Science Orbit: Working at Mercury

The MESSENGER mission had six specific science objectives for its primary mission:

  • Provide major-element maps of Mercury to 10% relative uncertainty on the 1000-km scale and determine local composition and mineralogy at the ~20-km scale.
  • Provide a global map with > 90% coverage (monochrome, or black and white) at 250-m average resolution and > 80% of the planet imaged stereoscopically. Also provide a global multi-spectral (color) map at 2-km/pixel average resolution, and sample half of the northern hemisphere for topography at 1.5-m average height resolution.
  • Provide a multi-pole magnetic-field model resolved through quadrupole terms with an uncertainty of less than ~20% in the dipole magnitude and direction.
  • Provide a global gravity field to degree and order 16 and determine the ratio of the solid-planet moment of inertia to the total moment of inertia to ~20% or better.
  • Identify the principal component of the radar-reflective material at Mercury's north pole.
  • Provide altitude profiles at 25-km resolution of the major neutral exospheric species and characterize the major ion species energy distributions as functions of local time, Mercury heliocentric distance, and solar activity.

To accomplish these science goals, the MESSENGER spacecraft obtained many types of observations from different portions of its orbit around Mercury. Some major constraints had to be met, including completing the observations within two Mercury solar days (equivalent to one Earth year) and keeping the spacecraft sunshade facing the Sun at all times. The observation plan also took into account MESSENGER’s orbit around Mercury. The orbit during the primary mission was highly eccentric (egg-shaped), with the spacecraft passing 200 kilometers (124 miles) above the surface at the lowest point and more than 15,193 kilometers (9,420 miles) at the highest. At the outset of the orbital phase of the mission, the plane of the spacecraft’s orbit was inclined 82.5° to Mercury’s equator, and the lowest point in the orbit was reached at a latitude of 60° North.

The spacecraft's orbit is elliptical rather than circular because the planet's surface radiates back heat from the Sun. At an altitude of 200 km, the re-radiated heat from the planet alone was 4 times the solar intensity at Earth. By spending only a short portion of each orbit flying this close to the planet, the temperature of the spacecraft was better regulated.

As Mercury moved around the Sun, the MESSENGER spacecraft stayed in an approximately fixed orientation with its sunshade facing the Sun, so effectively the planet rotated beneath the spacecraft. Different parts of the surface were illuminated depending on where Mercury was in its year, so the spacecraft could view the surface of the planet under every possible lighting condition.

Observing the Surface

MESSENGER’s 12-month primary orbital-phase mission covered two Mercury solar days; one Mercury solar day, from sunrise to sunrise, is equal to 176 Earth days. This means that the spacecraft passed over a given spot on the surface only twice during the primary mission, 6 months apart, making the time available to observe the planet’s surface a precious resource. The first solar day was focused on obtaining global map products from the different instruments, and the second focused on specific targets of scientific interest and completion of a global stereo map.

As Mercury moved around the Sun, the spacecraft’s orbit around the planet stayed in a nearly fixed orientation that allowed MESSENGER to keep its sunshade toward the Sun. In effect, Mercury rotated beneath the spacecraft and the surface illumination changed with respect to the spacecraft view. At some times, the spacecraft was traveling in an orbit that followed the terminator — the line that separates day from night. These trajectories are known as “dawn-dusk” orbits and are good for imaging surface features such as craters, because shadows are prominent and topography and texture can be clearly seen. At other times, the spacecraft followed a path that took it directly over a fully lit hemisphere of Mercury, then over a completely dark hemisphere. These trajectories are called “noon-midnight” orbits and are good for taking color observations on the dayside, because there are fewer shadows to obscure surface features.

Some instruments, such as Mercury Laser Altimeter (MLA), can operate whether the surface is lit or not, but others, such as Mercury Dual Imaging System (MDIS), need sunlight in order to acquire data. The low-altitude segments of the orbit over the northern hemisphere will allow MESSENGER to conduct a detailed investigation of the geology and composition of Mercury's giant Caloris impact basin — the planet's largest known surface feature, among other goals.

MESSENGER will operate in orbit around Mercury for one Earth year, equivalent to four Mercury years or two Mercury solar days. Different portions of the orbit are used by different instruments to acquire data.

Orchestrating the Observations

Different instruments are given priority in determining spacecraft pointing at different portions of the spacecraft orbit and as a function of the parts of Mercury’s surface that are illuminated at any given time. For example, MLA “drives” the spacecraft pointing whenever its laser can range to the planet’s surface (less than ~1500 km altitude), UVVS controls the pointing when no other instruments can “see” the planet, and MAG and EPPS primarily ride along and collect data regardless of what else is going on, since they generally don’t need to point at the planet’s surface. The two MDIS imagers are mounted on a common pivot, and so they can often look at the surface or at other targets when the rest of the instruments are pointed in a different direction.

This image shows a typical view from MESSENGER’s science planning software tool. The picture on the left shows the orientation of the spacecraft with respect to Mercury, and the table on the right shows details of the spacecraft’s orbit at that time. Views such as this one allow scientists to decide how best to take data to accomplish their science goals.

To meet the mission science objectives while taking into consideration the constraints associated with spacecraft safety and orbital geometry, the MESSENGER Project has planned the entire year of observations in advance of the orbital phase. Because of the large number of different science observations required to meet the science objectives, a special software tool has been developed to help carry out the complicated process of maximizing the scientific return from the mission and minimizing conflicts between instrument observations. This task is particularly challenging because most of the instruments are fixed on the spacecraft and are pointed in the same direction, but the different instruments may need to be pointed toward different locations at different times to meet the science goals.

Some observations also must be taken under specific observing conditions (such as taking color images when the Sun is high overhead), and the software tool works by finding the best opportunities for each of the instruments to make their measurements and then analyzing how those measurements contribute toward the science goals of the entire mission. Many iterations are necessary before a solution is found that satisfies all the science goals while staying within the limitations associated with the spacecraft’s onboard data storage and downlink capacity.

This planning-tool view shows MDIS image footprints (boxes) on Mercury’s surface after one orbit. The footprints vary in size depending on where the spacecraft is in its orbit and which of the two imagers are used. Here, the footprints are smallest at high northern latitudes, when MESSENGER is closest to the planet, and are largest near the bottom of the view because at that time the spacecraft is much farther from Mercury.

Although a baseline plan for the entire year has been formulated, commands to execute the plan will be sent up to the spacecraft on a weekly basis. Each “command load” contains all the commands that the spacecraft will need to execute during a given week. Because each command load is different and contains many tens of thousands of commands, the mission operations engineers start each load three weeks ahead of time. This schedule permits the command load to be thoroughly tested and reviewed before it is sent up to the spacecraft. Because of this process, mission operations personnel at any given time will be working on several command loads, each of which is at a different stage of development.

The Science Team has also developed the capability to regenerate the plan at short notice in order to respond to any anomalies that might occur in flight, such as an instrument problem, or on the ground, such as a missed Deep Space Network track.

Under this plan, each instrument will obtain the data needed to fulfill MESSENGER’s science objectives. Once in orbit, MDIS will build on the imaging it acquired during the three Mercury flybys to create global color and monochrome image mosaics during the first six months of the orbital mission phase. Emphasis during the second six months will shift to targeted, high-resolution imaging with the NAC and repeated mapping at a different viewing geometry to create a stereo map. MLA will measure the topography of the northern hemisphere over four Mercury years. GRNS and XRS will build up observations that will yield global maps of elemental composition. MAG will measure the vector magnetic field under a range of solar distances and conditions. VIRS will produce global maps of surface reflectance from which surface mineralogy can be inferred, and UVVS will produce global maps of exospheric species abundances versus altitude.

EPPS will sample the plasma and energetic particle population in the solar wind, at major magnetospheric boundaries, and throughout the environment of Mercury at a range of solar distances and levels of solar activity. The radio science experiment will extend topographic information to the southern hemisphere by making occultation measurements of planet radius, and the planet’s obliquity and the amplitude of the physical libration will be determined independently from the topography and gravity field.

Each orbit is 12 hours in duration, so MESSENGER orbits Mercury twice every Earth day. Once a day, the spacecraft stops making measurements and turns its antenna toward Earth for 8 hours, in order to send data back to the Deep Space Network, from which it will be sent on to the MESSENGER Mission Operations Center.