— Saturn’s rings glow especially bright in this enhanced-colour mosaic from the Cassini spacecraft, assembled from images taken while the Sun was hidden behind Saturn itself (Image: NASA/JPL/Space Science Institute)
The year 2006 was one of things lost and found. The solar system lost its former ninth planet and NASA lost a long-serving Mars probe, but scientists found good evidence for dark matter, signs of liquid water flows on present-day Mars, and a planet just a few times more massive than Earth around another star.
The year opened with the spectacular return to Earth on 15 January of the Stardust mission, which had spent years travelling to and from Comet Wild 2 to collect samples to be examined in the laboratory.
Early analysis of the samples led to the surprising finding that although the comets were formed in the frigid outer solar system, some of the building blocks must have been transported there from very close to the Sun, because they appear to have been heated above 1000°C.
In other comet news this year, the close passage of a disintegrating comet by the Earth in April gave astronomers a rare view of what may be a common fate for comets.
Some say Pluto is just an overgrown comet, and the International Astronomical Union controversially voted to redefine the term "planet" in a way that excludes Pluto, relegating the former ninth planet to a second class of "dwarf planets".
Pluto's demotion was partly prompted by the confirmation earlier in the year that at least one object in the distant reaches of the solar system is bigger than Pluto. Initially called Xena, or the "tenth planet", it was given the official name Eris, after the Greek goddess of discord.
Amidst all the controversy, the New Horizons spacecraft continued towards its planned 2015 encounter with Pluto, following its January launch.
Land of lakes
NASA's Cassini spacecraft went on dazzling scientists and the general public with its investigation of Saturn and its moons. About 100 lakes of liquid methane or ethane, or both, were revealed on Saturn's largest moon, Titan, making it only the second body after the Earth known to have surface liquids fed by rain and rivers. Vast fields of dunes were also revealed on the Titan's surface, probably made of frozen hydrocarbon particles.
Cassini also found a giant storm raging at Saturn's south pole, somewhat reminiscent of a hurricane, a faint new ring around the planet, and ripples in a previously known ring perhaps resulting from a comet or asteroid strike in the 1980s.
A new phase in the exploration of Venus began with the arrival in orbit of the European Space Agency's Venus Express spacecraft, which promptly returned images of a curious double vortex structure in the clouds above the planet's south pole. There were also hints that Venus's surface might be older than previously believed, preserving a much longer record of its history.
ESA's SMART-1 spacecraft ended its lunar observing mission in a deliberate crash landing on the Moon, destroying itself in a flash of light and producing a small crater.
Two new Sun-observing missions were launched in 2006, NASA's Solar Terrestrial Relations Observatory (STEREO) mission and JAXA's Hinode (formerly Solar-B), which has already returned some initial results, including video of evolving plasma loops in the Sun's atmosphere.
A fleet of robotic probes at Mars delivered many new discoveries this year. A new NASA spacecraft, the Mars Reconnaissance Orbiter arrived in orbit in March, and began returning stunning images of the Martian surface, including a portrait of the rover Opportunity next to Victoria crater and a view of sand dunes carved with gullies.
But NASA experienced disappointment at Mars as well this year, with the disappearance of its 10-year-old Mars Global Surveyor. Before it disappeared, however, it returned images showing changes suggestive of recent water flow in gullies that it had been monitoring.
Radar sounding suggested there is a lot of water locked up in the form of ice buried beneath the surface near the planet's south pole.
NASA's unstoppable Spirit and Opportunity rovers surpassed the 1000 Martian day mark in 2006, despite having been originally rated for only 90 days on the Red Planet.
One of Spirit's wheels seized up permanently, but even that led to a new discovery by gouging a track in the Martian soil and revealing a buried layer of sulphates, yet more evidence of past water on the Red Planet.
While parked for six months during Mars's southern hemisphere's winter, Spirit created the most detailed panoramic view of the planet ever made. Opportunity finally arrived at the 800-metre-wide Victoria crater, returning some beautiful pictures of its own, and was looking for a good way into the crater when the year ended.
There were also many new discoveries about planets beyond our solar system. Astronomers found the smallest planet yet around a normal star, with just 3 to 11 times the mass of Earth. There were also some oddities, including a puffed up planet with a density less than that of a wine cork, two objects with the mass of planets orbiting each other instead of a star, and dusty discs around two hypergiant stars, suggesting planets might form even in the turbulent environment near these enormous suns.
Beyond our own galaxy, more progress was made in understanding gamma-ray bursts - the most powerful explosions in the universe. Analysis of an unusual gamma-ray burst called GRB 060218 suggested it was not powered by a star collapsing to form a black hole thought to be the case for most of the observed long gamma-ray bursts but may have been a less massive star collapsing to form a highly magnetised neutron star instead.
And two more GRBs detected in May and June may result from an entirely unknown process.
Big bang leftovers
The larger-scale universe made headlines this year as well. Physicists John Mather and George Smoot were awarded a Nobel prize for their work with the COBE satellite, which detected the first variations in the cosmic microwave background leftover from the big bang.
Scientists saw the gravitational effects of dark matter in isolation for the first time by studying a region of space where a colossal collision between galaxy clusters separated it from ordinary matter, results that were hailed as proof of dark matter's existence.
Of course, other scientists put forward arguments against dark matter, saying that modified gravity theories could explain astronomical observations.
A survey of the most distant supernova ever seen revealed some precious new information about dark energy, the mysterious force that is speeding up the universe's expansion and whose properties make it very difficult to study.
The new observations showed that dark energy has been present for at least the past 9 billion years and that its strength cannot have varied much during that time.
Solar system in a can
Among the more bizarre things announced this year, scientists proposed building a spacecraft carrying a miniature solar system to test for subtle gravitational effects due to hidden extra dimensions, and a team of astronomers suggested observations of a quasar indicated it was powered by an exotic ball of plasma called a MECO rather than a black hole.
The future appeared to hold promise as well, as the European Southern Observatory approved plans to build a giant 42-metre infrared and visible-light telescope, four times bigger than any existing telescope that observes at these wavelengths.
NASA announced that it would send a space shuttle mission to the Hubble Space Telescope in 2008 to extend the venerable observatory's life and install more powerful instruments. Hubble recovered twice in 2006 from the temporary shutdown of its main camera.