[meteorite-list] MESSENGER's Endgame: Hover Campaign Promises Bird's-Eye View of Mercury's Surface

[Ron Baalke]

http://messenger.jhuapl.edu/news_room/details.php?id=276

MESSENGER Mission News
March 18, 2015

MESSENGER's Endgame: Hover Campaign Promises Bird's-Eye View of Mercury's Surface

MESSENGER will not go gentle into that good night. The mission will end 
sometime this spring, when the spacecraft runs out of propellant and the 
force of solar gravity causes it to impact the surface of Mercury. But 
the team initiated a "hover" observation campaign designed to gather scientific 
data from the planet at ultra-low altitudes until the last possible moment. 
Engineers have devised a series of orbit-correction maneuvers (OCMs) over 
the next five weeks -- the first of which was carried out today -- designed 
to delay the inevitable impact a bit longer.

A highly accurate OCM executed on January 21 targeting a 15-kilometer 
periapsis altitude -- the lowest to date -- set the stage for the hover 
campaign, in a short extension of the Second Extended Mission termed XM2-Prime 
(XM2'). The top science goals for XM2' will be carried out with the Magnetometer 
(MAG) and the Neutron Spectrometer (NS), and each instrument will target 
different objectives in different regions, explained MESSENGER Deputy 
Project Scientist Haje Korth, of The Johns Hopkins University Applied 
Physics Laboratory (APL), in Laurel, Md.

"With MAG, we will look for crustal magnetic anomalies," he said. "For 
instance, we have seen hints of crustal magnetization at higher altitudes 
(~70 kilometers) over the northern rise in Mercury's northern smooth plains. 
We will revisit this region at lower altitudes during XM2'. There may 
be other regions where such signals can be observed, and we will be looking 
for them."

"With NS, scientists will hone in on shadowed craters at northern high 
latitudes to search for water ice," Korth said. "We have found such evidence 
previously in the mission, but we hope to find more at low altitudes and 
spatially resolve the distribution within individual craters if we are 
lucky."

According to Korth, the observations enabled by this "saving throw" are 
no less significant than earlier ones. "Establishing the presence of crustal 
magnetic anomalies on Mercury would be a huge result, because it would 
extend the known temporal baseline for Mercury's internal magnetic field 
by eight orders of magnitude," he said. "Moreover, observing any such 
anomalies at different altitudes will allow the depth of the source to 
be determined."

"Since the periapsis altitude during the hover campaign is ~30 kilometers 
or less throughout XM2', we will have the opportunity to map half the 
planet with a magnetic magnifying glass, so to speak," he continued. "There 
are regions we have never seen at such low altitudes, and multiple areas 
of magnetic anomalies may be detected."
 
Staying Aloft

The ever-present tug of the Sun's gravity continues to perturb the spacecraft's 
orbit and drive closest approach downward toward the planet surface. For 
the last few weeks MESSENGER's altitude at closest approach has remained 
between 13 and 17 kilometers. To extend this hover campaign as long as 
possible, MESSENGER's mission design team optimized the trajectory design 
and the placement of each orbit-correction maneuver.

"We decided on a strategy that includes five maneuvers in as many weeks 
to keep the spacecraft within a tight altitude range of 5 to 39 kilometers 
above the surface of Mercury at closest approach," said APL's Jim McAdams, 
MESSENGER's Mission Design Lead Engineer.

Four of these five maneuvers occur in situations different from the dawn-dusk 
orbit orientation used for all earlier orbit-correction maneuvers in the 
mission, McAdams said. "During the interplanetary cruise phase, we designed 
similar course-correction maneuvers consisting of two or three separate, 
closely spaced maneuvers accomplished with different thruster sets. For 
XM2', we simplified the design and implementation of the final maneuvers, 
so that each will be executed at a single spacecraft orientation using 
one thruster set to maximize the orbit altitude change per unit mass of 
propellant consumed."

The maneuvers are not without risk, McAdams explained. "Increased uncertainty 
associated with effects on the spacecraft orbit of Mercury's gravity field 
at lower-than-ever altitudes, challenges in accurately predicting the 
spacecraft orbit when the Sun is near the spacecraft-to-Earth communications 
direction, and implementation of frequent OCMs make for a challenging 
final few weeks of flight operations," he said. "Depending on how each 
maneuver goes and on how Mercury's gravity field affects the minimum orbital 
altitude, we may need to plan and implement a contingency maneuver. Inserting 
a contingency maneuver will increase the likelihood of the hydrazine propellant 
running out earlier than planned."

So Far, So Good

This first maneuver went as planned. At the time of this most recent operation, 
MESSENGER was in an orbit with a closest approach of 11.6 kilometers (7.2 
miles) above the surface of Mercury. With a velocity change of 3.07 meters 
per second (6.87 miles per hour), the spacecraft's four largest monopropellant 
thrusters (with a small contribution from four of the 12 smallest monopropellant 
thrusters) nudged the spacecraft to an orbit with a closest-approach altitude 
of 34.5 kilometers (21.4 miles).

This maneuver also increased the spacecraft's speed relative to Mercury 
at the maximum distance from Mercury, adding about 1.1 minutes to the 
spacecraft's eight-hour, 16.5-minute orbit period. OCM-13 used propellant 
from the small auxiliary fuel tank. This view shows MESSENGER's orientation 
at the start of the maneuver.

MESSENGER was 185.6 million kilometers (115.4 million miles) from Earth 
when the 32-second maneuver began at 11:00 a.m. EDT. Mission controllers 
at APL verified the start of the maneuver 10.3 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 2, will again raise the spacecraft's minimum 
altitude, allowing scientists to continue to collect images and data from 
MESSENGER's instruments.

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 conclude in March 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.