Quotes of the Day
Mercury ain't for sissies. Say what you will about the difficulty of sending a spacecraft to a distant place like Saturn, it's Mercury, the first rock from the sun — with its crush of solar gravity and its inferno of solar heat — that presents the greatest challenges. NASA has dared go to Mercury only once before, 30 years ago, when Mariner 10 made three flybys. Now NASA is set to try again, with a spacecraft called MESSENGER, which is scheduled to take off as early as this week on a mission to become the first probe to orbit the planet.
Even to astronomers, Mercury is a cosmic oddity. It spins so slowly and scoots around the sun so fast that a Mercury year — just 88 Earth days — is half as long as a Mercury day. On the planet's illuminated side — where the sun looks three times as big as it does from Earth and is 11 times as bright — temperatures climb to 840°F. When that side rotates into darkness, the thermometer plunges to --300°F. Eons of this rotisserie roll have cooked Mercury down to a nub with a metal core that represents three-quarters of its diameter. Yet there may be water ice in permanently shadowed polar craters.
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MESSENGER will study these and other Mercurian curiosities when it arrives at the planet in 2011. Completing a full orbit in 12 hours and swooping as close as 124 miles to the surface, it will need just six months to photograph the entire globe. After a three-decade wait for a return visit, that ought to seem like no time at all.
A Scorched Surface ...
Mercury is closer to the sun than any other planet, but it is not quite the hottest of the group. That distinction goes to nearby Venus, where the temperature climbs to 900°F (482°C), owing to the planet's dense, heat-trapping atmosphere
Reasons for Mercury's wild temperature swings are its lack of any appreciable atmosphere and its reflective surface that bounces 90% of the sun's heat back into space
... with Mysteries Below
Mercury has a large core of mostly iron, making the planet extraordinarily dense. This means that although Mercury is about 75% the diameter of Mars, it has about the same gravity
CORE COMPARISONS
Diameter -- Core as % of diameter
Mercury -- 3,031 miles (4,878 km) -- 75%
Venus -- 7,504 miles (12,104 km) -- 50%
Earth -- 7,909 miles (12,756 km) -- 55%
Mars -- 4,208 miles (6,787 km) -- 45%
Moon -- 2,155 miles (3,476 km) -- 20%
OUT OF THE DARKNESS
Mercury's crater-pocked surface looks much like that of our moon. Its most prominent feature is the Caloris Basin, an 800-mile (1,300-km)-wide crater probably formed by an asteroid impact when the planet was still young. MESSENGER will provide the first picture of the whole basin, half of which was in darkness when the Mariner 10 probe flew by 30 years ago
A BRIGHT SPOT
Mercury's average distance from the sun is 36 million miles (58 million km), about two-thirds closer than Earth. From Mercury's surface, the sun appears as much as three times as large and 11 times as bright as it does on Earth
SOME MERCURY MYSTERIES
Is there ice on the surface? Radar imaging has detected what appears to be ice reflections in craters near the planet's poles. The walls of these craters may be so steep that sunlight never reaches their floors, putting them in permanent deep freeze
WHY IS THERE A MAGNETIC FIELD?
Earth's magnetic field is believed to be generated by the movement of molten iron in the core. But Mercury is so much smaller than Earth that its core should have solidified long ago. MESSENGER will try to determine whether the field is just a dying remnant or is still being driven by a molten core
A Roundabout Route
Launch — Aug. 2004
Earth — flyby
Venus — flybys
Mercury — flybys
Arrival — March 2011
MESSENGER isn't due to enter Mercury's orbit until March 2011. But the probe will be busy all along the way, getting a gravity boost from Earth next year, then Venus twice in an ever tightening spiral toward the first planet from the sun
WE DON'T KNOW THE HALF OF IT
This picture of Mercury is a mosaic of 500 frames taken by Mariner 10, which saw only half the planet up close during its three flybys. The Hubble Space Telescope has not dared to snap some pictures of its own (training its gaze so close to the sun could fry its electronics) MESSENGER will need just 12 hours to complete an orbit of Mercury. With the planet spinning slowly below, it should be able to survey the entire surface within six months
THE MESSENGER PROBE
A nimble little spacecraft, MESSENGER weighs just 2,400 lbs. (1,100 kg), 55% of which is fuel. It carries a dual camera system and six other scientific instruments, all of which operate on solar power-something of which there is no shortage so close to the sun. A sunshade made of ceramic cloth keeps the hardware running at a balmy room temperature
--Energetic particle and plasma spectrometers Will measure charged particles in Mercury's magnetosphere
--Magnetometer Will map Mercury's magnetic field
--X-ray, gamma-ray and neutron spectrometers Will detect types of elements on Mercury's surface and within its crust
--Laser altimeter Will map Mercury's terrain
--Dual imaging system Wide-and narrow-angle cameras
--Atmospheric and surface composition spectrometer Will detect and measure atmosphere and surface composition
--Insulating blanket
--Sunshade
--Star trackers
--Propellant tank (1 of 3)
--Solar panel (back)
Sources: NASA; JPL; Mark Robinson, Northwestern University Center for Planetary Sciences; Johns Hopkins University Applied Physics Lab TIME
- Jeffrey Kluger