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The MESSENGER spacecraft
FROM MESSENGER PRESS KIT
Posted: July 28, 2004

After Mariner 10's visits to Mercury the space science and engineering communities yearned for a longer and more detailed look at the innermost planet - but that closer look, ideally from orbit, presented formidable technical obstacles. A Mercury orbiter would have to be tough, with enough protection to withstand searing sunlight and roasting heat bouncing back from the planet below. The spacecraft would need to be lightweight, since most of its mass would be fuel to fire its rockets and slow the spacecraft down enough for Mercury's gravity to capture it. And it would have to be compact enough to lift off on a conventional and cost-effective rocket.


An illustration shows the MESSENGER spacecraft and denotes its instruments. Credit: JHU-APL
 
Designed and built by the Johns Hopkins University Applied Physics Laboratory - with contributions from organizations in 24 states and six countries - the MESSENGER spacecraft tackles each of these challenges. A ceramic-fabric sunshade, heat radiators and a mission design that limits time over the planet's hottest regions protect MESSENGER without expensive and impractical cooling systems. The spacecraft's graphite composite structure - strong, lightweight and heat tolerant - is integrated with a low-mass propulsion system that efficiently stores and distributes the approximately 600 kilograms (1,323 pounds) of propellant that accounts for 55 percent of MESSENGER's total launch weight.

To fit behind the 8-foot by 6-foot sunshade MESSENGER's wiring, electronics, systems and instruments are packed into a small frame that could fit inside a large sport utility vehicle. And the entire spacecraft is light enough to launch on a Delta II rocket, the largest launch vehicle allowed under NASA's Discovery Program of lower-cost space science missions.

Science Payload

MESSENGER carries seven scientific instruments and a radio science experiment to accomplish an ambitious objective: return the first data from Mercury orbit. The miniaturized payload - designed to work in the extreme environment near the Sun - will image all of Mercury for the first time, as well as gather data on the composition and structure of Mercury's crust, its geologic history, the nature of its active magnetosphere and thin atmosphere, and the makeup of its core and the materials near its poles.

Mercury Dual Imaging System

Mass: 7.9 kilograms (17.4 pounds)
Peak Power: 10 watts
Development: Johns Hopkins University Applied Physics Laboratory, Laurel, Md.

The multispectral MDIS has wide- and narrow-angle imagers - both based on charge-coupled devices (CCDs), similar to those found in digital cameras - to map the rugged landforms and spectral variations on Mercury's surface in monochrome, color and stereo. The imager pivots, giving it the ability to capture images from a wide area without having to repoint the spacecraft and allowing it to follow the stars and other optical navigation guides.

The wide-angle camera has a 10.5-degree field of view and can observe Mercury through 12 different filters across the wavelength range 400 to 1,100 nanometers (visible and near-infrared light). Multispectral imaging will help scientists investigate the diversity of rock types that form Mercury's surface. The narrow-angle camera can take black-and-white images at high resolution through its 1.5-degree field of view, allowing extremely detailed analysis of features as small as 18 meters (about 60 feet) across.

Gamma-Ray and Neutron Spectrometer

Mass: 13.1 kilograms (31 pounds)
Peak Power: 23.6 watts
Development: Johns Hopkins University Applied Physics Laboratory

GRNS packages separate gamma-ray and neutron spectrometers to collect complementary data on elements that form Mercury's crust.

The Gamma-Ray Spectrometer measures gamma rays emitted by the nuclei of atoms on Mercury's surface when struck by cosmic rays. Each element has a signature emission, and the instrument will look for geologically important elements such as hydrogen, magnesium, silicon, oxygen, iron, titanium, sodium and calcium. It may also detect naturally radioactive elements such as potassium, thorium and uranium.

The Neutron Spectrometer will map variations in the fast, thermal and epithermal neutrons Mercury's surface emits when struck by cosmic rays. "Fast" neutrons shoot directly into space; others collide with neighboring atoms in the crust before escaping. If a neutron collides with a small atom (like hydrogen), it will lose energy and be detected as a slow (or thermal) neutron. Scientists can look at the ratio of thermal to epithermal (slightly faster) neutrons across Mercury's surface to estimate the amount of hydrogen - possibly locked up in water molecules - and other elements.

X-Ray Spectrometer

Mass: 3.4 kilograms (7.5 pounds)
Peak Power: 11.4 watts
Development: Johns Hopkins University Applied Physics Laboratory

XRS will map the elements in the top millimeter of Mercury's crust using three gas-filled detectors pointing at the planet and one silicon solid-state detector pointing at the Sun. The planet-pointing detectors measure fluorescence, the X-ray emissions coming from Mercury's surface after solar X-rays hit the planet.

XRS detects emissions from elements in the 1-10 kiloelectron-volt range - specifically, magnesium, aluminum, silicon, sulfur, calcium, titanium and iron. Two detectors have thin absorption filters that help distinguish among the lower-energy X-ray lines of magnesium, aluminum and silicon.

Beryllium copper honeycomb collimators give XRS a 12-degree field of view, which is narrow enough to eliminate X-rays from the star background even when MESSENGER is at its farthest orbital distance from Mercury. A small, thermally protected, solar-flux monitor mounted on MESSENGER's sunshade tracks the X-rays bombarding the planet.

Magnetometer

Mass (including boom): 4.4 kilograms (9.7 pounds)
Peak Power: 4.2 watts
Development: NASA Goddard Space Flight Center, Greenbelt, Md., and the Johns Hopkins University Applied Physics Laboratory

A three-axis, ring-core fluxgate detector, MAG will characterize Mercury's magnetic field in detail, helping scientists determine the field's exact strength and how it varies with position and altitude. Obtaining this information is a critical step toward determining the source of Mercury's magnetic field.

The MAG sensor is mounted on a 3.6-meter (nearly 12-foot) boom that keeps it away from the spacecraft's own magnetic field. While in orbit at Mercury the instrument will collect magnetic field samples at 50-millisecond to one-second intervals; the rapid sampling will take place near Mercury's magnetosphere boundaries.

Mercury Laser Altimeter

Mass: 7.4 kilograms (16.3 pounds)
Peak Power: 38.6 watts
Development: NASA Goddard Space Flight Center

MLA will map Mercury's landforms and other surface characteristics using an infrared laser transmitter and a receiver that measures the round-trip time of individual laser pulses. The data will also be used to track the planet's slight forced libration - a wobble about its spin axis - which will tell researchers about the state of Mercury's core.

MLA data combined with Radio Science Doppler ranging will be used to map the planet's gravitational field. MLA can view the planet from up to 1,000 kilometers (620 miles) away with an accuracy of 30 centimeters (about one foot). The laser's transmitter, operating at a wavelength of 1,064 nanometers, will deliver eight pulses per second. The receiver consists of four sapphire lenses, a photon-counting detector, a time-interval unit and processing electronics.

Mercury Atmospheric and Surface Composition Spectrometer

Mass: 3.1 kilograms (6.8 pounds)
Peak Power: 8.2 watts
Development: University of Colorado, Boulder

Combining an ultraviolet spectrometer and infrared spectrograph, MASCS will measure the abundance of atmospheric gases around Mercury and detect minerals in its surface materials.

The Ultraviolet Visible Spectrometer will determine the composition and structure of Mercury's exosphere - the low-density atmosphere - and study its neutral gas emissions. It will also search for and measure ionized atmospheric species. Together these measurements will help researchers understand the processes that generate and maintain the atmosphere, the connection between surface and atmospheric composition, the dynamics of volatile materials on and near Mercury, and the nature of the radar-reflective materials near the planet's poles. The instrument has 25-kilometer resolution at the planet's limb.

Perched atop the ultraviolet spectrometer, the Visible-Infrared Spectrograph will measure the reflected visible and near-infrared light at wavelengths diagnostic of iron and titanium-bearing silicate materials on the surface, such as pyroxene, olivine and ilmenite. The sensor's best resolution is 3 kilometers.

Energetic Particle and Plasma Spectrometer

Mass: 3.1 kilograms (6.8 pounds)
Peak Power: 7.8 watts
Development: University of Michigan, Ann Arbor, and the Johns Hopkins University Applied Physics Laboratory

EPPS will measure the mix and characteristics of charged particles in and around Mercury's magnetosphere using an Energetic Particle Spectrometer (EPS) and a Fast Imaging Plasma Spectrometer (FIPS). Both are equipped with time-of-flight and energy-measurement technologies to determine particle velocities and elemental species.

From its vantage point near the top deck of the spacecraft, EPS will observe ions and electrons accelerated in the magnetosphere. EPS has a 160- by 12-degree field of view for measuring the energy spectra, atomic composition and pitch-angle distribution of these ions and electrons. Mounted on the side of the spacecraft, FIPS will observe low-energy ions coming from Mercury's surface and sparse atmosphere, ionized atoms picked up by the solar wind, and other solar wind components. FIPS provides nearly full hemispheric coverage.

Radio Science observations - gathered by tracking the spacecraft through its communications system - will precisely measure MESSENGER's speed and distance from Earth. From this information, scientists and engineers will watch for changes in MESSENGER's movements at Mercury to measure the planet's gravity field, and to support the laser altimetry investigation to determine the size and condition of Mercury's core. NASA's Goddard Space Flight Center leads the Radio Science experiment.

Spacecraft Systems and Components

Thermal

While orbiting Mercury, MESSENGER will "feel" significantly hotter than spacecraft that orbit Earth. This is because Mercury's elongated orbit swings the planet to within 46 million kilometers (29 million miles) of the Sun, or about two-thirds closer to the Sun than Earth's orbit. The Sun also shines up to 11 times brighter at Mercury than we see from our own planet.


An illustration shows the MESSENGER spacecraft and denotes its systems. Credit: JHU-APL
 
MESSENGER's first line of thermal defense is a heat-resistant and highly reflective sunshade, fixed on a titanium frame to the front of the spacecraft. Measuring about 2.5 meters (8 feet) tall and 2 meters (6 feet) across, the thin shade has front and back layers of Nextel ceramic cloth - the same material that protects sections of the space shuttle - surrounding several inner layers of Kapton plastic insulation. While temperatures on the front of the shade could reach 370 degrees C (698 degrees F) when Mercury is closest to the Sun, behind it the spacecraft will operate at room temperature, around 20 degrees C (68 degrees F). Multilayered insulation covers most of the spacecraft.

Radiators and one-way heat pipes are installed to carry heat away from the spacecraft body, and the science orbit is designed to limit MESSENGER's exposure to heat re-radiating from the surface of Mercury. (MESSENGER will only spend about 25 minutes of each 12-hour orbit crossing Mercury's broiling surface at low altitude.) The combination of the sunshade, thermal blanketing and heat-radiation system allows the spacecraft to operate without special high-temperature electronics.

Power

Two single-sided solar panels are the spacecraft's main source of electric power. To run MESSENGER's systems and charge its 23-ampere-hour nickel-hydrogen battery, the panels, each about 1.5 meters (5 feet) by 1.65 meters (5.5 feet), will support between 385-485 watts of spacecraft load power during the cruise phase and 640 watts during the orbit at Mercury. The panels could produce more than two kilowatts of power near Mercury, but to prevent stress on MESSENGER's electronics, onboard power processors take in only what the spacecraft actually needs.

The custom-developed panels are 67 percent mirrors (called optical solar reflectors) and 33 percent triplejunction solar cells, which convert 28 percent of the sunlight hitting them into electricity. Each panel has two rows of mirrors for every row of cells; the small mirrors reflect the Sun's energy and keep the panel cooler. The panels also rotate, so MESSENGER's flight computer will tilt the panels away from the Sun, positioning them to get the required power while maintaining a normal surface operating temperature of about 150 degrees Celsius, or 302 degrees Fahrenheit.

Propulsion

MESSENGER's dual-mode propulsion system includes a 660-newton (150-pound) bipropellant thruster for large maneuvers and 16 hydrazine-propellant thrusters for smaller trajectory adjustments and attitude control. The "large velocity adjust" thruster requires a combination of hydrazine and an oxidizer, nitrogen tetroxide. Fuel and oxidizer are stored in custom-designed, lightweight titanium tanks integrated into the spacecraft's composite frame. Helium pushes the fuel and oxidizer through the system to the engines.

At launch the spacecraft will carry about 600 kilograms (1,323 pounds) of propellant - and use nearly 30 percent of it during the maneuver that starts the orbit at Mercury. The small hydrazine thrusters play several important roles: four 22-newton (5-pound) thrusters are used for small course corrections and help steady MESSENGER during large engine burns. The dozen 4.4-newton (1-pound) thrusters are also used for small course corrections and serve as a backup for the reaction wheels that maintain the spacecraft's orientation during normal cruise and orbital operations.

Communications

MESSENGER's X-band coherent communications system includes two high-gain, electronically steered, phased array antennas - the first ever used on a deep space mission; two medium-gain fanbeam antennas; and four low-gain antennas. The circularly polarized phased arrays, located with the fanbeam antennas on the front and back of the spacecraft, are the main link for sending science data to Earth. For better reliability the antennas are fixed; they "point" electronically across a 45-degree field without moving parts, and during normal operations at least one of the two antennas will point at Earth.

Higher gain antennas send radio signals through a narrower, more concentrated beam than lower gain antennas. High-gain antennas are used primarily to send larger amounts of data over the same distance as a low-gain antenna. The fanbeam and low-gain antennas, also located on MESSENGER's front and back sides, are used for lower-rate transmissions such as operating commands, status data or emergency communications. MESSENGER's downlink rate ranges from 9.9 bits per second to 104 kilobits per second; operators can send commands at 7.8 to 500 bits per second. Transmission rates vary according to spacecraft distance and ground-station antenna size.

Command and Data Handling

MESSENGER's "brain" is its Integrated Electronics Module (IEM), a space- and weight-saving device that combines the spacecraft's core avionics in a single box. The spacecraft carries a pair of identical IEMs for backup purposes; both house a 25-megahertz main processor and 10-MHz fault protection processor. All four are radiation-hardened RAD6000 processors, based on predecessors of the PowerPC chip found in some models of Macintosh computer. The computers, slow by current home-computer standards, are state of the art for the radiation tolerance required on the MESSENGER mission.

Programmed to monitor the condition of MESSENGER's key systems, both fault protection processors are turned on at all times and protect the spacecraft by turning off components and/or switching to backup components when necessary. The main processor runs the Command and Data Handling software for data transfer and file storage, as well as the Guidance and Control software used to navigate and point the spacecraft. Each IEM also includes a solid-state data recorder, power converters and the interfaces between the processors and MESSENGER's instruments and systems.

Intricate flight software guides MESSENGER's Command and Data Handling system. MESSENGER receives operating commands from Earth and can perform them in real time or store them for later execution. Some of MESSENGER's frequent, critical operations (such as propulsive maneuvers) are programmed into the flight computer's memory and timed to run automatically.

For data, MESSENGER carries two solid-state recorders (one backup) able to store up to 1 gigabyte each. Its main processor collects, compresses and stores images and other data from MESSENGER's instruments on the recorder; the software sorts the data into files similar to how files are stored on a PC. The main processor selects the files with highest priority to transmit to Earth, or mission operators can download data files in any order the team chooses.

Antenna signal strength (and downlink rate) varies with spacecraft-Earth distance and ground-station antenna size. While orbiting Mercury MESSENGER will store most of its data when it's farther from Earth, typically sending only information on its condition and the highest-priority images and measurements during daily eighthour contacts through NASA's Deep Space Network. The spacecraft will send most of the recorded data when Mercury's path around the Sun brings it closer to Earth.

Guidance and Control

MESSENGER is well protected against the heat, but it must always know its orientation relative to Mercury, Earth and the Sun and be "smart" enough to keep its sunshade pointed at the Sun. Attitude determination - knowing in which direction MESSENGER is facing - is performed using star-tracking cameras, digital Sun sensors and an Inertial Measurement Unit (containing gyroscopes and accelerometers). Attitude control for the 3-axis stabilized craft is accomplished using four internal reaction wheels and, when necessary, MESSENGER's small thrusters.

The Inertial Measurement Unit accurately determines the spacecraft's rotation rate, and MESSENGER tracks its own orientation by checking the location of stars and the Sun. Star-tracking cameras on MESSENGER's top deck store a complete map of the heavens; 10 times a second, one of the cameras takes a wide-angle picture of space, compares the locations of stars to its onboard map, and then calculates the spacecraft's orientation. The Guidance and Control software also automatically rotates the spacecraft and solar panels to the desired Sun-relative orientation, making sure the panels produce sufficient power while maintaining safe temperatures.

Six Sun sensors back up the star trackers, continuously measuring MESSENGER's angle to the Sun. If the flight software detects that the Sun is "moving" out of a designated safe zone it can initiate an automatic turn to ensure the shade faces the Sun. Then ground controllers can analyze the situation while the spacecraft turns its antennas to Earth and awaits instructions - an operating condition known as "safe" mode.

Spacecraft Hardware Suppliers

Structure: ATK Composite Optics, Inc., San Diego; Propulsion: Aerojet, Sacramento, Calif.; Transponder: General Dynamics, Scottsdale, Ariz.; Solid State Power Amplifier Converters: EMS Technologies, Montreal, Canada; Inertial Measurement Unit: Northrop Grumman, Woodland Hills, Calif.; Star Trackers: Alenia Spazio, Rome, Italy; Sun Sensors: Adcole Corporation, Marlborough, Mass.; Reaction Wheels: Teldix GmbH, Heidelberg, Germany; Solar Array Drives: Moog Inc., East Aurora, N.Y.; Solar Arrays: Northrop Grumman Space Technology, Redondo Beach, Calif.; Battery (with APL): Eagle Picher Technologies, Joplin, Mo.; Integrated Electronics Module (with APL): BAE systems, Manassas, Va.; Heat Pipes: Swales Aerospace, Beltsville, Md.

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