[meteorite-list] Latest Maneuver Illustrates Critical Role Telecommunications System Plays in Delaying MESSENGER's Mercury Impact

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 8 Apr 2015 14:56:47 -0700 (PDT)
Message-ID: <201504082156.t38Lulnj009103_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/news_room/details.php?id=279

MESSENGER Mission News
April 7, 2015

Latest Maneuver Illustrates Critical Role Telecommunications System Plays
in Delaying MESSENGER's Mercury Impact

MESSENGER's orbit-correction maneuver on April 6 was a nail biter. It
was the 15th such maneuver since the spacecraft entered orbit about Mercury
in 2011, and the third in a series of increasingly risky "burns" designed
to delay MESSENGER's inevitable impact onto Mercury's surface. Each maneuver
illustrates the critical role that the spacecraft's radio frequency (RF)
telecommunications system plays in its operation.

The RF telecommunications system is used to receive operational commands
at the spacecraft from Earth, and to transmit data acquired in making
science observations, and data indicative of spacecraft health, from the
spacecraft to Earth. The RF subsystem also supports MESSENGER navigation
by providing precise observations of the spacecraft's Doppler velocity
and range in the line of sight to Earth.

The system consists of redundant General Dynamics Small Deep Space Transponders,
solid-state power amplifiers, phased-array antennas, and medium- and low-gain
antennas. The phased-array antennas, the first electronically steered
antennas ever to be used in deep space, have no moving parts, thus reducing
the likelihood of failure in the extreme thermal environment of Mercury.
These antennas are designed to work at the 350 degrees Celsius ambient
temperature.

Onboard Insight

"The RF system provides our only insight into what's going on aboard the
spacecraft," explained MESSENGER Communications Engineer Dipak Srinivasan,
of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel,
Md. Such situational awareness is particularly important as the team attempts
-- in a series of increasingly risky maneuvers -- to raise the spacecraft's
minimum altitude sufficiently to extend orbital operations as long as
possible.

"We use data from the RF system to confirm whether a maneuver has started
and completed properly," he said. "We can also look at the change in the
signal's frequency -- caused by the spacecraft's changing motion and the
resulting Doppler effect -- to provide instantaneous assessments on the
maneuver status."

The frequent, almost back-to-back orbit-correction maneuvers (OCMs) of
MESSENGER's "hover campaign" present a challenge to the RF system. "During
OCMs, the spacecraft has to be oriented in a way that best supports the
propulsion system and keeps the sunshade between the Sun and the spacecraft's
thermally sensitive payload," Srinivasan added. "This requirement means
that communications can suffer when we are forced to use our low-gain
antennas to support communications with Earth. In such situations, the
signal strength we receive on the ground is quite low, so we must optimize
our ground configuration to maintain communications throughout the burn."

The Latest Maneuver

MESSENGER was nearly at its farthest point from Mercury in its eccentric
orbit about the planet when Monday's maneuver was executed. The burn raised
the spacecraft's minimum altitude above Mercury from 13.1 kilometers (8.1
miles) to 25.7 kilometers (16.0 miles). It increased the spacecraft's
speed relative to Mercury and also added about 0.55 minutes to the spacecraft's
eight-hour, 18.9-minute orbit period. Four of the 12 smallest monopropellant
thrusters imparted a change in velocity of 1.77 meters per second (3.97
miles per hour).

The operation used all of MESSENGER's remaining usable hydrazine propellant
from the small auxiliary fuel tank. It was completed over its final six
minutes with helium pressurant being expelled through the same thrusters
that were used with the first part of the maneuver.

Although no problems were reported during the maneuver, the usable propellant
was depleted sooner than predicted. The MESSENGER flight operations team
is planning a "clean-up" maneuver for April 8 (with a backup scheduled
for April 11) that will again use helium pressurant to put the spacecraft
back on schedule for OCM-16 on April 14.

Solar Conjunction Adds to the Thrill

The spacecraft is about to enter a superior solar conjunction, during
which Mercury and MESSENGER will be on the far side of the Sun from Earth.
"As we approach superior solar conjunction, the RF signal has to travel
through more of the solar plasma," Srinivasan explained. "The plasma causes
scintillations in the signal, disrupting it in both phase and amplitude.
This phenomenon introduces noise in our received signal, deteriorating
our signal-to-noise ratio and making it harder to decode the information
from the spacecraft. As the angle between the spacecraft and Sun gets
smaller and smaller, the signal eventually drops out completely, and we
won't pick it up again until the spacecraft emerges from behind the Sun
on the other side."

Having an OCM just before a superior solar conjunction is cutting it close,
he admits. "Conjunctions always cause a slight worry, as the Sun prevents
us from contacting the spacecraft for relatively long periods of time.
But we have several conjunctions and maneuvers under our belt and we are
confident the spacecraft will pull through okay," he said. "Fortunately,
we have a proven fault-protection system on board, as well as seasoned
mission operations and engineering teams ready to solve problems that
may arise."

"Our engineering team continues to pull rabbits out of this mission's
hat," said MESSENGER Deputy Principal Investigator Larry Nittler, of the
Carnegie Institution of Washington. "Their efforts to keep our little
spacecraft going long past all original expectations are truly heroic.
They are working to keep the craft flying at low altitudes for a few extra
weeks -- fighting against the gravitational pull of the Sun -- with empty
fuel tanks, by blowing helium into space. The observations we make these
last few weeks will add importantly to the long list of scientific discoveries
from this amazing mission."

--------------------------------------------------------

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 Wed 08 Apr 2015 05:56:47 PM PDT