Having outlasted its original one-year mission as the first spacecraft to orbit the planet Mercury, the MESSENGER spacecraft is still hard at work investigating the chemistry, composition and geological history of the solar system's innermost planet.
Sean Solomon, Director of the Lamont-Doherty Earth Observatory at Columbia University, discussed his work as a principal investigator for the MESSENGER spacecraft.
Sean, you've called Mercury, around which the unmanned probe MESSENGER is now orbiting, "one of the last frontiers of the inner solar system." What are three of the most important things MESSENGER hopes to learn about the planet?
Mercury was "one of the last frontiers of the inner solar system" because prior to MESSENGER no spacecraft had ever been placed in orbit about the innermost planet and an entire hemisphere had never been seen at close range. Among the top goals of the MESSENGER mission were to determine Mercury's surface composition and learn what that chemistry tells us about how Mercury formed, to determine Mercury's geological history and how that history compares with those of the other inner planets, and to ascertain the nature of Mercury's global magnetic field and why the smallest inner planet retained a global field even though the larger bodies Mars and Venus have not done so.
Why did you and your team decide to call the probe MESSENGER? Was it a reference to the Roman deity Mercury, messenger to the gods?
The name MESSENGER is an acronym, chosen before the full mission name was selected: MErcury Surface, Space ENvironment, GEochemistry, and Ranging. The MESSENGER name was meant not only to evoke the role of the planet's Roman namesake on Olympus but also to convey the idea that this spacecraft would be returning important messages about how the inner planets formed and evolved.
How does Mercury compare in mass to Earth? Can you tell me more about similarities between the four "terrestrial" planets in our solar system?
Mercury's mass is about 5 percent that of Earth. For comparison, the mass of Mars is about 10 percent that of Earth, and Venus and Earth are nearly the same in mass. The inner planets differ more in density, particularly if the density is corrected for the differences in pressures achieved at the planetary centers. The "uncompressed" density of Mars is the least, those of Venus and Earth are similar, and that of Mercury is the greatest.
How long will MESSENGER orbit Mercury? What will be its final end?
MESSENGER successfully completed its primary mission to acquire observations of Mercury from orbit for one Earth-year. About this time last year, NASA approved an extended mission for another Earth-year of observations that will be completed in March 2013.
The project is in negotiation with NASA over a second extended mission that will permit additional observation through calendar year 2013 or until early 2014. The ultimate fate of MESSENGER is that the altitude of closest approach will decrease progressively with each orbit until the probe crashes onto Mercury's surface some time in 2014 or 2015.