After a voyage of nearly eight billion kilometres that took over six and a half years, the Messenger spacecraft has settled down to its task of scientifically surveying Earth’s enigmatic sibling, the planet Mercury. Recently, the American space probe began sending back images of that far away world, the first to be taken from a spacecraft in orbit around it. It is now 37 years since the Mariner-10 probe, which too had been despatched by the United States’ National Aeronautics and Space Administration, provided images of that terra incognita as it thrice flew past the innermost planet in the Solar System. While the Mariner-10 could image less than half the planet, the Messenger (an acronym for MErcury Surface, Space ENvironment, GEochemistry and Ranging) will cover all of it with 75,000 high-resolution images and also send back data from a variety of onboard instruments. The first such image shows in great detail a 90-km-wide crater called ‘Debussy’ located in the South Pole, an area that was never seen before. Protected by a shade from the Sun’s scorching heat, the Messenger’s science mission will last a year as time is measured on Earth. But that is equivalent to just two Mercury days because the planet takes 176 Earth days to complete one rotation. So, although the probe is circling the planet once every 12 hours, it will be able to view any place on the surface under similar lighting conditions only twice during its mission. By combining information from images taken from two different angles, the topography of the planet can be mapped.
Mercury, one of the four inner rocky planets like Earth, could hold the key to understanding the formation and evolution of the planets in the Solar System. It is the smallest and densest of these planets; has the oldest surface; and experiences the largest daily surface temperature variations. Aside from imaging the planet, the mission has been designed to provide answers to six key questions: the planet’s density; its geologic history; the internal magnetic field; the materials at its poles; its exosphere; and the structure of its core. The answers to these questions will help unlock many scientific secrets. For instance, knowing the size and nature of Mercury’s core will make it possible to decipher its high density and possession of an internal magnetic field that is akin to the Earth’s. Diverse data from Mariner-10’s flyby of Mercury provided insights into its unknown features and raised questions about the then-prevalent theories. The next year promises to be rich in planetary surprises, as vast amounts of data from the Messenger come flooding back.