Sean Solomon on the Climactic End of the MESSENGER Mission to Mercury

Episode Summary

Executive Summary: This episode centers on the end of NASA’s MESSENGER mission, with Sean Solomon reflecting on its final impact, scientific triumphs, and legacy for Mercury science. The show also previews New Horizons’ early Pluto imagery, discusses future Mercury exploration, and includes Bruce Betts’ night-sky update and trivia segment.

Main Topics: MESSENGER mission ends at Mercury: The show opens with the spacecraft’s planned impact on Mercury, describing the final telemetry, beacon mode, and the team’s emotional farewell at APL. MESSENGER’s major scientific legacy: Sean Solomon explains why the mission was a success: it returned unprecedented data, especially the first orbital measurements of Mercury’s surface composition. Mercury’s puzzling composition and formation: MESSENGER found Mercury unexpectedly rich in volatile elements, undermining prevailing theories of how the planet formed. Future exploration of Mercury: Discussion turns to BepiColombo and possible future landers, especially the scientific promise of Mercury’s polar ice deposits and dark material. New Horizons and early Pluto views: Emily Lockdawala summarizes the excitement around newly processed Pluto images and the release of raw New Horizons images online. Night sky, space history, and trivia: Bruce Betts covers visible planets, Alan Shepard’s first American spaceflight anniversary, LightSail details, and a trivia contest about near-Earth comets.

Key Arguments: MESSENGER exceeded expectations by mapping Mercury globally and returning science far beyond its original promise. The mission’s end was driven by fuel exhaustion; remaining helium was repurposed to generate small thrust for orbit adjustments. Even at the end, MESSENGER was still producing valuable science through beacon mode, laser altimetry, and its final close-range imaging. Mercury’s surface composition was the mission’s most fundamental breakthrough because it could not be measured remotely before MESSENGER. The discovery of high sulfur, potassium, sodium, and chlorine content contradicts standard formation models that predicted volatile depletion. The mismatch between theory and observation means the origin of Mercury—and by extension inner-planet formation—remains incompletely understood. A future Mercury lander would be technically difficult because of extreme day-night temperature swings and the lack of an atmosphere, but polar deposits could be highly valuable targets. MESSENGER will continue to matter scientifically for decades, including comparisons with Mariner 10 and BepiColombo. New Horizons’ current Pluto images are still blurry smudges, but they already reveal rotationally varying surface features and will improve rapidly. Near-Earth comet counts matter for planetary defense and public understanding, but they are much smaller than the total near-Earth object population.

Data Points: MESSENGER mission duration: nearly 4,000 days in space - Describes the spacecraft’s lifespan before impact on Mercury Mercury orbits completed by MESSENGER: more than 4,000 orbits - Marks the scale of orbital science returned Impact date: April 30, 2015 - Date MESSENGER crashed into Mercury Launch year: late 2004 - Sean Solomon says he has been on the show since just before launch Closest approach before final partial orbit: just a couple of kilometers off the surface - Sean estimates how low MESSENGER flew before the end Temperature variation on Mercury: 600 degrees Celsius / 1100 degrees Fahrenheit - Used to illustrate challenges for a future lander Sulfur abundance on Mercury: 10 times that at Earth’s surface - A key example of Mercury’s unexpected volatile richness Known near-Earth comets: 101 - Bruce Betts gives the accepted count as of April 11, 2015 Near-Earth objects total: 11,000–12,000 - Distinguished from the smaller comet subset in trivia discussion LightSail launch timing: Wednesday, May 20, 2015 - Matt Kaplan notes the expected launch date at the end of the show Contest deadline: May 12, 2015 at 8 a.m. Pacific Time - Submission deadline for the trivia question about LightSail sail thickness Alan Shepard’s flight year: 1961 - This Week in Space History segment New Horizons raw image release cadence: every couple of days - Emily notes fresh raw Pluto images are being posted regularly

Pivotal Quotes: "They really got every last possible bit of science they could out of this tiny little mission." — Emily Lockdawala: Her reaction to the end of MESSENGER "The surprise from MESSENGER was that as we went through the inventory of elements that we could measure with our spectrometers, one by one, the volatile species showed up in very high abundance." — Sean Solomon: Explaining why Mercury’s composition upended formation theories "We know rather precisely where it should be, and it's one of the youngest features on Mercury." — Sean Solomon: Referring to MESSENGER’s impact site as a future science target

Implications: MESSENGER’s data will shape Mercury science for decades, especially theories of planetary formation and volatile retention. The mission also sets up future comparisons with BepiColombo and possible landers, while New Horizons and LightSail show the continuing pace of planetary exploration.