Messenger heads for close encounter with Mercury
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: January 13, 2008
NASA's innovative Messenger spacecraft is closing in on Mercury for a high-speed, low-altitude flyby Monday to give eager scientists their first close-up glimpse of the half-frozen, half-baked planet in more than three decades. The flyby is the first of three close encounters planned over the next two years that will use Mercury's gravity to slow the craft enough to slip into orbit around the innermost planet in 2011.
"It's really an awesome mission," said Marilyn Lindstrom, Messenger program scientist at NASA Headquarters. "It's goal is to understand the surface, the interior, the magnetosphere and the atmosphere of this innermost planet. But in the process of doing that, we hope to be able to apply that to understanding how all four of the terrestrial planets, the four Earth-like planets, formed.
"To understand the excitement of the scientists, you have to think of this as the first superbowl in 30 years. We've been waiting to go back to Mercury for that long and people are just that jazzed up. ... Our spacecraft is lined up and ready to go. The team's been working on this a long time, the team is ready and really pumped. And Mercury, here comes Messenger!"
Halfway through its 4.9-billion-mile, 6.5-year journey, Messenger is on a precise trajectory that will carry it within 126 miles of Mercury's tortured surface at 2:04 p.m. EST Monday. Streaking past the planet at more than 16,000 mph, Messenger's suite of instruments will collect some 700 gigabytes of data, including more than 1,300 photographs.
Based on the latest tracking data, "the spacecraft will fly by the planet within (2.5 miles) of the target altitude - bull's eye!" Eric Finnegan, Messenger systems engineer with the Applied Physics Laboratory at Johns Hopkins University, said Saturday in a NASA statement.
"Operations has confirmed that the core Mercury command load sequence was on-board the spacecraft Thursday night, and all subsystems and instruments are operating nominally," he said. "On Friday night, the Gamma-Ray Spectrometer turned on its cooler in preparation for the flyby and the Mercury Laser Altimeter was powered up this morning. The spacecraft is now fully configured for the encounter. All systems are 'go' for flyby."
Designed and built by the Applied Physics Laboratory, the solar-powered Messenger was launched from the Cape Canaveral Air Force Station atop a Delta 2 rocket on Aug. 3, 2004. If all goes well, the spacecraft will brake into orbit around Mercury on March 18, 2011, for a full year of scientific observations. Total cost of the project is $446 million.
Messenger measures 4.7 feet tall, 6.1 feet wide and 4.2 feet deep. Its two side-mounted solar panels extend 20 feet tip to tip. The spacecraft is equipped with seven miniaturized scientific instruments, a computer system, maneuvering thrusters, a main engine, communications equipment and a large sunshade, all crammed into a half-ton dry-weight package.
Only one other spacecraft - NASA's Mariner 10 - has ever visited Mercury, flying past the heavily cratered world three times in 1974 and 1975. But Mariner 10 was strictly a flyby mission, photographing just one side of the planet while collecting valuable but limited scientific data.
Messenger will photograph Mercury's other side Monday and "we're extremely excited about that," said Faith Vilas, a Messenger scientist with the MMT Observatory at Mt. Hopkins, Ariz. "Probably the first thing most of us want to see is what the other 55 percent of Mercury's surface looks like. ... Are we going to see large volcanic structures or are we going to see volcanism that's extrusive, sort of like the moon? What surface processes have changed the planet? ... We're expecting some pretty major surprises out of this."
The solar system was born some 4.5 billion years ago when a vast cloud of rotating gas and dusty debris reached a critical density and collapsed under its own gravity to form a flattened disk. At its heart, pressures and temperatures eventually became extreme enough to ignite fusion reactions and the sun flashed to life.
Material in the outer disk, through complex, little-understood chemical and physical processes, ultimately formed the planets, asteroids and comets known today. Four dense terrestrial worlds - Mercury, Venus, Earth and Mars - formed close in to the sun while four gas giants - Jupiter, Saturn, Uranus and Neptune - formed much farther out. Remote Pluto is part of a vast cloud of icy debris that makes up what is known as the Kuiper belt.
With a diameter of 3,031 miles, Mercury is only slightly larger than Earth's moon and smaller than Saturn's moon, Titan. It is the densest planet in the solar system and circles the sun in a highly elliptical orbit with a high point of 43 million miles and a low point of 29 million miles. Earth orbits the sun at an average distance of 92 million miles.
The aptly named Mercury is the fastest planet in the solar system, completing an orbit around the sun every 88 Earth days and moving at an average speed of 108,000 mph. It rotates on its axis every 59 days but because of its high orbital speed and slow rotation, a solar day - sunrise to sunrise at the same point on the surface - lasts 176 Earth days.
"At certain latitudes, an observer on the surface could watch the sun rise, move directly overhead and stop, then retrograde back a bit," according to a National Oceanic and Atmospheric Administration web site. "After retrograding, the sun would then proceed back on its westward track. In addition, since Mercury has virtually no atmosphere to scatter light, the sky would be black, even though the sun disk itself would be over twice as large as what we observe from the Earth."
On the planet's sunlit side, temperatures can exceed 750 degrees Fahrenheit while the night side can reach 350 degrees below zero, a swing of some 1,100 degrees. And at the poles, radar data indicate possible deposits of water ice in permanently shaded craters. It is the only terrestrial planet other than Earth with a global magnetic field and it features a huge, oversized iron core.
"Mercury is, of course, one of the family of inner planets that includes our Earth," Sean Solomon, Messenger principal investigator, said last week. "But Mercury, as a family member, is a real oddball. It is so dense that more than two thirds of the planet have to be iron metal residing in a central core, a much higher fraction than any other planet."
It is possible that metals naturally concentrated close to the sun before the planets coalesced. Another theory holds that after Mercury formed, a sudden increase in the sun's energy output boiled off the planet's rocky outer layers. Yet another theory holds that a body one sixth the size of the original planet crashed into Mercury near the dawn of the solar system and blew off its outer crust.
Data collected by Messenger may resolve the question once and for all.
"Mariner 10 showed us a surface that was so heavily cratered that it looked like geological activity on Mercury ended very early in the history of the solar system," Solomon said. "And yet, Mercury is the only other inner planet which, like Earth, has a magnetic field, which we believe means it must have a very dynamic, molten iron core.
"How to reconcile this ancient surface with this modern-day internal dynamic activity is one of the mysteries we hope to solve. All the inner planets formed about the same time by common processes, and yet Mercury ended up with this extreme outcome and we really need better information to make sure that our ideas for how the Earth and sister planets formed can be generalized to account for all the outcomes that we see."
Solomon said Messenger would give scientists their first data on the "elemental composition of Mercury's surface, we're going to make the first spacecraft measurements of surface reflectivity of visible light, ultraviolet light, near infrared light, to give us important clues to the mineral make up of Mercury's surface materials.
"We're going to make the first detailed view of the hemisphere of Mercury that Mariner 10 missed," he said. "Mariner 10 only saw 45 percent of the planet, more than half the planet has never been seen. That will change on Monday. Messenger is going to make the first laser altimetric measurements of the shape and topography of Mercury and it's going to improve our understanding of the gravity field (for the first time) since Mariner 10. That information will, in turn, tell us about the internal structure of Mercury and in particular, the nature of its huge core."
While the scientific observations are important, the primary goal of the flyby is to use Mercury's gravity to change the spacecraft's trajectory. The net effect will be to reduce Messenger's velocity by about 5,000 mph and shorten its orbital period around the sun by 11 days.
At a news briefing last week, Finnegan said the flyby will set up a sort of race between Messenger and Mercury.
"Using its internal engine and future gravity assists, the spacecraft, after being lapped by Mercury many times in this race around the sun, will ultimately match the 88-day orbital period of the innermost planet," Finnegan said. "In order to facilitate this change in velocity, the spacecraft will speed over the uncharted surface of Mercury at a relative velocity of over 16,000 miles per hour and pass within (126) miles of the surface, the closest proximity any man-made object has made with this planet."
Early Sunday, Messenger began executing stored computer commands that will orchestrate the flyby observations.
"The entire instrumentation suite will be operating during the flyby, taking over 1,300 images and gathering other scientific observations, filling the on-board data recorder with over 700 gigabytes of historic measurements within the period of 55 hours," Finnegan said.
"Fifty minutes prior to closest approach, signals from the spacecraft will go quiet as Messenger passes behind Mercury, out of Earth's view. Forty minutes later, engineers and scientists on the ground will attempt to witness the gravitational pull of the planet firsthand by re-acquiring the transmitted signals from the spacecraft within minutes of the closest approach point. On Tuesday, at noon EST, 22 hours after the flyby, Messenger will take one last look at Mercury before turning back to Earth to start returning the treasure stored on board."
It will take Messenger about a week to play back the stored pictures and other data. A news conference to discuss the results of the flyby is tentatively planned for Jan. 30. If all goes well, Messenger will zoom past Mercury again in October and a third time in September 2009 before braking into orbit on March 18, 2011. The orbital phase of the mission will continue for a full Earth year.