Saturday, April 25, 2015

MESSENGER Executes Last Orbit-Correction Maneuver, Prepares for Impact

MESSENGER spacecraft. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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).

The previous maneuver, completed on April 14, raised MESSENGER's minimum altitude above Mercury from 6.5 kilometers (4.0 miles) to 13.3 kilometers (8.3 miles). But because of progressive changes in the orbit over time, the spacecraft's minimum altitude continued to decrease. 

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). 

Mission controllers at APL verified the start of the maneuver 9.4 minutes later, when the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network (DSN) tracking station in Goldstone, California. This was the third MESSENGER maneuver designed to adjust the course of the spacecraft using just helium gas.

Since MESSENGER's launch in 2004, mission engineers have been working in lockstep with KinetX Aerospace to conduct such maneuvers. KinetX, based in Simi Valley, California, is the first commercial company to navigate any spacecraft to distant planetary bodies. The team processes radiometric tracking measurements from NASA's DSN antennas to perform orbit determination for MESSENGER. 

The KinetX team was key to successfully navigating the spacecraft to arrive at the planet, and then for maintaining precise knowledge of the spacecraft's position while in orbit, including these last two months during MESSENGER's "hover campaign."

"Navigating a spacecraft so close to a planet's surface had never been attempted before, but it was a risk worth taking given mission success had already been met, and the novel science observation opportunities available only at such very low altitudes," said Bobby Williams, who leads the KinetX Space Navigation and Flight Dynamics group. "The MESSENGER mission presented new technical challenges for mission design and navigation that were successfully met through close cooperation and innovation of the APL and KinetX flight operations teams." 

MESSENGER Principal Investigator Sean Solomon, Director of Columbia University's Lamont-Doherty Earth Observatory, commented on yesterday's maneuver on behalf of the project's Science Team as the end of the mission draws near.

"Operating a spacecraft in orbit about Mercury, where the probe is exposed to punishing heat from the Sun and the planet's dayside surface as well as the harsh radiation environment of the inner heliosphere, would be challenge enough," he said. "But MESSENGER's mission design, navigation, engineering, and spacecraft operations teams have done much more. They've fought off the relentless action of solar gravity, made the most of every usable gram of propellant, and devised novel ways to modify the spacecraft trajectory never before accomplished in deep space. They've extended the duration of MESSENGER's orbital observations by more than a factor of four over the original plan, and an amazing set of scientific discoveries has been enabled by their creative efforts. This latest maneuver is icing on a multi-tiered cake of spectacular accomplishment. The MESSENGER mission will soon end, but its legacy of scientific knowledge and technical innovation will endure for as long as we study the planets and explore the Solar System."

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