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Countdown to Mercury

Artist's impression of MESSENGER spacecraft in orbit at Mercury

Artist's impression of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit at Mercury.

  • Mercury is the smallest, hottest and densest planet in the Solar System
  • MESSENGER spacecraft has travelled 7.6 billion kilometres to reach it
  • Will study the planet’s surface, thin atmosphere and geologic history

NASA’S MESSENGER SPACECRAFT is scheduled to slide into orbit around the closest planet to the Sun on March 18 (Sydney time zone). The mission is an effort to study Mercury’s geologic history, magnetic field, surface composition and other mysteries.

The findings are expected to broaden our understanding of rocky planets, more and more of which are being discovered in other Solar Systems.

At 11:45am on March 18, Sydney time (8:45pm US EDT on March 17) the MESSENGER spacecraft will execute a 15-minute braking manoeuvre that will place it into orbit around Mercury, making it the first craft ever to do so, and initiating a one-year science campaign to understand the innermost planet.

MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging.

MESSENGER image of Mercury

It might look like the Moon, but Mercury is very different. It is the smallest, hottest and densest planet in the Solar System.

Mercury is an extreme among the rocky planets in our Solar System—it is the smallest, the densest (after correcting for self-compression) and the one with the oldest surface and largest daily variations in surface temperature and the least explored.

Understanding this “end member” among the terrestrial planets is crucial to developing a better understanding of how the planets in our Solar System formed and evolved.

“Now that so many new planets are being discovered around stars in other Solar Systems, we need to know the effects of space weathering on rocky surfaces so we can accurately interpret telescopic and other remote sensing data we obtain from other rocky or dusty worlds,” says Ann Sprague, a research scientist at the University of Arizona’s Lunar and Planetary Laboratory.

The heat is on

When MESSENGER streaked into the early morning sky over Cape Canaveral on August 3, 2004, very little was known about Mercury. No spacecraft had approached the planet since the Mariner 10 space probe performed three fly-by manoeuvres over the course of 1974 and 1975, imaging the planet’s surface. However, Mariner 10 sent back photos of only one side of the planet, leaving the other shrouded in mystery.

One of the mysteries scientists are hoping to solve with the MESSENGER mission surrounds Mercury’s magnetic field. At a diameter only slightly larger than that of the moon (about 4,800 kilometres), Mercury should have solidified to the core. However, the presence of a magnetic field suggests the planet’s innards are partially molten.

Artist's impression of a rupes (cliff) on Mercury

Artist's impression of a rupes (cliff) on Mercury. These giant cliffs are believed to have formed when Mercury¹s interior cooled and the entire planet shrank slightly as a result.

During its long journey toward Mercury, MESSENGER passed the planet several times, filling in the imaging gaps left by Mariner 10.

Now, the entire planet with the exception of about five percent has been observed. MESSENGER will focus its cameras on getting the best possible images of the remaining portions, mostly in the polar regions.

One of the great challenges MESSENGER will face is the intense heat due to Mercury’s proximity to the Sun. At the planet’s equator, surface temperatures become hot enough to melt lead. The heat reflected from the planet’s surface is so intense that the spacecraft’s instruments need to be shielded against the glare.

Follow the live webcast of MESSENGER’s arrival from 10:55am Sydney time on Friday, March 18 (7:55pm US EDT on March 17): MESSENGER arrival webcast

Adapted from information issued by the University of Arizona. Images courtesy NASA / JHU APL / CIW. Rupes artwork: Michael Carroll/Alien Volcanoes by Lopes and Carroll, The Johns Hopkins University Press, 2008.

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Solar System portrait, inside looking out

MESSENGER “family portrait” of our Solar System

The MESSENGER spacecraft collected a series of images (here split into two halves) to complete a “family portrait” of our Solar System as seen from the inside looking out. All of the planets are visible except for Uranus and Neptune, which at distances of 3.0 and 4.4 billion kilometres were too faint to detect. (Pluto, smaller and even farther away, would have been even more difficult to observe). Earth’s Moon and Jupiter’s Galilean satellites (Callisto, Ganymede, Europa, and Io) can be seen in the image insets. See the link in the main text below for a full-size version of the image.

NASA’s MESSENGER spacecraft has captured the first portrait of our Solar System from the inside looking out.

Comprised of 34 images, the mosaic provides a complement to the Solar System portrait—that one from the outside looking in—taken by Voyager 1 in 1990 (see below).

“Obtaining this portrait was a terrific feat by the MESSENGER team,” says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington.

“This snapshot of our neighbourhood also reminds us that Earth is a member of a planetary family that was formed by common processes four and a half billion years ago,” he added.

“Our spacecraft is soon to orbit the innermost member of the family, one that holds many new answers to how Earth-like planets are assembled and evolve.”

MESSENGER’s Wide Angle Camera (WAC) captured the images on November 3 and 16, 2010. In the mosaic, all of the planets are visible except for Uranus and Neptune, which—at distances of 3.0 and 4.4 billion kilometres—were too faint to detect.

Earth’s Moon and Jupiter’s Galilean satellites (Callisto, Ganymede, Europa, and Io) can be seen in the NAC image insets. The Solar System’s perch on a spiral arm of the Milky Way galaxy also afforded a beautiful view of a portion of the galaxy in the bottom centre.

See the full-size image here.

Assembling this portrait was no easy feat, says Solomon. “It’s not easy to find a moment when many of the planets are within a single field of view from that perspective, and we have strong Sun-pointing constraints on our ability to image in some directions.”

Outside looking in

On February 14, 1990, NASA’s Voyager 1 spacecraft had sailed beyond the outermost planet in our Solar System and turned its camera inward to snap a series of final images that would be its parting valentine to the string of planets it called home.

Mercury was too close to the Sun to see, Mars showed only a thin crescent of sunlight, and Pluto was too dim, but Voyager was able to capture cameos of Neptune, Uranus, Saturn, Jupiter, Earth and Venus from its unique vantage point. These images, later arranged in a large-scale mosaic, make up the first family portrait of our planets arrayed about the Sun.

Voyager portrait of the Solar System

The cameras of Voyager 1 on February 14, 1990, pointed back toward the Sun and took a series of pictures of the Sun and the planets, making the first ever "portrait" of our Solar System as seen from the outside. See the link in the main text for a full-size version of the image.

See the full-size image here.

Candy Hansen, a planetary scientist based at NASA’s Jet Propulsion Laboratory, who worked with the Voyager imaging team at the time, remembers combing through the images and finally finding the image of Earth. She had seen so many pictures over the years that she could distinguish dust on the lens from the black dots imprinted on the lens for geometric correction.

There was our planet, a bright speck sitting in a kind of spotlight of sunlight scattered by the camera. Hansen still gets chills thinking about it.

“I was struck by how special Earth was, as I saw it shining in a ray of sunlight,” she said. “It also made me think about how vulnerable our tiny planet is.”

This was the image that inspired Carl Sagan, the Voyager imaging team member who had suggested taking this portrait, to call our home planet “a pale blue dot.”

Voyager images of six planets

These six narrow-angle colour images were made from the first ever 'portrait' of the Solar System taken by Voyager 1 in 1990, when it was more than 6 billion kilometres from Earth. Left to right and top to bottom are: Venus, Earth, Jupiter, and Saturn, Uranus, Neptune. The background features in the images are artefacts resulting from the magnification.

As he wrote in a book by that name, “That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. … There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world.”

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.

MESSENGER was launched on August 3, 2004, and—having completed flybys of Earth, Venus, and Mercury—will start a yearlong study of its target planet in March 2011.

Adapted from information issued by NASA / JPL-Caltech. Image credits: (MESSENGER) NASA / JHU APL / Carnegie; (Voyager) NASA / JPL.

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2011: The year in space

Artist's impression of the Juno spacecraft at Jupiter

Artist's impression of the Juno spacecraft investigating Jupiter. Juno is set for launch later this year.

THERE ARE LOTS OF EXCITING happenings coming up in space this year. Here’s just a sample of what we can expect.

On February 14-15, NASA’s Stardust probe will do a fly-by of comet Tempel 1. It’ll be looking for damage done by the Deep Impact spacecraft, which fired a projectile into the comet back in 2005.

Also there’ll be the launch of Glory, an Earth-orbiting spacecraft that’ll make readings of black carbon and aerosols in the atmosphere, and measure the amount of incoming sunlight. Plus there’ll be the final flight of the space shuttle Discovery, on a mission to the space station.

March will see NASA’s MESSENGER spacecraft go into orbit around Mercury, the first probe to do so. A largely unknown world, the closest planet to the Sun is sure to hold some surprises. That month will also mark 25 years since Europe’s Giotto probe gave us our first close-up look at a comet, the famous Halley.

In April there’ll be a bunch of anniversaries, the 30th of the first space shuttle launch, the 40th of the first space station launch (which was the Soviet’s Salyut 1), and the biggie, the 50th anniversary of the flight of Vostok 1, carrying Yuri Gagarin, the first person to go into outer space.

Artist's impression of Salyut 1

Artist's impression of Salyut 1, the world's first space station.

April will also see the last flight of space shuttle Endeavour. And it could be the final shuttle mission of all. An extra flight by Atlantis in June has been approved but not yet funded, so we’ll just have to wait and see. Also in June, some parts of Australia will catch a short glimpse of a lunar eclipse.

July will see the second flight of the new, private Dragon spacecraft, designed to take cargo and eventually people to the International Space Station. Its first short test flight last year went perfectly. Dragon could end up being the replacement for the space shuttle.

Also in July, NASA’s Dawn spacecraft will sidle up to the 530km-wide asteroid Vesta, and go into orbit. It’ll spend a year investigating it before heading off to do the same thing with the even larger asteroid Ceres, which is actually known as a dwarf planet these days.

Artist's impression of the Dawn spacecraft

Artist's impression of the Dawn spacecraft studying asteroid Vesta

August will see the launch of Juno, NASA’s new unmanned mission to study the planet Jupiter. It’ll take about five years to get there. And the following month will see NASA launch GRAIL, a pair of satellites that’ll orbit the Moon and map its gravitational field, which will help scientists work out its inner structure.

In November, Russia will launch Fobos-Grunt, a mission to the larger of the two Martian moons, Phobos. All going well, it’ll touch down, grab some samples, and blast off back to Earth with them. The Chinese are piggybacking a small satellite too, which will orbit Mars and study its atmosphere, ionosphere and surface.

Finally, in December, there’ll be another launch of that Dragon capsule, plus the first launch of its competitor, called Cygnus. And to top it off, we’ll have another lunar eclipse.

Image credits: NASA / JPL-Caltech.

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Portrait of Earth and Moon

MESSENGER image of the Earth and Moon

NASA’s MESSENGER spacecraft took this image of the Earth and Moon from the distance of the orbit of Mercury, 183 million kilometres away.

Looking back from its orbit around Mercury, NASA’s Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft captured this view of Earth and the Moon on May 6, 2010.

The spacecraft was 183 million kilometres (114 million miles) from Earth at the time, farther than our average distance from the Sun (150 million kilometres, or 93 million miles) because Mercury and Earth were at different places in their orbits around the Sun.

The image was taken by the spacecraft’s Wide Angle Camera (WAC) on the Mercury Dual Imaging System (MDIS).

The view was a happy coincidence for the MESSENGER science team, as the probe was looking for vulcanoids, small rocky objects that have been postulated to hide in orbits between Mercury and the Sun.

MESSENGER is the first spacecraft to orbit Mercury since Mariner 10 in 1974-75. It is not, however, the first to get a long-distance shot of Earth. Below are some others, and you can see more of them on the Planetary Society’s site here.

Spirit rover image of Earth

In 2004, the Spirit rover on Mars snapped the first shot of our planet as viewed from the surface of another planet.

Cassini long-distance image of Earth

In 2006, Cassini sent back snapshots from 1.5 billion kilometres (930 million miles) from Earth as the spacecraft orbited Saturn. The Earth is small dot to the right of centre.

Voyager 1 pale blue dot image of Earth

And the operators of the venerable Voyager 1 spacecraft pieced together a family portrait of the entire Solar System in 1990, spying Earth from more than 6.4 billion kilometres (4 billion miles) away.

NASA image provided by the MESSENGER science team, NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Text adapted from information issued by Mike Carlowicz, NASA.

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