Astronomical milestones of 2015

Pluto stole the spotlight, but plenty of other stuff happened in space this year


FORGET PLUTO  The Mercury orbiter MESSENGER (shown) crash-landed into the planet in one of several monumental moments for space missions this year. 

NASA, JHUAPL, Carnegie Institution of Science

The New Horizons mission to Pluto might get all the attention, but 2015 had plenty of other amazing space mission firsts — and lasts, as scientists said good-bye to two orbiters.


The Dawn probe arrived at Ceres March 6, becoming the first spacecraft to orbit a dwarf planet (take that, Pluto!) (SN: 4/4/15, p. 9). Dawn quickly started mapping its new home. Bright patches sitting within craters (SN Online: 9/10/15), which at first glance looked like exposed ice, are probably salt deposits. The craters themselves are also puzzlingly scattered unevenly across the surface (SN: 9/5/15, p. 8).


A leak of oxygen, buried since the solar system’s start, was the last thing Rosetta mission researchers expected to detect at comet 67P/Churyumov-Gerasimenko. Finding such oxygen was a first in cometary chemistry (SN: 11/28/15, p. 6). The Philae lander, meanwhile, surprised the world when it awoke June 13 from a nearly seven-month slumber (SN Online: 6/14/15). Contact has since been spotty.


NASA’s premier planet hunter introduced us to Kepler 452b this year, possibly the most Earthlike world yet known (SN: 8/22/15, p. 16). Its 385-day orbit of a sunlike star would be comforting to humans. But at 1.6 times the width of Earth, the exoplanet might not have a solid surface on which they could enjoy it. That’s OK. With 1,030 confirmed exoplanets and counting, the Kepler space telescope keeps looking.


With a spectacular crash, the Mercury orbiter ended its four-year orbit April 30 when it ran out of fuel and smashed into the planet’s surface at over 14,000 kilo­meters per hour (SN Online: 4/30/15). One legacy: a new crater. MESSENGER was the first craft to orbit Mercury, to make complete maps and to spy water ice in the shadows at the planet’s poles.

Venus Express

The craft ran out of fuel late last year and began a death spiral into Venus’ atmosphere. It sent a final transmission to Earth in January, wrapping up nine years in orbit. But its data speak from beyond the grave. Flashes of infrared light from the planet’s surface add to evidence that volcanoes may be erupting there (SN Online: 6/19/15). 

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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