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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 20 Mar 2015 16:22:10 -0700 (PDT)
Message-ID: <201503202322.t2KNMABf008020_at_zagami.jpl.nasa.gov>

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.
Received on Fri 20 Mar 2015 07:22:10 PM PDT