Asteroids, Comets, Black Holes — Oh My! The Year 2019 in Astronomy

From asteroids and (interstellar) comets to black holes and the sun, 2019 has been full of amazing space science.

This past year has been a fantastic one for astronomy and planetary science. On New Year's Day, two spacecraft reached their targets, and things took off from there. Join us as we review some of the hottest science news from the last 12 months.

Related: The Greatest Spaceflight Moments of 2019
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 Farthest flyby kicks off year

On New Year's Day, 2019, NASA's New Horizons spacecraft zoomed by its target, 2012 MU69. This Kuiper Belt Object, which has since been officially named 'Arrokoth,' is the most distant object ever to ever be observed by a flyby from a spacecraft from Earth. 

New Horizons revealed that Arrokoth looked like a flat snowman, with two pancake-like lobes joined together. The incredible object immediately revealed new information about how planets and other objects formed in the early solar system, thanks to its near-pristine characteristics. While New Horizons moves onward on a journey that will eventually take it out of the solar system, it continues to send information back to Earth about Arrokoth and will do so until mid-2020. 

 Visiting an asteroid  

Also on New Year's Eve this year, NASA's Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (or the OSIRIS-REx spacecraft) entered into orbit around the asteroid Bennu. The craft arrived at Bennu in early December, and rang in the new year by firing its thrusters, which pushed it into the asteroid's orbit, making Bennu the smallest object ever orbited by a spacecraft. With this maneuver, OSIRIS-REx also set a world record for closest orbit of an object, a record the craft later broke again this past year.

But the spacecraft didn't spend the last 12 months just sitting in orbit around the asteroid. It began an in-depth study of the diamond-shaped object, searching for an ideal target area to grab a piece of Bennu in 2020. That spacecraft will then return the rocky sample to Earth for more in-depth study. 

From what the craft has found so far, it seems like Bennu displays some surprising activity, like jetting material from its surface. OSIRIS-REx has also found an interesting ridge and some intriguing boulders on the asteroid. As the year came to a close, mission scientists selected a landing place, 'Nightingale', as the sample return site. OSIRIS-REx will continue to orbit Bennu until 2021, when it will collect a sample and return to Earth. 

 Double diamonds 

Bennu wasn't the only asteroid that was visited by spacecraft in 2019. The Japanese mission Hayabusa2 was already orbiting the asteroid Ryugu when 2019 dawned. In February, the spacecraft used a sampler horn attached to its belly to gather material blown up from the surface by a bullet fired into the asteroid. 

In April, a free-flying, single-shot 'gun,' known as the Small Carry-on Impactor, fired a second bullet into the asteroid's surface after Hayabusa2 dropped a deployable camera and moved to the far side of the object. A third bullet shot into the asteroid in July, which excavated subsurface material that the spacecraft later collected in its horn.

On Nov. 12, packed with precious space rocks, Hayabusa2 bid Ryugu farewell and began its return trip to Earth. The spacecraft is expected to bring samples of the asteroid to Earth in late 2020. That may not be the end for Hayabusa2, however, as it has the potential to continue to study other asteroids. 

 A comet from another star 

In late August, astronomers caught a glimpse of a new comet, named Borisov. for its discoverer. The fast-moving object was quickly characterized as an interstellar comet, originally born around another star and making a quick tour around our sun. Unlike fellow interstellar visitor 'Oumuamua, which was only visible for a few short weeks, Borisov was discovered before it made its pass behind the sun and should be visible until late spring 2020, giving astronomers plenty of time to study it. Also unlike 'Oumuamua, a mysterious object scientists had trouble characterizing, Borisov is clearly a comet with observable surface activity and a glowing tail.

Not only is Borisov another interstellar treat for planetary scientists – it also suggests that interstellar objects may be more common than previously suspected. After 'Oumuamua's 2017 visit, astronomers didn't anticipate catching a sight of another interstellar object until the early-2020s, when the Large Synoptic Survey Telescope (LSST) goes online. The LSST should be capable of catching more faint objects, allowing it to better spot interstellar interlopers than current instruments. 

 Photographing a black hole

The year wasn't all about small bodies and planetary science. In 2019, astronomers made history by photographing a black hole. 

Using the Event Horizon Telescope, an instrument made up of multiple telescopes spread around the globe, astronomers snapped a photo of the supermassive black hole at the center of the nearby galaxy M87, which lies 53.5 million light-years away. The monster black hole weighs in at about 6.5 billion times the mass of our sun, and is even larger than the supermassive black hole at the center of our galaxy, the Milky Way.

Because the gravity of black holes swallows even light, the scientists didn't capture a picture of the black hole itself. Instead, they photographed the it's boundary, the event horizon, mapping out the black hole's silhouette against the background radiation of the material swirling around it. These researchers hope to photograph the Milky Way's own black hole in the near future. 

 Marsquakes 

In April, NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander felt the ground move under its robotic feet as the spacecraft sensed its first confirmed marsquake. The Martian equivalent of earthquakes, marsquakes come from seismic waves traveling through the planet's interior. Because Mars lacks tectonic plates, marsquakes occur less frequently than their terrestrial counterparts. Hopefully, the April marsquakes and other events will help the spacecraft on its eventual goal of tracing the interior of the Red Planet.

InSight also carried a mechanical mole with it to Mars. The instrument, a burrowing heat probe, was supposed to dig 10 to 16 feet (3 to 5 meters) beneath the planet's surface. Shortly after its February deployment, however, the "mole" became stuck about 1 foot (0.3) meters down. It was designed to dig through sandy soils like those seen around Spirit and Opportunity, but the ground under Insight is different from other landing sites. So, even though the experiment isn't going smoothly, it continues to teach scientists about the surface of Mars. 

 Probing the sun 

Launched in 2018, NASA's Parker Solar Probe is on a mission to "touch the sun" as it draws closer to the planet over its seven-year mission. 

Ultimately, the spacecraft will come within 3.9 million miles (6.2 million kilometers) of the sun's surface, though it hasn't gotten that close yet. The spacecraft made its second solar flyby between March 30 and April 19, 2019. Data from the first two flybys were released to the public earlier this year. The spacecraft made its third flyby in September, 2019. The next flyby will come just after the New Year, in January of 2020. 

 Opening Apollo 

In November, scientists opened up one of the last untouched Apollo samples, a tube containing 15 ounces of moon rocks collected during NASA's Apollo 17 mission. 

The sample, collected near the rim of Lara Crater, was the first untouched Apollo sample opened since the 1970s. A corresponding tube will be opened in January 2020. Scientists hope that, with new instruments and techniques, they will be able to gain more insights about the lunar surface and the moon overall. 

After January, only two tubes, one from Apollo 15 and one from Apollo 16, will make up the remaining untouched samples.

 Lost Opportunity 

While 2019 had many firsts, it also boasted a few lasts. In February, NASA declared its Opportunity Mars rover "dead," eight months after a massive Martian dust storm silenced the solar-powered rover. Scientists suspect that dust covering the rover's solar panels kept it from recharging, bringing an end to the longest running Martian mission ever.

Along with its sister rover Spirit, Opportunity landed on Mars in 2004. Each rover embarked on what were to be 90-day missions. However, over a decade and a half, Opportunity covered more than a marathon's worth of ground, finding conclusive evidence that Mars hosted large bodies of liquid water in the past. 

Opportunity analyzed clay materials, determining that large, kilometer-scale bodies of water once existed on the now-dry planet. The hard-working rover also determined that the Martian water was neither acidic nor basic, establishing the physical habitability of Mars during the same period that life on Earth was evolving. Traveling 28.06 miles (45.16 km) over its lifetime, Opportunity holds the record for distance traveled by any vehicle, robotic or crewed, on the surface of another world.

 Mercury transit of 2019

Astronomers also experienced a last of sorts in 2019. On Nov. 11, the tiny planet Mercury made its last transit of the sun until 2032. 

Planetary transits occur when a planet moves between Earth and the sun, and provide Earth-bound astronomers the opportunity to study the atmosphere of a world like Mercury, however thin it may be. To get in-depth observations like this, astronomers require the orbits of both worlds to line up precisely, a relatively rare occurrence. 

Astronomers used ground-based telescopes, as well as other space-based instruments to document and study the historic event. 

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