MESSENGER's Radio Science (RS) team recently completed a month-long orbit simulation test to measure Doppler performance using the probe's low-gain radio frequency (RF) antennas. The test was conducted in an environment similar to one MESSENGER will encounter during its Mercury orbit phase in 2011-12.

The Radio Science team will use MESSENGER's RF and telecommunications systems to study several aspects of Mercury. The frequency of MESSENGER's radio signal, as received by the Deep Space Network (DSN) ground stations, depends on the velocity of the spacecraft as seen from Earth. The large, powerful DSN antennas in California, Spain, and Australia are operated by NASA's Jet Propulsion Laboratory.

"By monitoring small changes in radio frequency, we can determine the velocity and acceleration of the MESSENGER spacecraft," explains Radio Science Instrument Scientist Mark Perry of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., adding that the frequency changes are called Doppler shifts. "Measuring the very slight changes in MESSENGER's velocity allows us to derive variations in Mercury's gravity with great precision. The gravity field then tells us how mass is distributed on and within Mercury, particularly variations in the thickness of the crust, the deep structure of such features as craters and mountains, and even the size of the planet's core."

During the orbit simulation test ‹ conducted from August 20 to September 19 ‹ MESSENGER's trajectory carried it nearly along the path of Mercury's orbit. On 10 separate occasions the team assessed the quality of Doppler observations by collecting an hour of tracking data using the low-gain antennas.

"This campaign has given us hard data on the performance of MESSENGER and the DSN ground system in the configuration that they will have during the orbital phase of the mission," says Perry. The team also hopes to see how variations in solar activity affect the Doppler measurements. The radio science team will use the data collected to refine MESSENGER's orbital operations plans.

RS Experiments to Reveal Mysteries of Mercury's Interior

One of the highest-level science goals for MESSENGER is to elucidate the structure of Mercury's deep interior. A major surprise from the first Mariner 10 flyby of Mercury in 1974 was that Mercury has an internal magnetic field, which may indicate that the planet has a liquid outer core. At that time, theories for planetary evolution predicted that Mercury's core would be frozen solid.

Earth-based radar measurements recently detected a variation in Mercury's spin rate over the planet's 88-day year. The amplitude of that variation, according to a theory worked out more than 30 years ago by MESSENGER Co-Investigator Stanton Peale, provides strong evidence that at least the outer part of Mercury's core is molten.

"MESSENGER's Radio Science investigation will measure the gravity field of Mercury and, in conjunction with other data, is expected to answer major questions about the size of Mercury's core, the extent to which the core is molten, and how the core is coupled to the overlying mantle, which in turn will help in understanding the thermal evolution of the planet," says David Smith, a planetary geodesist at NASA's Goddard Space Flight Center and a MESSENGER Co-Investigator.

Radio Science Cruise-Phase Calibrations

Throughout MESSENGER's cruise phase, the team has calibrated the radio frequency system to achieve the most accurate velocity measurements. Perry says that part of this calibration is to determine how much the Doppler frequency shifts are affected by the solar wind.

Because Mercury is so close to the Sun, this determination is more important for MESSENGER than for most missions. "A second part of the calibration is to verify that we can measure accurate Doppler shifts, even when using MESSENGER's small low-gain antennas," Perry explains. "We also tested our analysis techniques by extracting the shape of Venus's gravity field using the data from the second Venus flyby."

Mercury Flyby Operations

During MESSENGER's first flyby of Mercury on January 14, 2008, and the two subsequent passes of Mercury on October 6, 2008, and September 29, 2009, the Radio Science team will use the Doppler shifts to get an early read on Mercury's gravity field.

"MESSENGER's radio frequency system and the upgraded DSN antennas enable an accuracy 10 to 100 times better than obtainable from Mariner 10," Perry notes. "Our first glimpse of the shape of Mercury's gravity field next January may improve considerably our determination of the character of Mercury's core, achieving important science three years before MESSENGER's primary orbit phase begins!"

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 launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, 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.