Mercury fly-by a success

"Soon we will know how heavy Mercury's atmospheric particles are, how fast they were moving and in what direction," said Zurbuchen. "Our ion spectrometer is like a fish-eye camera for particles--its organizes particles according to mass but also sees what direction they are coming from and at what speed."

Mercury also has a weak but important magnetic field, indicating that it has a molten liquid core of iron like the Earth. FIPS will also be able to indirectly study the magnetosphere. "Mercury has a magnetosphere the likes of which we have never seen before," said Zurbuchen.

Earth's stronger magnetosphere prevents charged particles from the Sun from reaching the surface, except during periods of extreme solar activity. However, Mercury's weaker magnetosphere cannot prevent particles from the Sun--mostly protons and electrons--from reaching its surface.

"Solar particles actually hit the ground on Mercury--protons, alpha particles, even highly-charged heavier elements like oxygen and potassium. In fact many of the particles in the magnetosphere are likely to be from the Sun, while others will come from the planet directly," said Zurbuchen. "We will be able to tell the difference by their charge."

The final orbit around Mercury for Messenger will be elliptical so that it can have both close fly-bys for sampling the atmosphere and close-up photography, and distant viewing so the entire planet can be surveyed. Mission managers expect Messenger to be able to orbit Mercury for several years before running out of fuel for retrorockets that prevent the decay of its orbit.

Beside FIPS, Messenger has several other sensitive instruments. A dual-imaging system has both wide-angle and telephoto imaging chips for mapping and gathering topographic information. Two matched imagers enable stereoscopic imaging of the surface of Mercury in 3D. A Gamma-Ray and Neutron Spectrometer will detect radioactive elements on Mercury's surface, detect the effect of cosmic rays as well as determine whether there is ice at Mercury's poles (which are never exposed to sunlight).

An X-ray spectrometer will measure various elements in Mercury's crust. A magnetometer will map Mercury's magnetic field as well as detect magnetized rocks in its crust. A laser altimeter will bounce a laser off the planet's surface to accurately measure Mercury's topography. And a radio source will use the Doppler effect to study Mercury's mass distribution, including variations in the thickness of Mercury's crust.

Mercury is 36 million miles from the Sun and is the smallest planet in the solar system at just 3,000 miles in diameter at its equator.