Astronomy Now Home
Home Magazine Sky Chart Resources Store

On Sale Now!



The April 2014 issue of Astronomy Now is on sale! Order direct from our store (free 1st class post & to UK addresses). The Astronomy Now iPad/iPhone editions are now available worldwide in the iTunes store.



Top Stories



Earthshine used to test life detection method
...By imagining the Earth as an exoplanet, scientists observing our planet's reflected light on the Moon with ESO's Very Large Telescope have demonstrated a way to detect life on other worlds...
  READ MORE

Solid buckyballs discovered in space
...Astronomers using NASA’s Spitzer Space Telescope have detected a particular type of molecule, given the nickname “buckyball”, in a solid form for the first time...
  READ MORE

Steamy water-world gets the Hubble treatment
...Hubble Space Telescope observations of a 7 Earth-mass planet find an unusual water-rich world swathed in a thick, steamy atmosphere...
  READ MORE








MESSENGER enters orbit around Mercury
DR EMILY BALDWIN
ASTRONOMY NOW
Posted: 17 March 2011, update 18 March


Bookmark and Share

After a 6.5 year journey around the inner Solar System, the MErcury Surface, Space ENvironment, GEochemistry and Ranger spacecraft, MESSENGER, has become the first to ever enter orbit around innermost planet Mercury

After a period of systems checking, on 4 April its suite of instruments, including imaging cameras, spectrometers, a magnetometer and an altimeter, will be turned on and the science phase of the mission will commence. Throughout its one year nominal science mission, MESSENGER will orbit the planet once every 12 hours in order to glean information on Mercury's surface features, its magnetic field and exosphere.


The first image footprint will be captured on 29 March as part of the instrument commissioning phase, and will include previously unseen regions of the south pole. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

The spacecraft will have a highly elliptical orbit, passing between 200 kilometres and over 15,000 kilometres from the planet's surface. This extremely elliptical orbit allows MESSENGER's temperature to be better regulated – at an altitude of 200 kilometres, the re-radiated heat from the planet alone is four times the solar intensity at Earth. As Mercury orbits the Sun, the spacecraft will stay in a fixed orientation to keep its sunshield facing the Sun. Once a day the spacecraft will downlink its data via the Deep Space Network.

The mission aims to answer six primary science questions:

Why is Mercury so dense?
Earth, Mars and Venus are all known to posses dense iron-rich cores, but despite Mercury being the smallest of the four innermost planets, it has the highest density – so high that it implies that its core could dominate as much as 60 percent of the planet's volume. Is Mercury's density a result of the planet forming close to the Sun, its lighter rocky particles lost to the Sun? Could heat in the early phase of planet formation have vaporized Mercury's outer rocky layer? Or did a giant impact strip Mercury of its crust and upper mantle? MESSENGER will measure the composition of the planet's surface using X-ray, gamma-ray and neutron spectrometers, to determine which one of these theories is correct.

What is the geological history of Mercury?
Before MESSENGER made its first flyby of Mercury, over half of the planet had never been seen before. Now, including Mariner 10 observations and MESSENGER's three flybys, 98 percent of the planet has been observed, confirming that its cratered, ancient surface is dominant across the whole globe. Volcanic features also suggest that volcanism persisted in Mercury's early history, and scarps, or cliffs, are believed to have been formed when the entire planet contracted – shrunk – as it cooled, buckling up the surface. Topography measurements, mapping, and compositional analysis will help piece together the geological history of the planet.


A taste of what is to come spectacular colour images detailing the distribution of different rock types. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington.

What is the nature of Mercury's magnetic field?
It is thought that Mercury has a dynamic magnetosphere much like the Earth's, albeit on a smaller scale. How does Mercury's magnetosphere change with solar activity? How is it driven – from fluid motions in the planet's core? MESSENGER's magnetometer will characterize the planet's magnetic field to determine precisely its strength at different positions and altitudes, and how it interacts with the solar wind.

What is the structure of Mercury's core?
We know that Mercury has a large iron-rich core and a global magnetic field, but what is the structure of that core? A core of pure iron would be completely solid today, but if other elements such as sulphur, were present, portions of the core might remain molten. Determining the composition of the core will provide constraints on the planet's thermal history and evolution. MESSENGER's laser altimeter will be used to measure Mercury's libration – it's 'wobble' on its rotational axis; the libration of the rocky outer part of the planet will be twice as large if it is floating on a liquid outer core than if it is solid. Improved gravity field measurements will also help constrain the size and structure of the core.

What are the unusual materials at Mercury's poles?
Despite Mercury's proximity to the Sun, some polar craters remain in permanent shadow. Earth-based radar imaging shows a highly reflective substance on the floors of these large craters. Could water-ice be present on Mercury, or is the feature a result of low temperature silicates? MESSENGER's neutron spectrometer will search of the signatures of hydrogen in the polar deposits, and the ultraviolet spectrometer and energetic particle and plasma spectrometer will look for signatures from hydroxide or sulphur in the vapour over these deposits. The laser altimeter will also provide information about the topography of the craters.


MESSENGER has already seen 91 percent of Mercury from its three flybys, which has enabled scientists to plan observations during its orbital phase. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

What volatiles are important at Mercury?
Mercury is surrounded by a thin envelope of gas – an exosphere – containing hydrogen, helium, oxygen, sodium, potassium, calcium and magnesium. These components are not stable on long timescales so each element must have a source. Hydrogen and helium are supplied by the solar wind, while other elements may come from comet or micrometeoroid impacts. Processes such as vaporization of surface rocks during impacts, evaporation of elements from the surface when exposed to sunlight, sputtering from solar wind, or diffusion from the planet's interior, may all play a role. MESSENGER's spectrometers will determine the composition of the exosphere and compare it with details of the planet's surface. Variations in the composition will also be monitored to determine the processes that contribute to it.

MESSENGER has already completed three flybys, which not only revealed new images and details of the planet, but also provided the vital gravity assists to bring the spacecraft into orbit around the planet on 18 March.

For more information on the MESSENGER mission see our dedicated Focus in the March issue of Astronomy Now magazine.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
 GET YOUR COPY

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
 GET YOUR COPY

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
 GET YOUR COPY


HOME | NEWS ARCHIVE | MAGAZINE | SOLAR SYSTEM | SKY CHART | RESOURCES | STORE | SPACEFLIGHT NOW

© 2014 Pole Star Publications Ltd.