April 3, 2015 – MESSENGER mission controllers at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, conducted a maneuver yesterday to raise the spacecraft’s minimum altitude sufficiently to extend orbital operations and further delay the probe’s inevitable impact onto Mercury’s surface.
The previous maneuver, completed on March 18, raised MESSENGER to an altitude at closest approach from 11.6 kilometers (7.2 miles) to 34.4 kilometers (21.4 miles) above the planet’s surface. Because of progressive changes to the orbit over time in response to the gravitational pull of the Sun, the spacecraft’s minimum altitude continued to decrease.
At the time of yesterday’s maneuver, MESSENGER was in an orbit with a closest approach of 5.5 kilometers (3.4 miles) above the surface of Mercury. With a velocity change of 2.96 meters per second (6.63 miles per hour), four of the spacecraft’s 12 smallest monopropellant thrusters nudged the spacecraft to an orbit with a closest approach altitude of 27.5 kilometers (17.1 miles). This maneuver also increased the spacecraft’s speed relative to Mercury at the maximum distance from Mercury, adding about 1.2 minutes to the spacecraft’s eight-hour, 17.6-minute orbit period.
The second orbit-correction maneuver (OCM) in MESSENGER’s low-altitude hover campaign, also called the extension of the second extended mission, OCM-14 is the first propulsive course correction since December 2006 to use the two small thrusters that point sunward from the sunshade center panel.
MESSENGER was 200.6 million kilometers (124.6 million miles) from Earth when the 6.7-minute maneuver began at about 4:30 p.m. EDT. Mission controllers at APL verified the start of the maneuver 11.2 minutes later, after the first signals indicating spacecraft thruster activity reached NASA’s Deep Space Network tracking station in Goldstone, California.
The next maneuver, on April 6, will again raise the spacecraft’s minimum altitude, allowing scientists to continue to collect images and data from MESSENGER’s instruments. The 3.8 days between OCM-14 and OCM-15 will be the shortest time between any two MESSENGER maneuvers.
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun.
The MESSENGER spacecraft was launched on a United Launch Alliance Delta II on August 3, 2004 and was inserted into orbit around Mercury on March 18, 2011 (UTC), to begin its primary mission — a yearlong study of its target planet. MESSENGER’s first extended mission began on March 18, 2012, and ended one year later. MESSENGER is now in a second extended mission, which is scheduled to conclude this spring.
The spacecraft and science instruments remain remarkably healthy, but the propulsion system is running on fumes. The force of solar gravity continues to perturb the spacecraft orbit in a manner that drives the probe downward toward Mercury’s surface with each closest approach, and the tanks of propellant — needed to boost the spacecraft to higher altitudes — are running dry.
Engineers have devised a series of orbit-correction maneuvers (OCMs) which are designed to delay the inevitable impact a bit longer. The team has initiated this “hover” observation campaign to keep MESSENGER in orbit up to four weeks longer and to gather scientific data from the planet at ultra-low altitudes until the last possible moment.
Sean C. Solomon, the Director of Columbia University’s Lamont-Doherty Earth Observatory, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.
Simone Marchi of Boulder’s Southwest Research Institute has been an external collaborator of the NASA MESSENGER Geology Discipline Group. LASP contributed the Atmospheric and Surface Composition Spectrometer.