MESSENGER

• 1. Introduction

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  MESSENGER hit Mercury at 14,080 kilometers per hour on the 30th of April, 2015. That final, deliberate crash ended an eleven-year journey that had covered 7.9 billion kilometers and redrawn what humanity knew about the innermost planet in the solar system. Before MESSENGER, no spacecraft had ever circled Mercury. Before MESSENGER, the best maps of the planet came from a single mission nearly four decades earlier, and those maps covered less than half the surface. What would a full picture reveal? What was hiding in the permanently shadowed craters near Mercury's poles? And how does a spacecraft even get to Mercury, a world so deep in the Sun's gravitational grip that reaching it requires years of looping through the inner solar system? The answers would take six years just to arrive, and four more to collect.

• 2. The Long Road to Mercury

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  Mariner 10, launched in 1973, had been the only spacecraft to visit Mercury before MESSENGER. Its final flyby occurred on the 16th of March, 1975, and after that, no close-range observations of the planet took place for more than thirty years. The problem was not indifference. Mission proposals kept appearing in the years that followed, but they consistently looked too expensive. Getting a spacecraft into orbit around Mercury is extraordinarily difficult. A probe on a direct path from Earth would accelerate as it fell toward the Sun, arriving at Mercury moving far too fast to be captured by the planet's gravity without burning through impractical quantities of fuel. Mercury also has no substantial atmosphere, ruling out the aerocapture and aerobraking techniques that help spacecraft brake into orbit around Venus or Mars.

  The solution came from a trajectory designed by Chen-wan Yen in 1985. Yen's approach used consecutive gravity-assist maneuvers around Venus and Mercury, threading the spacecraft through a series of planetary flybys that progressively bled off speed. Combined with minor propulsive corrections, this made a Mercury orbiter achievable within the budget of a Discovery-class mission. A study in 1998 confirmed the plan was feasible and made the formal case for sending an orbiting spacecraft to Mercury, then described as the least-explored of the inner planets.

  The launch was originally scheduled for a twelve-day window beginning the 11th of May 2004. On the 26th of March, 2004, NASA pushed the window back to allow further testing of the spacecraft, a change that delayed Mercury arrival by two years and reshaped the entire flight path. The original plan included three Venus flybys; the revised trajectory called for one Earth flyby, two Venus flybys, and three Mercury flybys before orbital insertion.

• 3. Building the Machine

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  The Johns Hopkins University Applied Physics Laboratory designed and built the MESSENGER spacecraft, with Sean Solomon of the Carnegie Institution of Washington serving as principal investigator for science operations. The spacecraft bus stood 1.85 meters tall and was built primarily around four graphite fiber and cyanate ester composite panels. Those panels supported the fuel tanks, thrusters, antennas, and the instrument platform.

  The most distinctive feature was a large ceramic-cloth sunshade, 2.5 meters tall and 2 meters wide. Mercury orbits close enough to the Sun that temperatures on the planet's dayside can reach extremes that would destroy unprotected spacecraft electronics. The sunshade provided passive thermal protection, keeping the instruments in shadow while the probe worked.

  At launch, MESSENGER weighed approximately 1,100 kilograms, most of that mass being propellant. The main engine was a 645-newton thruster, the LEROS 1b model, manufactured at AMPAC-ISP's Westcott works in the United Kingdom. The spacecraft carried 607.8 kilograms of propellant and helium pressurizer for that engine. Solar power during Mercury orbit came from two rotating gallium arsenide and germanium panels that together provided an average of 450 watts. The onboard computers ran on a pair of IBM RAD6000 processors, one a 25-megahertz main processor and one a 10-megahertz fault protection processor, with two solid-state recorders of up to one gigabyte each for data storage. MESSENGER's total mission cost, including spacecraft construction, was estimated at under 450 million US dollars.

• 4. Seven Years of Flybys

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  MESSENGER launched on the 3rd of August, 2004, at 06:15:56 UTC from Space Launch Complex 17B at Cape Canaveral Air Force Station in Florida, aboard a Delta II 7925 rocket. The burn sequence lasted 57 minutes, placing the spacecraft into a heliocentric orbit at a final velocity of 10.68 kilometers per second.

  One year after launch, on the 2nd of August, 2005, MESSENGER swept past Earth at an altitude of 2,347 kilometers over central Mongolia at 19:13 UTC. The team used the pass to calibrate instruments and investigate the so-called flyby anomaly, in which some spacecraft have shown trajectories that deviate slightly from predictions. No anomaly was detected. That December, a 524-second thruster burn adjusted the path toward Venus by 316 meters per second.

  The first Venus encounter came on the 24th of October, 2006. MESSENGER passed behind Venus and entered superior conjunction, meaning the Sun lay directly between Earth and the probe, cutting off radio contact. No science observations were conducted. The second Venus flyby, on the 5th of June, 2007, came within 338 kilometers of the planet's surface, providing the greatest velocity reduction of the entire mission. During that encounter, the European Space Agency's Venus Express was also in orbit around Venus, enabling the first simultaneous measurement of particle-and-field characteristics at Venus by two spacecraft.

  Three Mercury flybys followed. The first, on the 14th of January, 2008, brought MESSENGER within 200 kilometers of the surface at 19:04:39 UTC, making it only the second mission in history to reach Mercury. The second flyby occurred on the 6th of October, 2008. During the third, on the 29th of September, 2009, the spacecraft unexpectedly entered safe mode near closest approach, causing the loss of planned science data. It recovered about seven hours later. A final deep space maneuver on the 24th of November, 2009, provided the 0.177-kilometer-per-second velocity change needed to target the orbital insertion date.

• 5. Into Orbit

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  At 00:45 UTC on the 18th of March, 2011, MESSENGER fired its main thruster for approximately 15 minutes, executing a 0.9-kilometer-per-second braking maneuver. Confirmation that the spacecraft had achieved Mercury orbit arrived at 01:10 UTC. Mission lead engineer Eric Finnegan described the orbit as near-perfect.

  The orbit was highly elliptical, swinging the probe to within 200 kilometers of Mercury's surface and then out to 15,000 kilometers every twelve hours. That shape was deliberate. A tighter orbit would have exposed the spacecraft to more of the heat radiating from Mercury's sunlit surface; the elongated path gave MESSENGER time to cool during the outbound leg and allowed measurements of the solar wind and magnetic fields at varying distances.

  After an eighteen-day commissioning phase to verify all instruments survived the journey intact, formal data collection began on the 4th of April, 2011. In its first six months, MESSENGER gathered science results presented in October 2011 at the European Planetary Science Congress in Nantes, France. Among the findings: unexpectedly high concentrations of magnesium and calcium in Mercury's nightside atmosphere, and the discovery that Mercury's magnetic field is offset significantly to the north of the planet's center. The primary mission concluded on the 17th of March, 2012, by which point the spacecraft had collected close to 100,000 images. Full mapping of Mercury's surface was achieved on the 6th of March, 2013.

• 6. Ice, Organics, and a Hidden Core

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  On the 3rd of July, 2008, the MESSENGER team announced the probe had detected large amounts of water in Mercury's exosphere, a finding no one had anticipated. That discovery opened a larger question: if water existed in Mercury's thin outer atmosphere, might it also persist on the surface?

  In November 2012, NASA confirmed that MESSENGER had found evidence of both water ice and organic compounds inside permanently shadowed craters near Mercury's north pole. Earth-based radar observations had long suggested that something highly reflective was lurking in those craters. The ice was preserved by the permanent shadow, where sunlight never reaches and temperatures remain cold enough to keep volatiles stable despite Mercury's proximity to the Sun.

  The mission also provided visual evidence of past volcanic activity on Mercury's surface and evidence pointing to a liquid iron planetary core. In February 2013, NASA published what it described as the most detailed and accurate three-dimensional map of Mercury yet assembled, built from thousands of MESSENGER images. After the mission ended, analysis of the probe's radio ranging data produced the first measurement of the rate of mass loss from the Sun itself, a result that required the precision tracking only a long-duration Mercury orbiter could provide.

• 7. Watching the Solar System From Within

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  In November 2010, MESSENGER's MDIS camera collected 34 images that were assembled into a mosaic published on the 18th of February, 2011. The image showed all the planets of the solar system except Uranus and Neptune, which lay too far from the Sun to appear. The MESSENGER family portrait was created as a companion to the Voyager 1 portrait photographed from the outer solar system on the 14th of February, 1990.

  In November 2013, MESSENGER joined numerous other spacecraft in imaging Comet Encke, also designated 2P/Encke, and Comet ISON, designated C/2012 S1, during their passages through the inner solar system.

  On the 8th of October, 2014, MESSENGER turned its narrow-angle camera toward Earth. Between 9:18 UTC and 10:18 UTC, it captured 31 images taken two minutes apart as the Moon underwent a total lunar eclipse. The spacecraft was 107 million kilometers from Earth at the time. Earth measured about 5 pixels across in the camera's field of view; the Moon registered just over 1 pixel. Researchers increased the Moon's brightness by a factor of roughly 25 in the processed images to reveal its disappearance into shadow. This was the first time in history that a total lunar eclipse of Earth's Moon was observed from the surface of another planet.

• 8. The Final Impact

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  By late 2014, MESSENGER's maneuvering propellant was nearly gone. The probe's low orbit around Mercury had been slowly decaying, requiring periodic reboosts to avoid an uncontrolled crash. Its final reboost burns were executed on the 24th of October, 2014, and the 21st of January, 2015. With no propellant left for further corrections, the team extended the spacecraft's life by venting helium from the pressurizer tanks and using it as reaction mass. MESSENGER continued returning science data through this period, including close-up photographs of ice-filled craters and landforms at Mercury's north pole.

  On the 30th of April, 2015, at 3:26 p.m. EDT, MESSENGER struck Mercury at 14,080 kilometers per hour. Scientists estimated the impact created a crater approximately 16 meters wide on Suisei Planitia, near the crater Janacek, at coordinates 54.4 degrees north and 149.9 degrees west. The impact site was on the side of Mercury facing away from Earth at that moment, so no telescope or instrument detected the collision directly. NASA confirmed the mission's end at 3:40 p.m. EDT, after the Deep Space Network failed to receive a signal from the spacecraft after it should have emerged from behind the planet.