[meteorite-list] MESSENGER Executes Last Orbit-Correction Maneuver, Prepares for Impact

[Ron Baalke]

http://messenger.jhuapl.edu/news_room/details.php?id=282

MESSENGER Mission News
April 25, 2015

MESSENGER 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).

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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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 August 3, 2004, and entered orbit 
about Mercury on March 18, 2011, to begin 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 operate through April 2015. Dr. 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.