Sodium gas tail, image from http://messenger.jhuapl.edu/image_highlights.html
Executes Last Orbit-Correction Maneuver, Prepares for Impact
MESSENGER mission controllers at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., conducted the last of six planned maneuvers on April 24 to raise the spacecraft’s minimum altitude sufficiently to extend orbital operations and further delay the probe’s inevitable impact onto Mercury’s surface.
With the usable on-board fuel consumed, this maneuver expelled gaseous helium — originally carried to pressurize the fuel, but re-purposed as a propellant. Without a means of boosting the spacecraft’s altitude, the tug of the Sun’s gravity will draw the craft in to impact the planet on April 30, at about 8,750 miles per hour (3.91 kilometers per second), creating a crater as wide as 52 feet (16 meters).
At the start of yesterday’s maneuver, at 1:23 p.m. EDT, MESSENGER was in an orbit with a closest approach of 8.3 kilometers (5.1 miles) above the surface of Mercury. With a velocity change of 1.53 meters per second (3.43 miles per hour), the spacecraft’s four largest monopropellant thrusters released gaseous helium to nudge the spacecraft to an orbit with a closest approach altitude of 18.2 kilometers (11.3miles).