Threading its path through an aim point 209 miles above the surface of Venus, MESSENGER will use the pull of the planet's gravity to guide it closer to Mercury. During this flyby, Venus's gravity will change the spacecraft's direction around the sun and decelerate it from 22.7 to 17.3 miles per second.

"Typically, spacecraft have used planetary flybys to speed toward the outer solar system," said Andy Calloway, MESSENGER mission operations manager, Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md. "MESSENGER, headed in the opposite direction, needs to slow down enough to slip into orbit around Mercury."

This will be MESSENGER's second pass by Venus. During its first flyby of the planet, in October 2006, no scientific observations were made. Venus was at superior conjunction, placing it on the opposite side of the sun from Earth, leading to a two-week radio contact blackout between the spacecraft and its operators. This upcoming encounter offers opportunities for new observations of Venus's atmosphere, cloud structure, space environment and perhaps even its surface. The spacecraft will train most of its instruments on Venus during the upcoming encounter.

"During the flyby we'll ensure that the spacecraft and payload remain healthy, calibrate several of the science instruments, and practice many of the observations planned for the Mercury flybys," said Sean Solomon, MESSENGER principal investigator and planetary scientist at the Carnegie Institution of Washington.

The team plans to image the upper cloud layers at visible and near-infrared wavelengths for comparison with earlier spacecraft observations. Magnetic field and charged particle observations will be made to characterize the solar wind interaction with Venus and search for solar wind pick-up ions. Ultraviolet-visible and X-ray spectrometry will permit detailed observations of the composition of the upper atmosphere, and MESSENGER will search for lightning on the Venus night side.

MESSENGER will join the European Venus Express spacecraft, currently orbiting Venus, to make new observations of the Venus environment. To understand fully how solar wind plasma affects and controls the Venus ionosphere and nearby plasma dynamics, simultaneous measurements are needed of the interplanetary conditions and the particle-and-field characteristics at Venus. The combined MESSENGER and Venus Express observations will be the first opportunity to conduct such two-spacecraft measurements.

"By coordinating and comparing these observations, we will be able to maximize the science from both missions and potentially learn things that would not be revealed by one set of observations alone," said APL's Ralph McNutt, MESSENGER project scientist.

MESSENGER is only the second spacecraft to set sights on Mercury. NASA's Mariner 10 sailed past the planet three times in 1974 and 1975 and took detailed images of about 45 percent of the surface. Carrying seven scientific instruments on its compact and durable composite frame, MESSENGER will provide the first images of the entire planet. The mission also will collect detailed information on the composition and structure of Mercury's crust, its geologic history, the nature of its thin atmosphere and active magnetosphere, as well as the makeup of its core and polar materials.

Launched in August 2004, MESSENGER has completed more than 40 percent of its 4.9-billion mile journey to Mercury, which includes 15 loops around the sun. An Earth flyby one year after launch and a large propulsive maneuver in December 2005 set the spacecraft on course for the first Venus flyby in October 2006.

Next up for MESSENGER is a trio of swings past Mercury, in January and October 2008 and September 2009. During these flybys, the probe will map most of the planet and determine surface and atmospheric composition. These data will be used to help plan priorities for the yearlong orbital mission, which begins in March 2011.

The MESSENGER project is the seventh in NASA's Discovery Program of lower-cost, scientifically focused space missions. The Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages the mission for NASA's Science Mission Directorate, Washington.

earlier related report
CU-Boulder Space Scientists Gear Up For Mercury Mission Flyby Of Venus
Boulder (SPX) Jun 05 - University of Colorado at Boulder researchers will scan Venus during a spacecraft flyby this week using an $8.7 million instrument they designed and built for NASA's MESSENGER Mission, launched in 2004 and speeding toward Mercury.

Built by CU-Boulder's Laboratory for Atmospheric and Space Physics, the instrument will make measurements of the thick clouds and shrouded surface of Venus during the June 5th flyby, said LASP Senior Research Associate William McClintock, a mission co-investigator who led the CU-Boulder instrument development team. Known as the Mercury Atmospheric and Surface Composition Spectrometer, or MASCS, the instrument will compare the atmosphere of Venus with data from other spacecraft that have visited the planet in the past four decades.

"This is our first opportunity for a close flyby of a solar system object with MESSENGER, and we should be able to tell if the atmosphere of Venus has been changing in recent years, " said McClintock. "As importantly, we are using Venus as a test case to learn more about our instrument performance in preparation for the spacecraft's ultimate destination of Mercury."

Carrying seven instruments, MESSENGER will be the first spacecraft ever to orbit Mercury and the first to return data from the hot, rocky planet in more than 30 years. The circuitous, 4.9 billion mile journey to Mercury, which requires more than seven years and 13 loops around the sun, is using the gravity of Venus during its flyby this week to guide it closer to Mercury's orbit.

MESSENGER will make its first flyby of Mercury in January 2008, zipping by it again at a top speed of 141,000 miles per hour in October 2008 before flying by a third time in September 2009 and finally settling into orbit in March 2011. "This is a mission that requires some patience," said Mark Lankton, LASP's program manager for the MASCS instrument. "We are anticipating a brief symphony of action at Venus, and we have a lot of data to take in a hurry."

Dozens of CU-Boulder undergraduate and graduate students will be involved in data analysis from MESSENGER in the coming years, said Lankton.

MASCS's ultraviolet and visible spectrometer will be looking at the cloud composition of Venus. While the surface of Venus is hot enough to melt lead and its atmosphere is filled with noxious carbon dioxide gases and acid rain, Earth and Venus were virtual twins at birth, scientists believe.

The miniaturized MASCS instrument, which took more than three years to develop, weighs less than seven pounds and was built to last, said McClintock. "Many space instruments have a lifetime of only three to four years," he said. "But we knew we had to make this one robust enough to work for more than a decade under harsh conditions."

The MESSENGER spacecraft is about the size of a small economy car and is equipped with a semi-cylindrical thermal shade to protect it from the sun. More than half of the weight of the 1.2-ton spacecraft consists of propellant and helium. "We like to call it the little spacecraft that could," said McClintock.

"This event at Venus will be a very good tune-up for our first flyby of Mercury next January," said LASP Director Daniel Baker, also a co- investigator on the MESSENGER team. "The first encounter with Mercury will be extremely valuable, as it will essentially double the amount of information we now have about the planet."

A space physicist, Baker is interested in the magnetic field of Mercury and its interaction with the solar wind, including "substorms" associated with Mercury's magnetic field that occur in the planet's vicinity. Understanding Mercury's surface, tenuous atmosphere and magnetic field are the keys to understanding the evolution of the inner solar system, he said.

Mercury was visited only once before by a spacecraft, in 1974 and 1975, when NASA's Mariner 10 spacecraft made three flybys and mapped roughly 45 percent of the planet's rocky surface at the time.

MASCS will probe the mineral composition of Mercury's surface, the distribution of gases in its tenuous atmosphere and the workings of a giant, comet-like sodium gas cloud enveloping the planet, said McClintock. The researchers also hope to determine if Mercury ever had volcanoes on its surface and if the permanently shadowed craters at Mercury's poles contain water-ice.

MESSENGER is equipped with a large sunshield and heat-resistant ceramic fabric because Mercury is about two-thirds of the way nearer to the sun than Earth and is bombarded with 10 times the solar radiation. Sandwiched by the sun and Mercury -- which has daytime temperatures of about 800 degrees Fahrenheit -- the spacecraft will "essentially be on a huge rotisserie," said Baker.

Managed by Johns Hopkins University's Applied Physics Laboratory for NASA, MESSENGER involves about 20 scientists. Sean Solomon of the Carnegie Institution in Washington, D.C., is MESSENGER's principal investigator.

Data from MESSENGER will be sent to NASA's Deep Space Network tracking antennas, then to Johns Hopkins and finally to LASP's Space Technology Building in the CU Research Park.

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