Last week, the MESSENGER spacecraft became the first man-made satellite to orbit Mercury, the closest planet to our Sun. MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is operated by the Johns Hopkins Applied Physics Lab and NASA.
Launched in August of 2004, MESSENGER is designated as a Discovery Class mission; that is to say it is a low-cost science mission operating within the solar system.
Initially, the cost of a mission to Mercury was thought to far outweigh the potential knowledge to be gained.
However, in 1985 scientist Chen-wan Yen devised an orbital flight path that would minimize cost and make the mission possible. It was not until 1998 that a proposal to use this scheme was finally made.
In orbital mechanics, once a spacecraft is launched, it is subject to the laws of Newtonian mechanics, primarily the gravitational attraction of the Sun and planets.
Once set in an orbit, the spacecraft will follow that path indefinitely unless some alteration is made. Any alteration, whether it is changing the orbit characteristics around the same planet or moving to a new planet entirely, as is the case with the MESSENGER spacecraft, requires certain amounts of fuel to be stored.
Unlike with cars, where the cost of fuel itself is the largest factor, the issue in space flight is designing a vehicle that can carry the necessary amount of fuel, and launching the vehicle with enough thrust to get it out of Earth’s atmosphere.
Fortunately, once the spacecraft has left Earth, the engines must only be turned on for a matter of minutes to hours over the course of a multiple year mission.
Furthermore, the length of a mission directly impacts the cost. The more commands that must be sent, the more expensive the mission becomes, and the more the engine burns, the more commands that are sent.
MESSENGER’s flight path took it on a tour of the inner planets before finally arriving at Mercury. This served both to reduce the necessary maneuvers by relying on “slingshotting” off other planets, as well as gathering data from them as it went by. Otherwise, MESSENGER only had to make five “deep-space maneuvers” before reaching Mercury.
On March 17, MESSENGER made the final insertion burn to enter into Mercury’s orbit. It now orbits the planet around its north and south poles, allowing it to get sweeping pictures of the entire surface as Mercury rotates on its own axis.
“The first tasks,” David Blewett, a MESSENGER participating scientist, wrote in an email to The News-Letter, “are for the engineers to evaluate the spacecraft’s performance in its new thermal environment, and for the instrument scientists and instrument engineers to turn on the science instruments and verify that they are operating as expected.”
The instrumentation for MESSENGER was constrained to a low 50 kilograms because of the weight of the fuel carried. Still, the spacecraft is outfitted with eight systems, which all route through data processing units to relay information back to Earth.
Instruments include the Mercury Dual Imaging System, Gamma-Ray and Neutron Spectrometer, X-Ray Spectrometer, Magnetometer, Mercury Laser Altimeter, Mercury Atmospheric and Surface Composition Spectrometer and Energetic Particle and Plasma Spectrometer.
“[Mercury] is the smallest, the most dense, has the most eccentric orbit, we don’t know what kind of rocks form the surface and it may have polar ice caps despite its position close to the Sun,” Blewett wrote. “We must figure out Mercury’s origin and evolution if we are to
fully comprehend the general processes that operated to produce the diverse family of planets in our solar system.”
MESSENGER aims to answer six key questions during its planned year-long sciencemission that will help scientists address these problems.
According to the APL’s MESSENGER website, they are one, why is Mercury so dense? Two, what is the geologic history of Mercury? Three, what is the nature of Mercury’s magnetic field? Four, what is the structure of Mercury’s core? Five, what are the unusual materials at Mercury’s poles and six, what volatiles (atmospheric gases) are important at Mercury?
“My own interests are in Mercury’s geology,” Blewett wrote. “Among other things . . . [I will] try to determine what minerals are present in the surface rocks, and make comparisons to lunar samples and meteorites. It will also be exciting to see if high-resolution images show volcanic features like lava flow fronts or lava channels similar to those on the Moon and the Earth.”
Within six months, MESSENGER will have imaged the entire Mercury surface in sunlight, but the mission is expected to last beyond its stated one-year phase.
