New Moon Discovered Orbiting Mercury

This discovery image provides the first evidence that Mercury has a small natural satellite or moon. Visible as a small bright spot in an image taken yesterday by the Mercury Dual Imaging System (MDIS) Wide Angle Camera (WAC), the moon is approximately 70 meters (230 feet) in diameter and orbits Mercury at a mean distance of 14,300 km (8,890 miles). A proposal to name the moon "Caduceus," after the staff carried by the Roman god Mercury, has been submitted by the MESSENGER team to the International Astronomical Union, the body responsible for assigning names to celestial objects.

This discovery presents an unprecedented opportunity for a return of samples from the Mercury system, as Project Scientist Nat MacRulf explains. "We have yet to identify a sample from Mercury in any of the meteorite collections we have here on Earth. Such a sample would give us critical insight into the chemical composition of Mercury and the timing of crustal formation on that body, leading to a better understanding of how the planet formed and evolved. If we could obtain a sample of Caduceus, it would enhance the scientific return of the MESSENGER mission beyond our wildest dreams!"

Work on designing a scenario for sample return is already underway. MESSENGER Project Manager Burt Panini held an emergency meeting with the MESSENGER mission operations and navigation teams yesterday evening to determine if the spacecraft could be targeted toward the diminutive moon. After an intensive discussion, a unanimous decision was taken to abandon the orbit-correction maneuvers that had been planned for later this month to place the spacecraft in an eight-hour orbit. Instead, the new plan is to use the remaining propellant to crash MESSENGER into Caduceus. "Our detailed analysis tells us that if we act now, and with the right trajectory, MESSENGER will impart just enough momentum to the moon to break it free of Mercury's gravity well and set it on an Earth-crossing trajectory suitable for recovery as a Mercury meteorite," said Panini.

This action will form the basis of a new request to NASA by the MESSENGER team for an extended extended mission, tentatively called "MESSENGER Infinitesimally Nudging Caduceus," or MIN-C for short. Once MIN-C is approved by NASA, the spacecraft will be targeted for a collision trajectory. If Caduceus is successfully released from the pull of Mercury and placed on a course to reach Earth, we can expect the moon to arrive at Earth by 2014. "The risk to the public is reassuringly small", offers MESSENGER mission design lead Adam McJames. "We have designed a trajectory that will bring the moon to Earth at a remote location on the Wilkes Land ice sheet in Antarctica. This trajectory will avoid all population centers and will put the moon's impact site within reach for retrieval by the scientific staff at the U.S.-operated McMurdo Station."

If successful, MESSENGER's extended extended MIN-C mission will mark the first instance of the documented arrival to Earth of material from the Mercury system. Moreover, it will serve as the basis for a new Discovery-class mission proposal currently in development by the Applied Psychics Laboratory for a Mercury lander mission for in situ X-ray analysis of surface composition. That mission is to be named the Hermean On-surface Analysis with X-rays.

Date acquired: March 31, 2012
Image Mission Elapsed Time (MET): 131766564
Image ID: 6418
Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 38.15°
Center Longitude: 66.18°
Resolution: 410 meters/pixel (0.25 miles/pixel) in the lower left corner of the image
Scale: The large crater in the center of the image (Copland) is about 210 kilometers (130 miles) in diameter.
Spacecraft Altitude: 16,200 kilometers (10,070 miles)
Incidence Angle: 69.1°
Emission Angle: 80.8°
Phase Angle: 138.2°

Photo credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington