April 7, 2015 – 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.
“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, Maryland.
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 said. “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 said. “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.”