"You bet, I'm really excited about this flyby," said William C. Feldman, a Senior Scientist at the Tucson-based Planetary Science Institute and Cognizant Co-Investigator for the Neutron Spectrometer flying aboard the MESSENGER spacecraft.

MESSENGER will make its third and final pass around Mercury before returning in March 2011 to go into orbit around the solar system's innermost planet. The current flyby is designed to slow the spacecraft and to prepare its trajectory for orbital insertion.

Feldman is excited about taking a close look at the side of Mercury that MESSENGER flew by on its second encounter with the planet in October 2008 and about seeing another five percent of the planet's surface that is being imaged for the first time.

During the second flyby, the Neutron Spectrometer was not favorably positioned and the spacecraft was flying faster than it will be on the third pass. The slower encounter speed on the this third flyby will allow the spacecraft to remain close to the planet for a longer time and will yield better Neutron Spectrometer data than the instrument recorded on the first two encounters, Feldman said.

Feldman is excited both by what scientists expect to measure and by what will turn up serendipitously. Flying to another planet is like a road trip, Feldman explained. You have a good idea of what you want to see, but there are often those unexpected surprises that can become trip highlights.

MESSENGER's Gamma-Ray and Neutron Spectrometer detected iron and/or titanium during the spacecraft's first flyby. But Feldman expects to get an even better assessment of the iron and titanium on the planet's surface this time around because the Neutron Spectrometer will be pointed in the direction the spacecraft is flying, and it will be going about twice as fast as the neutrons travel. This will allow it to scoop up many more neutrons than if it were looking straight down at the planet's surface.

Examining the spectrometer data will then help scientists answer two important questions: What form does iron take on Mercury and how much is there? The planet's surface is much darker than the Moon's, indicating that there should be a high iron and/or titanium content, Feldman said.

This could be in the form of ilmenite, an iron-titanium oxide, or it might be nanophase iron, which gets distributed on the surface because of space weathering, Feldman said. Space weathering is the beating that rocky bodies with little or no atmosphere take from cosmic rays, the solar wind and meteorites.

For the Neutron Spectrometer, iron and titanium are the known attractions on this part of MESSENGER's road trip. The unknown attraction is, well, unknown, Feldman said.

"We will sample a composition on this side of the planet that could be very different from what we saw on the other side of the planet during the first flyby," he said. "It would be very surprising if we found the exact same composition that we saw on that first flyby."

Neutron spectrometers on earlier missions to the Moon and Mars "really paid off," Feldman noted. "We found the water molecule deposits near the Moon's poles and made the best maps of water content and depth on the surface of Mars." He's hoping for similar payoffs from MESSENGER's Neutron Spectrometer.

Once the spacecraft goes into orbit, its Gamma-Ray and Neutron Spectrometer also will be searching for hydrogen as a possible indicator of water deposits in Mercury's polar regions, which never get full sunlight, he said.

While highly sensitive infrared spectrometers on Earth-based telescopes and the Hubble Space Telescope gather a great deal of information about terrestrial planets, the only way to get neutron spectrometer data is to take a road trip, Feldman explained. That's because neutrons have a mean life of less than 15 minutes. After that, they decay into an electron, a proton and an electron neutrino. So they never have a chance to reach Earth.

Neutrons are electrically neutral particles that are freed when cosmic rays collide with the nuclei of atoms. Some of these free neutrons travel high above the planet, where they can be captured by the Neutron Spectrometer, Feldman explained. The upward current and amount of energy carried by a particular neutron indicates the conditions of its origin, which tells scientists what kinds of elements are present.

Data from MESSENGER's Neutron Spectrometer will help scientists better understand the planet's origin, evolution and volcanic processes.

The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.