This year was full of discovery throughout the cosmos.
We were dazzled by beautiful images from space telescopes; marveled at the discovery of planets, stars and objects; were intrigued by a lunar mystery solved by missing Apollo mission data; and saw the first confirmed image of the birth of a planet.
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Things were just waiting to be found in our own corner of the universe, like 12 new moons around Jupiter, Earth-like characteristics on Pluto and a possible super-Earth orbiting a neighboring star. More studies suggested water on Mars and the moon. And astronomers found the fastest-growing black hole ever.
Of course, speculation abounded over where signs of life may be found outside Earth.
Here are some of the most amazing discoveries and space happenings of 2018.
'Oumuamua and other interstellar visitors
Although it's been over a year since a cigar-shaped object came rapidly tumbling through our solar system, we learned even more about this interstellar visitor in 2018.
The object, nicknamed 'Oumuamua, was discovered in October 2017 by the Pan-STARRS 1 telescope in Hawaii. The name is Hawaiian for "a messenger that reaches out from the distant past."
Studies based on the observations made during its "flyby" have deemed it a new class of cometary interstellar object, although researchers are still debating how the long, dark-red object accelerated. The surface of it looked like a comet's core, but it didn't have a "coma," the atmosphere and dust around comets as they melt and release gases.
And this year, Harvard researchers mentioned in a research paper that it was possibly a probe sent from an ancient civilization -- although other experts were skeptical of this suggestion.
Meanwhile, an interstellar immigrant that originated outside our solar system was found hiding around Jupiter in May. The exo-asteroid, named 2015 BZ509, was captured by the gas giant's orbit during the early days of our solar system 4.5 billion years ago. It moves in a retrograde orbit around Jupiter and serves as a warning to other "visitors."
" 'Oumuamua is a visitor to the solar system," said Helena Morais, study author and professor of statistics at Sao Paulo State University in Brazil. "That was a nice and important confirmation that interstellar objects can pass by. If they pass by, then they may also be captured in a stable orbit, as it is the case of 2015 BZ509."
Repeating fast radio bursts from space
The only known repeating fast radio burst in the universe keeps sporadically flaring.
These radio flashes usually last a millisecond and have unknown physical origin. People love to believe that they're from an advanced extraterrestrial civilization, and this hypothesis hasn't been ruled out entirely by researchers at Breakthrough Listen, a scientific research program dedicated to finding evidence of intelligent life in the universe.
The newest detections allowed researchers to discover that the radio bursts themselves are polarized and coming from an environment that contains an incredibly strong magnetic field. They were also able to detect the radio bursts at a higher frequency than ever.
The radio burst itself releases a "monstrous" amount of energy in each millisecond, comparable to what our sun releases in an entire day, the researchers said.
So is it coming from a black hole, a powerful nebula or a neutron star? Or is it something else? Only time, and more detections, will tell.
"We can not rule out completely the ET hypothesis for the FRBs in general," said University of California, Berkeley, postdoctoral fellow Vishal Gajjar of Breakthrough Listen and the Berkeley SETI Research Center.
'Ghost particle' from space found on Earth
For the first time, scientists were able to trace the origins of a ghostly subatomic particle that traveled 3.7 billion light-years to Earth. The tiny, high-energy cosmic particle is called a neutrino, and it was found by sensors deep in the Antarctic ice in the IceCube detector. The discovery was announced in July.
Scientists and observatories around the world were able to trace the neutrino to a galaxy with a supermassive, rapidly spinning black hole at its center, known as a blazar.
"What we've found is not only the first evidence of a neutrino source, but also evidence that this galaxy is a cosmic ray accelerator," Gary Hill, a study co-author, associate professor at the University of Adelaide's School of Physical Sciences and member of the IceCube collaboration, said in a statement. "I have been working in this field for almost 30 years and to find an actual neutrino source is an incredibly exciting moment. Now that we've identified a real source, we'll be able to focus in on other objects like this one, to understand more about these extreme events billions of years ago which set these particles racing towards our planet."
Scientists say the discovery heralds a new era of space research, allowing the use of these particles to study and observe the universe in an unprecedented way. And the finding suggests that scientists will be able to track the origin of mysterious cosmic rays for the first time.
A combination of observations and data across the electromagnetic spectrum, provided by observatories on Earth and in space, makes this a prime example of how "multimessenger" astronomy is helping make discoveries possible. Multimessenger astronomy also contributed to the discovery of the neutron star collision that created light, gravitational waves and gold in October 2017.
Why is Tabby's Star flickering?
More than 1,000 light-years away, there is a star that has been baffling astronomers since it was first observed in data collected by the Kepler mission. It's now largely known as Tabby's Star, named for Tabetha Boyajian, a Louisiana State University Department of Physics and Astronomy assistant professor.
For no obvious reason, Tabby's Star has been dimming and brightening in strange and unpredictable ways. It has dimmed for a few days or a week at a time. And then there's the fact that it grew fainter over the past century. It's an F star, which is supposed to maintain constant brightness. So what was causing the dips in light?
Hint: not an alien megastructure. That theory has been debunked by the latest data set released in January.
"Dust is most likely the reason why the star's light appears to dim and brighten," Boyajian said. "The new data shows that different colors of light are being blocked at different intensities. Therefore, whatever is passing between us and the star is not opaque, as would be expected from a planet or alien megastructure."
If anyone feels disappointed that the main culprit is most likely dust, rather than an alien megastructure, Boyajian offers this: "This is definitely something new and exciting. Even if it is dust, what kind of dust does this?"
Kepler and Dawn come to an end
Kepler, a nine-year planet-hunting mission, discovered 2,899 exoplanet candidates and 2,681 confirmed exoplanets in our galaxy, revealing that our solar system isn't the only home for planets.
Kepler allowed astronomers to discover that 20% to 50% of the stars we can see in the night sky are likely to have small, rocky, Earth-size planets within their habitable zones -- which means liquid water could pool on the surface, and life as we know it could exist on these planets.
Dawn's 11-year mission sent it on a 4.3 billion-mile journey to two of the largest objects in our solar system's main asteroid belt. Dawn visited Vesta and Ceres, becoming the first spacecraft to orbit two deep-space destinations.
Vesta and Ceres are considered to be like time capsules from the beginning of our solar system. The experiments Dawn carried out enabled astronomers to look at the different ways Vesta and Ceres formed and evolved, as well as revealing that dwarf planets can also host oceans.
New beginnings for new missions
Although we bid farewell to historic missions, 2018 was an exciting time for groundbreaking new ones to launch. NASA's TESS, InSight and Parker Solar Probe all had successful launches this year and are already sending back new science, with the promise of discoveries in 2019.
TESS, a planet-hunting satellite, launched in April. The Transiting Exoplanet Survey Satellite is NASA's next mission in the search for exoplanets, or those that are outside our solar system, and TESS will be on the lookout for planets that could support life.
It picked up where Kepler left off. TESS will survey an area 400 times larger than what Kepler observed. NASA expects TESS to allow for the cataloging of more than 1,500 exoplanets, but it has the potential to find thousands. These exoplanets will be studied so that NASA can determine which are the best targets for missions like the James Webb Space Telescope.
The Mars InSight lander launched in May and landed on the Red Planet on November 26. The lander is already sending back photos and will begin science operations after a few months, when all of its instruments are on the surface.
It will be the first lander to investigate the deep interior of Mars. This will tell us not only about the history of Mars but about other rocky planets in our solar system like Earth.
The Parker Solar Probe, named for pioneering astrophysicist Eugene Parker, launched in August and has come closer to the sun than any spacecraft. This is the agency's first mission to the sun and its outermost atmosphere, the corona.
The mission will last seven years and provide data to answer key questions about the sun. The observations and data could provide insight about the physics of stars, change what we know about the mysterious corona, increase understanding of solar wind and help improve forecasting of major space weather events.
And OSIRIS-REx, NASA's first asteroid sample return mission, just reached the asteroid Bennu after traveling through space for two years.
Red Planet rovers
In June, the Curiosity rover found organic matter in Martian soil samples taken from 3 billion-year-old mudstone and detected methane in the atmosphere.
"With these new findings, Mars is telling us to stay the course and keep searching for evidence of life," said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters. "I'm confident that our ongoing and planned missions will unlock even more breathtaking discoveries on the Red Planet."
And while Curiosity has had another great year of photos and science on the Red Planet, it's been a sad one for the Opportunity rover.
On May 30, a dust storm began on Mars. By mid-June, the storm became "planet-encircling." Although Curiosity was largely unaffected, Opportunity was stranded in the dark and has maintained radio silence ever since.
NASA is hoping that winds will knock dust off of Oppy's solar panels so she can recharge and start communicating again. Until then, all they can do is try paging her each day with the hope of a response.
And in November, NASA selected the landing site for the next Martian mission: the Mars 2020 rover.
Is that an exomoon?
In October, astronomers announced the discovery of what could be an exomoon, a moon outside our solar system. The exomoon, which is estimated to be the size of Neptune, was found in orbit around a gigantic gas planet 8,000 light-years from Earth. This would be the first exomoon ever found.
Although moons are common in our solar system, which has nearly 200 natural satellites, the long search for interstellar moons has been an empty one. Astronomers have had success locating exoplanets around stars outside our solar system, but exomoons are harder to pinpoint because of their smaller size.
The scientists behind this discovery are hesitant to confirm that the new find is an exomoon due to some of its peculiarities and the fact that more observation is needed.
However, the finding is both promising and intriguing. The moon, which orbits a giant exoplanet called Kepler-1625b, is incredibly large, comparable to the size of the gas giant Neptune in our solar system. There's no analog for such a large moon in our own system. In our sky, it would appear two times bigger than Earth's moon, the researchers said.
Christa McAuliffe's lessons finally taught
Christa McAuliffe never got to realize her dream of teaching from space.
The 37-year-old social studies teacher from Boston was selected above nearly 11,000 educators as the primary candidate for the first Teacher in Space Mission. But the Space Shuttle Challenger exploded 73 seconds after launching on January 28, 1986, taking McAuliffe's life and those of the six astronauts aboard.
McAuliffe's lessons have remained untaught and forgotten, until now. Astronauts filmed some of her original lessons on the International Space Station, continuing McAuliffe's legacy 32 years after they were planned. It's fitting that the two astronauts, Joe Acaba and Ricky Arnold, are both former educators.
The lessons touch on liquids in zero gravity, Newton's laws, effervescence (bubbles or fizz in liquid) and chromatography, or the separation of a mixture. The first of McAuliffe's lessons has been completed, and the lesson plans are available through the Challenger Center's website.
"Filming Christa McAuliffe's lessons in orbit this year is an incredible way to honor and remember her and the Challenger crew," said Mike Kincaid, associate administrator for NASA's Office of Education. "Developed with such care and expertise by Christa, the value these lessons will have as new tools available for educators to engage and inspire students in science, technology, education and math is what will continue to advance a true legacy of Challenger's mission."
Life could be on these water worlds
Last year, astronomers announced that ocean worlds like Jupiter's moon Europa and Saturn's Enceladus may be the best chance for finding life outside Earth in our solar system. Both are icy with subsurface oceans.
Now, the discovery of complex organic molecules in plumes that rise from Enceladus' subsurface ocean further suggests that the moon could support life as we know it.
And old data from NASA's Galileo mission to Jupiter in 1997 revealed some of the best observations to date that plumes of water vapor and icy materials erupt from a literal hot spot on Europa.
NASA plans to further explore ocean worlds in our solar system through the Europa Clipper mission, the first to explore an alien ocean. The Europa Clipper, named for the innovative, streamlined ships of the 1800s, will launch in the 2020s and arrive at Europa after a few years.
Europa Clipper's instruments will be capable of "sniffing" the atmosphere of Europa, with more than 40 planned flybys. The flybys will be less than 228 miles above the surface, in the observed range of the plumes, which can reach 124 to 228 miles above the surface.
And although the Cassini mission made close flybys of Enceladus before coming to an end in 2017, proposals for missions to further study Enceladus have been submitted to NASA. Detecting complex organic molecules in its plumes needs further investigation, researchers said.
"Specific identification of these organic compounds is the next step in our search for life in Enceladus' ocean," said Hunter Waite, program director at the Southwest Research Institute and Cassini's Ion and Neutral Mass Spectrometer principal investigator. "The complexity of the organic compounds identified was beyond our wildest expectations: nonsoluble complex organics floating as a film on an alien ocean. Wrap your head around that."