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Two mysterious worlds explored for the first time. Liquid water seen flowing on Mars. A global ocean discovered hiding inside a moon of Saturn. Even during our Era of audacious solar system exploration, 2015 stands out. Here are a few highlights:

1. New Horizons Reveals the Face of Pluto

Whether or not you call it a planet, Pluto entranced the people of Earth when it sent a love note from three billion miles away via our New Horizons spacecraft.

2. Dawn Comes to Ceres

The dwarf planet Ceres, the largest object in the main asteroid belt, teased explorers with its bizarre bright spots before finally giving up some of its secrets to the Dawn spacecraft. HERE are the latest findings.

3. Cassini Marks Discoveries and Milestones at Enceladus

When the Cassini spacecraft performs its final close flyby of Saturn’s icy moon Enceladus on Dec. 19, it will be a true milestone. Scientists using data from Cassini’s instruments have uncovered astounding secrets about this small moon, including (confirmed this year) the fact that its underground ocean of liquid water is global, and is home to hydrothermal vents.

4. We Confirmed Evidence that Liquid Water Flows on Today’s Mars

Findings from our Mars Reconnaissance Orbiter (MRO) provided the strongest evidence yet that liquid water flows intermittently — on present-day Mars.

5. Rosetta Passes Perihelion

The European Space Agency’s Rosetta mission had a remarkable year, re-establishing contact with the Philae lander and following comet 67P/Churyumov-Gerasimenko as it swung near the sun.

6. Mars Explorers Confirm Lakes Once Dotted Mars

A study from the team behind our Mars Science Laboratory/Curiosity Rover confirmed that Mars was once, billions of years ago, capable of storing water in lakes over an extended period of time.

7. MAVEN Finds a Culprit in the Loss of Mars’ Atmosphere

The Mars Atmosphere and Volatile Evolution (MAVEN) mission identified the process that appears to have played a key role in the transition of the Martian climate from an early, warm and wet environment that might have supported surface life to the cold, arid planet that Mars is today.

8. Akatsuki Gets a Second Chance at Venus

Five years after a mishap sent the spacecraft off course, the Japan Aerospace Exploration Agency (JAXA) successfully inserted the Venus Climate Orbiter “Akatsuki” into orbit around Venus. While the mission is not funded by NASA, an agency partnership with JAXA provides an opportunity for eight of our scientists to work with the Akatsuki team and study data from the spacecraft over the next year or so.

9. A Trailblazing Mission Sends Its Final Message from Mercury

After a flight of nearly 11 years, the highly successful MESSENGER mission ended when, as planned, the spacecraft slammed into the surface of Mercury.

10. Mars Reconnaissance Orbiter Completes 40,000 Orbits

Mars Reconnaissance Orbiter, at Mars since 2006, has orbited the Red Planet more than 40,000 times. The mission, which studies the whole planet from space, has shown that Mars is diverse and dynamic by way of many thousands of spectacular images and other kinds of data.

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