It's been a busy, exciting and bittersweet year in spaceflight.
The private spaceflight industry made some big leaps, the Trump administration announced a familiar destination for U.S. astronauts, and a venerable NASA spacecraft that fundamentally altered our understanding of habitable worlds met its fiery end.
Let's just jump right in, because this is going to be a long one: Here are Space.com's picks for the top spaceflight stories of 2017. [See Space.com's top science stories of 2017]
1. Cassini says farewell
NASA's Cassini spacecraft plunged intentionally into Saturn's thick atmosphere on Sept. 15, bringing the probe's storied mission to an end.
That mission — the $3.2 billion Cassini-Huygens project, a joint effort of NASA, the European Space Agency and the Italian Space Agency — launched in October 1997 and arrived in the Saturn system on June 30, 2004.
On Dec. 25, 2004, the Huygens lander separated from the Cassini mothership and spiraled its way toward Saturn's huge moon Titan. Three weeks later, Huygens parachuted through Titan's thick, hazy atmosphere, becoming the first spacecraft ever to land softly on the surface of a world in the outer solar system.
Huygens collected data at Titan for just a few hours. But the Cassini orbiter kept zooming through the Saturn system, studying the gas giant, its famous rings and its many moons for more than a decade. The mission's discoveries have reshaped astronomers' understanding of the Saturn system and our solar system's ability to host life.
For example, Cassini's radar observations revealed lakes and seas of liquid hydrocarbons on Titan's surface — potential abodes for life-forms, albeit exotic ones that would be very different from the water-dependent organisms of Earth.
And the mission discovered geysers of water ice, organic materials and other stuff blasting from the south pole of another Saturn moon, the ice-covered Enceladus. Further Cassini observations strongly suggested that Enceladus harbors an ocean of liquid water beneath its icy shell, and that this ocean contains energy sources that could sustain life. (The Titan and Enceladus discoveries shaped the endgame for Cassini, which was running out of fuel: Mission managers wanted to dispose of the spacecraft properly, to make sure it never contaminated Titan or Enceladus with microbes from Earth.)
That's just a small sampling of the incredible science that Cassini delivered over its 13-plus years at Saturn. And then there were the images, thousands of them, that brought the mystery and exotic beauty of the Saturn system to the world daily. RIP, Cassini. You are missed. [Cassini's Greatest Hits: The Spacecraft's Best Images of Saturn]
2. Private spaceflight makes big strides
This year made it clear that we're in the middle of a private spaceflight revolution.
Let's start with SpaceX, whose Falcon 9 rockets flew 18 missions in 2017 — more than twice the company's previous record for a calendar year. Fourteen of those liftoffs featured landings by a Falcon 9 first stage; five of the launches involved pre-flown boosters, and two of them lofted used Dragon cargo spacecraft toward the International Space Station (ISS).
So, SpaceX hit some big milestones this year in its quest for full and rapid reusability, a key priority for company founder and CEO Elon Musk. Indeed, reusability is at the core of SpaceX's planned Mars-colonizing spaceflight system, the latest blueprints of which Musk unveiled in September at a conference in Australia.
A number of other private-spaceflight players also made significant progress in 2017. For example, Jeff Bezos' Blue Origin launched the new version of its New Shepard suborbital vehicle on its maiden flight, laying the foundation for possible crewed missions next year. Sierra Nevada Corp.'s Dream Chaser space plane conducted its first-ever "glide flight" in Earth's' atmosphere; Stratolaunch took its huge, satellite-lofting aircraft out for a maiden test drive on the runway; Rocket Lab and Vector flew their small-sat launchers for the first time, and Virgin Galactic's latest suborbital spaceship, VSS Unity, conducted four glide flights in the skies over Southern California.
SpaceX and Virginia-based company Orbital ATK made a series of uncrewed resupply runs to the ISS, and SpaceX and Boeing continued to develop their ISS astronaut taxis, with the goal of flying crews for the first time next year.
Oh, and California startup Made In Space sent to the ISS a machine designed to manufacture exotic optical fiber in orbit, which the company eventually aims to sell here on Earth. This little space factory's Dec. 15 launch may be a preview of spaceflight's future: It flew to the orbiting lab aboard a pre-flown Dragon, which itself flew atop a Falcon 9 with a used first stage.
3. U.S. astronauts are going back to the moon
On Dec. 11, President Donald Trump signed "Space Policy Directive 1," which instructs NASA to send astronauts back to the moon — rather than to a near-Earth asteroid, as former President Barack Obama had ordered — as a stepping stone to Mars.
"This time, we will not only plant our flag and leave our footprint — we will establish a foundation for an eventual mission to Mars," Trump said of the directive, which makes official a decision announced by Vice President Mike Pence in October at the inaugural meeting of the revived National Space Council. (The NSC had last been active in the early 1990s, during the presidency of George H.W. Bush.)
Neither Pence nor Trump gave any details about the new moon plan, however. It's unclear when the next crewed lunar landings will take place, or how much it will cost to make them happen. [From Ike to Trump: Presidential Visions for Space Exploration]
4. North Korea tests intercontinental ballistic missiles
North Korea's missile program is advancing faster than experts had predicted. The nuclear-armed rogue nation test-flew intercontinental ballistic missiles (ICBMs) three times this year, showcasing the apparent ability to hit the U.S. East Coast.
It's unclear if North Korea has mastered all the requisite ICBM technology, such as a re-entry vehicle, which would protect a nuclear warhead during an operational launch. And experts don't know for sure whether the Hermit Kingdom has miniaturized its nukes, allowing them to fit them on ICBMs. But this year shows that we probably shouldn't underestimate North Korea or its technical skills.
5. Peggy Whitson breaks U.S. spaceflight record
And now, back to the good news: This year, Peggy Whitson broke the record for the most total time spent in space by a U.S. astronaut.
That record — 534 days — was set just last year, by Jeff Williams. Whitson smashed the old record while aboard the ISS in April, finally landing on Earth in September after a 288-day orbital stint that took her lifetime spaceflight tally to 665 days.
Whitson holds several other spaceflight marks as well. In 2007, she became the first woman ever to command the ISS, and on her latest mission, she became the first woman to do so twice. She has also spent more time on spacewalks (60 hours and 21 minutes) than any other female astronaut. And during her latest ISS stint, Whitson became the oldest woman to travel in space, at 57.
The world record for most time spent in space is 879 days, which cosmonaut Gennady Padalka accumulated over the course of five missions, four of them to the ISS and one to the Soviet-Russian station Mir.
6. NASA chooses new astronaut class
NASA unveiled its 12 newest astronaut candidates (or "ascans," as the agency calls them) in June. The seven men and five women were chosen from an initial pool of 18,300 applicants — more than twice as many as the previous record of 8,000, which was set back in 1978. (The most recent astronaut class, which was announced in 2013, had an applicant pool of about 6,400, NASA officials said.)
The new ascans are now engaged in a two-year training program, at the end of which they'll become full-fledged astronauts. These spaceflyers are earning their wings at a pretty exciting time: Some of them may ride private spaceships to and from the ISS, and others may fly to the moon or Mars.
7. OSIRIS-REx flies by Earth
NASA's OSIRIS-REx asteroid-sampling spacecraft zoomed past Earth on Sept. 22, a little over a year after launching off the planet. But the probe didn't return because it was homesick; it came back to use Earth's gravity to boost its speed and refine its course toward the potentially hazardous asteroid Bennu.
If all goes according to plan, OSIRIS-REx (whose name is short for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) will rendezvous with Bennu in August 2018, snag some samples of the space rock two years later and return these samples to Earth in September 2023.
8. China sends cargo craft to space lab
China launched the uncrewed Tianzhou-1 cargo spacecraft to Earth orbit on April 20, and the freighter docked with the nation's Tiangong-2 space lab two days later. Over the next five months, the two vehicles performed a series of robotic refueling exercises, demonstrating technologies that China will need in order to build and maintain an orbiting space station — a goal the nation hopes to achieve by the mid-2020s.
Tianzhou-1 was intentionally de-orbited into Earth's atmosphere on Sept. 22, the very day that OSIRIS-REx flew by.
9. 104 satellites!
India's Polar Satellite Launch Vehicle (PSLV) lofted 104 satellites to Earth orbit on Feb. 14, shattering the previous single-launch record of 37 spacecraft, which had been held by Russia's Dnepr booster.
The satellites aboard the PSLV came from six different nations: India, the United States, the Netherlands, Israel, Kazakhstan and Switzerland. The vast majority — 88 of the 104 craft — were small, Earth-observing cubesats built by the San Francisco-based company Planet (formerly called Planet Labs).
10. The X-37B launches again
On Sept. 7, the U.S. Air Force's robotic X-37B space plane launched on another mystery mission to Earth orbit, the craft's fifth overall. The vehicle may stay aloft for a while; each successive X-37B mission has set a new program duration record, and the most recent one orbited Earth for 718 days.
The X-37B rode a Falcon 9 rocket to orbit on Sept. 7, marking the first time SpaceX had provided the space plane's lift. All four previous X-37B launches had come atop United Launch Alliance Atlas V rockets. [In Photos: SpaceX Launches X-37B Space Plane, Lands Falcon 9 Rocket]
11. Happy 60th anniversary, Sputnik 1
This year occasioned some historical reflection, because some big spaceflight anniversaries came in 2017. For example, the first-ever artificial satellite, the Soviet Union's Sputnik 1, launched to Earth orbit on Oct. 4, 1957, kicking off the space age.
And NASA's twin Voyager probes marked 40 years in space in 2017: Voyager 2 launched on Aug. 20, 1977, and Voyager 1 lifted off on Sept. 5 of that year. Both probes are still going strong, exploring previously uncharted regions. Voyager 1 reached interstellar space in August 2012, and its sibling should reach that exotic realm in the next few years, NASA officials have said.
Also this year, Voyager 1 fired its backup thrusters for the first time since 1980. The little engines performed well, and their utility should help extend the spacecraft's life by a couple of years, mission officials said.