched and landed the same suborbital New Shepard rocket four times this year, finally retiring the booster after a successful October test flight. (Dating back to 2015, SpaceX now has a total of six successful rocket landings, and Blue Origin has five.)
Both SpaceX and Blue Origin — which are headed by billionaire entrepreneurs Elon Musk and Jeff Bezos, respectively — aim to develop fully reusable rockets as a way to slash the cost of spaceflight and open up the heavens to exploration. As the above successes show, this year brought both companies closer to that ambitious goal. [Reusable Rocket Launch Systems: How They Work (Infographic)]
2. Juno arrives at Jupiter
NASA's $1.2 billion Juno probe launched in August 2011, on a mission to study Jupiter's atmosphere, composition, and gravitational and magnetic fields. The spacecraft finally arrived at the giant planet this year, slipping into Jupiter orbit after acing a make-or-break, 35-minute engine burn on the night of July 4.
The road has been a bit bumpy for Juno since then. An apparent valve issue prevented the spacecraft from performing a planned engine burn in October, so Juno remains in a long and looping 53-day orbit rather than its envisioned 14-day science orbit. In addition, a glitch caused the probe to go into a protective safe mode shortly before its Oct. 19 Jupiter close approach; as a result, Juno didn't gather data during the flyby.
But mission team members are working through these issues. Juno's latest close Jupiter flyby, on Dec. 11, went well, and the spacecraft is healthy overall, Juno principal investigator Scott Bolton, of the Southwest Research Institute in San Antonio, said this month.
3. NASA asteroid-sampling mission lifts off
Shortly after Juno reached its destination, another NASA spacecraft began a long deep-space journey. On Sept. 8, the $800 million Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) probe lifted off from Cape Canaveral Air Force Station in Florida.
If all goes according to plan, OSIRIS-REx will rendezvous with the 1,640-foot-wide (500 meters) near-Earth asteroid Bennu in August 2018. The probe will study the space rock from orbit for two years, then snag at least 2 ounces (60 grams) of asteroid material in July 2020. In September 2023, this sample will make it back to Earth, where scientists will examine the material, searching for the carbon-based building blocks of life and other interesting molecules. [Blastoff! OSIRIS-REx Launches to Asteroid Bennu (Video)]
4. European Mars mission arrives at the Red Planet
The first phase of the European-led ExoMars mission launched in March, sending a lander and an orbiter streaking toward the Red Planet. The duo got there in October, but only one of them lived to tell the tale.
The Trace Gas Orbiter (TGO) — which will sniff the Martian atmosphere for methane, a possible sign of Red Planet life — slipped into orbit successfully on Oct. 19. But earlier that day, the lander, known as Schiaparelli, crashed on Mars. Schiaparelli's computer apparently thought the lander was much closer to the planet's surface than it actually was, and as a result, the vehicle didn't fire its descent-slowing thrusters for nearly long enough, European Space Agency (ESA) officials have said.
Schiaparelli's main task was to prove out landing technologies that will put the life-hunting ExoMars rover down on the Red Planet in 2021. And data gathered during Schiaparelli's final minutes of life should indeed help in this aim, ESA officials have said.
5. Chinese astronauts dock with another space lab
China aims to have its own space station up and running in Earth orbit by the early 2020s, and the nation made a lot of progress toward that goal this year.
China launched its second-ever space lab, known as Tiangong-2, on Sept. 15, to help test spacecraft docking and rendezvous technologies. Then, on Oct. 18, two astronauts aboard the Shenzhou-11 spacecraft docked with the 9.5-ton (8.6 metric tons) Tiangong-2 and stayed aboard the lab for a month. The pair's stint more than doubled the previous Chinese record for longest crewed space mission.
Tiangong-2 is following in the footsteps of Tiangong-1, which launched in September 2011 and hosted three docking missions (two of which were crewed) before ending its operational life this past March.
6. Rosetta comet probe dives to its death
Europe's epic Rosetta comet mission came to an end on Sept. 30, when the probe dove into the surface of Comet 67P Churyumov-Gerasimenko. This was a planned suicide: The comet was streaking far from the sun, and the solar-powered Rosetta would not have been able to stay operational for much longer, ESA officials said. [Gallery: Rosetta's Last Comet Photos During Crash-Landing]
Rosetta launched in March 2004 and arrived at 67P in August 2014, in the process becoming the first spacecraft ever to orbit a comet. The mission achieved another first that November, when the Rosetta mothership dropped a lander called Philae onto Comet 67P's surface. That touchdown didn't go entirely as planned; Philae's anchoring harpoons failed to fire, and the lander bounced twice before finally coming to rest in a location that remained mysterious for nearly two years. (The Rosetta team didn't find Philae until early September, just weeks before Rosetta's death dive.)
The 1.3-billion-euro ($1.36 billion at current exchange rates) Rosetta mission captured the best-ever looks at a comet, and the mission's data should help scientists better understand these icy wanderers and the solar system's early days, mission team members have said.
7. SpaceShipTwo back in action
On Oct. 31, 2014, Virgin Galactic's first SpaceShipTwo vehicle, known as VSS Enterprise, broke apart during a rocket-powered test flight, killing co-pilot Michael Alsbury and injuring pilot Peter Siebold. The tragedy grounded the company — until this year.
In February, the company unveiled its new SpaceShipTwo, which is called VSS Unity. The suborbital space plane lifted off with its mothership in a "captive carry" test for the first time in September, then made its maiden free-flight test on Dec. 3, gliding back down to Earth in a solo runway landing. Unity will perform a series of such "glide flights" before beginning the rocket-powered phase of its test campaign.
SpaceShipTwo is designed to carry six passengers to an altitude of 62 miles (100 km) or so, then bring them back down to Earth. Tickets to ride the suborbital space plane currently sell for $250,000.
8. Spaceflight is hard: 2016 edition
As SpaceShipTwo's story indicates, spaceflight is a tough proposition, and the world got two more reminders of that fact this year.
On Sept. 1, a SpaceX Falcon 9 rocket exploded on the pad during a routine pre-launch test, destroying the booster and the $200 million AMOS-6 communications satellite. SpaceX engineers traced the anomaly to the interaction between oxygen and a carbon-composite helium container in the Falcon 9's upper stage. Falcon 9s have been grounded since the incident, but one of them should return to flight next month, SpaceX representatives have said.
Then, on Dec. 1, Russia's uncrewed Progress spacecraft failed during a cargo launch toward the International Space Station (ISS). A problem with the third stage of the Progress' Soyuz rocket apparently doomed the freighter, which burned up over southern Russia.
Such cargo-ship incidents aren't terribly uncommon. Another Progress fell back to Earth in May 2015 without reaching the station, and Orbital ATK and SpaceX — both of which signed ISS re-supply contracts with NASA — suffered their own failures in October 2014 and June 2015, respectively. (Both companies have flown successful cargo missions since those incidents.)
9. Elon Musk unveils Mars-colonization plans, and another team shoots for the stars
Elon Musk has long dreamed of colonizing Mars. Now we know how he plans to do it.
At a conference in Mexico in September, the SpaceX chief unveiled blueprints for the planned Interplanetary Transport System (ITS) — a reusable rocket-and-spaceship duo that Musk said could begin ferrying colonists to the Red Planet by the mid-2020s, if everything goes according to plan. ITS could potentially also fly astronauts to more far-flung destinations, such as Jupiter's ocean-harboring moon Europa, Musk said. [Images: SpaceX's Interplanetary Transport System]
Musk wasn't the only person to reveal a bold spaceflight vision this year. In April, famed cosmologist Stephen Hawking, several other scientists and billionaire investor Yuri Milner announced the $100 million Breakthrough Starshot project, which aims to develop a laser-sailing spaceflight system that will accelerate tiny probes to 20 percent the speed of light or so. The long-term goal is to send flotillas of such spacecraft to study Proxima Centauri and other nearby star systems up close, team members said.
10. Yearlong ISS mission ends
The longest-ever ISS mission came to an end this year, with the March touchdown of NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko.
Kelly and Kornienko spent 11 months aboard the ISS, allowing doctors and scientists to gather a wealth of data about the effects of long-duration spaceflight on human physiology and behavior. Such data will inform preparations to send astronauts to Mars, NASA officials have said. (It takes six to nine months to get to Mars with currently available propulsion technology.)