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The First Spacecraft To Orbit Mercury Will Die Today, Alone

After years of studying the planet’s craters, MESSENGER is about to become one.

At 3:46 PM EST today, NASA's MESSENGER orbiter will crash on Mercury, capping off its productive 11-year-long mission with a bang.

The spacecraft ran out of propellant on April 24, and has been losing altitude ever since. Mission leads expect the orbiter to impact Mercury's North Pole at the breakneck speed of 14,000 kilometers per hour, which will leave a brand new 16-meter-wide crater on the planet's surface. It seems fitting that after years of studying the planet's pockmarked surface features, MESSENGER is about to become one of them.

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It's always sad to lose an interplanetary explorer, but there's no doubt that MESSENGER has earned its meteoric retirement. In March 2011, it became the first spacecraft ever to orbit Mercury, the least explored terrestrial planet in the solar system.

Since then, the orbiter has comprehensively mapped the planet, discovered evidence of water ice at its poles, and challenged theories about its formation and composition. It has also transmitted about 270,000 pictures of Mercury, including this incredible composite image released by NASA this week.

Composite images of Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

MESSENGER is such a workhorse that it will continue making observations even as it plunges to its cataclysmic doom.

"One of the large [Deep Space Network] antennas will be in communication with MESSENGER up until the time the spacecraft passes behind the planet for the last time," Sean Solomon, the principal investigator of the MESSENGER mission, told me over email.

"The radio transmission time between Mercury and Earth is sufficiently long that the final data collected by MESSENGER will still be en route when the spacecraft impacts Mercury's surface," he said.

The orbiter's swan song of final dispatches is eagerly anticipated by MESSENGER researchers, because it will provide the first up-close observations of the planet. The team will be looking for more evidence of ice as the spacecraft spirals down, as well as signs of complex organic molecules like carbon.

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"Carbon is difficult to definitively detect with most remote sensing methods," planetary scientist Megan Bruck Syal told me in a recent interview about Mercury's carbon content.

"However, I believe that the Gamma Ray and Neutron Spectrometer instruments on-board the MESSENGER spacecraft have been looking for carbon," she continued. "The final days of the MESSENGER mission will include low-altitude data, which could possibly shed light on the carbon abundance at Mercury's surface."

Clearly, MESSENGER will go out doing what it does best, capturing valuable data about this underexplored planet. Even its death will be a kind of victory, enshrining it as the first spacecraft ever to land on Mercury's surface—technically, crash land, but it will be a groundbreaking achievement nonetheless.

To that point, Dr. Solomon hopes that MESSENGER's success will encourage steps towards a soft-landing on Mercury, with a probe that can sample the surface.

Yellow markings on MESSENGER image indicate water ice. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

"The next logical step in the exploration of Mercury will be a lander," he told me. "Such a mission will pose considerable technological challenges, because of the absence of an atmosphere to decelerate a spacecraft, the six-month-long solar day, the extreme diurnal variation in temperature, and the harsh radiation environment at the surface."

That's an intimidating bundle of hurdles, and it will take years to tackle them. But Solomon is optimistic it can be done, and has already selected the geological target that he would shoot for.

"My vote for a landing site would be on a polar deposit in the permanently shadowed floor of a high-latitude impact crater," he said. "A lander could provide a chemical assay of the dark volatile layer, hypothesized to consist of organic material delivered from the outer solar system."

That is a tantalizing prospect, and it wouldn't be possible without pioneering missions like MESSENGER. So at 3:46 PM EST today, be sure to take a moment of silence for this lonely probe as it crashes into the Shakespeare Quadrangle region of Mercury. To quote the Bard for whom MESSENGER's final resting place is named: "The elements be kind to thee, and make / Thy spirits all of comfort! Fare thee well."