Top Ten Astronomy Stories 2015

Top Ten Astronomy Stories 2015
Dubbed The Year of the Dwarf Planet, 2015 saw investigations of two dwarf planets that provided astronomers and the public with amazing pictures and insights. There were also other missions in space, and on Earth in the far north, we had a solar eclipse.

1. Pluto flyby
The New Horizons visit to Pluto produced surprise after surprise. Pluto's largest moon Charon was a bonus. Its unexpectedly diverse surface features include an enormous rift that would dwarf the Grand Canyon. With data from New Horizons still on the way home, who knows what's yet to come?

2. Dawn orbited Ceres
In March, Dawn went into orbit around Ceres, the only dwarf planet in the inner Solar System. From the start there was an intriguing mystery – what were the bright spots scattered about the surface of Ceres? Ice was the best guess, but they turned out to be mineral deposits left behind when ice sublimes, i.e., turns from solid to vapor. It strongly suggests that there's a layer of salty ice under the surface.

3. Rosetta
There was excitement in June when Philae, which is still on Comet Churyumov-Gerasimenko, woke up briefly. Sadly, communications couldn't be sustained. On August 13, Rosetta witnessed the perihelion (closest approach to the Sun) of the comet. Having studied it warming up as it approached the Sun, the mission has been extended to see what happens as the comet retreats.

4. MAVEN and the Martian atmosphere
NASA's MAVEN probe went into Martian orbit in December 2014. Its mission: find out how a planet with a thick atmosphere and surface water turned into the cold, arid body we know today. MAVEN's results show that the transformation began 4.2 billion years ago when Mars lost its magnetic field. With no magnetic field to protect the planet's atmosphere, the solar wind took about 500 million years to strip it away. When it was gone, surface water evaporated into space.

5. NASA finds water on Mars
NASA has been excitedly discovering water on Mars for many years now, but the latest discovery is different. It's not evidence of water in the past, but evidence of water in the present. Mars Reconnaissance Orbiter has been photographing long dark narrow streaks on steep slopes. They appear in summer and disappear in winter. They also contain water-soluble minerals, so it seems that there's ice near the surface that's melting, flowing and refreezing.

6. Farewell to Messenger
The Messenger probe was the first one to orbit Mercury. The original one-year mission turned into four productive years. Besides studying the planet's surface composition, Messenger learned about its geological history, and also found frozen water. On April 30th, the mission ended when Messenger ran out of propellant and was allowed to crash onto the planet, forming a new impact crater just north of Shakespeare basin.

7. Beagle 2 found
The British astrobiology lander Beagle 2 was lost on Christmas Day 2003. At last, on January 16 the UK Space Agency announced that through studying images from the HiRise camera on the Mars Reconnaissance Orbiter (MRO), Beagle 2 had been found on Isidis Planitia. Beagle landed, but some of the solar panels didn't deploy, so the antenna couldn't transmit. End of mission. So near, and yet so far.

8. Discovery of the most distant known object in the Solar System
Until October, dwarf planet Eris was the most distant known object in the Solar System, its orbit taking it out to 98 AU, i.e., 98 times as far from the Sun as the Earth is. The newly-discovered object V774104 lies at 103 AU. Astronomers think it's an icy object 500-1000 km across, and may be part of the Oort cloud, a spherical cloud of icy bodies surrounding the Solar System.

9. Refsdal supernova found as predicted
A supernova explosion is observed after it happens, but it can't be predicted. However on December 11, the Hubble Space Telescope took the very first picture of an expected supernova. The supernova was named for Norwegian astronomer Sjur Refsdal who did pioneering work on gravitational lenses.

Galaxy cluster MACS J1149.5+2223, which lies between us and the supernova's galaxy, is gravitationally lensing it. It magnified the supernova and created multiple images, because its warping of space means light comes along different paths that take different amounts of time to reach us. Astronomers monitored the galaxy cluster for over a year, and used their previous sightings and models of lensing events to – correctly – predict the December sighting. The red circles show three spots where the supernova was detected at different times. The middle one is the December image.

10. Equinox eclipse
There was a total solar eclipse on March 20th, the northern spring equinox. But no more than a few thousand people managed to see it. The only inhabited land in the path of totality was two archipelagos. The Faroe Islands are about halfway between Norway and Iceland, and Svalbard is about halfway between northern Norway and the North Pole. However many people in Europe saw the partial eclipse. Luc Jamet's evocative photograph from Svalbard won the Astronomy Photographer of the Year 2015 competition.

You Should Also Read:
ABC of Astronomy - G is for Gravitational Lens
Rosetta the Comet Chaser
Eris and Pluto - They're Not Twins

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