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

[Ron Baalke]

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."

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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 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.