About
Spacecraft and Instruments
Spacecraft   Instruments

Learn about the details of the spacecraft and how the spacecraft systems were designed to address the challenges of becoming the first spacecraft to orbit Mercury. Find details about MESSENGER’s science payload, which included seven scientific instruments and a radio science experiment, all designed to operate in the extreme environment near the Sun.

Spacecraft

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 to slow the spacecraft down enough to be captured by Mercury’s gravity. And the probe would have to be sufficiently compact to be launched on a conventional and cost-effective rocket. Designed and built by the Johns Hopkins University Applied Physics Laboratory – with contributions from research institutions and companies around the world – the MESSENGER spacecraft tackled each of these challenges and was the first spacecraft to orbit Mercury.

Spacecraft Overview

Spacecraft Overview

Designed and built by the Johns Hopkins University Applied Physics Laboratory – with contributions from research institutions and companies around the world – the MESSENGER spacecraft tackled the challenges associated with orbiting Mercury. A ceramic-fabric sunshade, heat radiators, and a mission design that limited time over the planet’s hottest regions protected MESSENGER without expensive and impractical cooling systems. The spacecraft’s graphite composite structure – strong, lightweight, and heat tolerant – was integrated with a low-mass propulsion system that efficiently stored and distributed the approximately 600 kilograms (about 1,320 pounds) of propellant that accounted for 54% of MESSENGER’s total launch weight.

To fit behind the 2.5-meter by 2-meter (roughly 8-foot by 6-foot) sunshade, MESSENGER’s wiring, electronics, systems, and instruments were packed into a small frame that could fit inside a large sport utility vehicle. And the entire spacecraft was light enough for launch on a Delta II 7925-H (“heavy”) rocket, the largest launch vehicle allowed under NASA’s Discovery Program of lower-cost, space science missions.

Thermal Design

Thermal Design

While orbiting Mercury, MESSENGER “felt” 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. As a result, the Sun shines up to 11 times brighter at Mercury than we see from our own planet.

MESSENGER’s first line of thermal defense was 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 had 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 were predicted to reach 370° C (about 700° F) when Mercury was closest to the Sun, behind it the spacecraft was designed to operate at room temperature, around 20° C (about 70° F). Multilayered insulation covered most of the spacecraft.

Radiators and diode (“one-way”) heat pipes were installed to carry heat away from the spacecraft body, and the science orbit was designed to limit MESSENGER’s exposure to heat re-radiating from the surface of Mercury. (MESSENGER only spent 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 allowed the spacecraft to operate without special high-temperature electronics.

Power

Power

Two single-sided solar panels were 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), supported between 385 and 485 watts of spacecraft load power during the cruise phase and 640 watts during the orbit at Mercury. The panels could have produced more than two kilowatts of power near Mercury, but to prevent stress on MESSENGER’s electronics, onboard power processors took in only what the spacecraft actually needed.

The custom-developed panels were two-thirds mirrors (called optical solar reflectors) and one-third triple-junction solar cells, which converted 28 percent of the sunlight hitting them into electricity. Each panel had two rows of mirrors for every row of cells; the small mirrors reflected the Sun’s energy and kept the panel cooler. The panels also rotated, so MESSENGER’s flight computer tilted the panels away from the Sun, positioning them to get the required power while maintaining a normal surface operating temperature of about 150°C, or about 300°F.

Propulsion

Propulsion

MESSENGER’s dual-mode propulsion system included 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 (LVA) thruster required a combination of hydrazine fuel and an oxidizer, nitrogen tetroxide. Fuel and oxidizer were stored in custom-designed, lightweight titanium tanks integrated into the spacecraft’s composite frame. Helium pressurized the system and pushed the fuel and oxidizer through to the engines.

At launch the spacecraft carried just under 600 kilograms (about 1,320 pounds) of propellant, and it used nearly 30 percent of it during the maneuver that inserted the spacecraft into orbit about Mercury. The small hydrazine thrusters played several important roles: four 22-newton (5-pound) thrusters were used for small course corrections and helped steady MESSENGER during large engine burns. The dozen 4.4-newton (1-pound) thrusters were also used for small course corrections and served as a backup for the reaction wheels that maintained the spacecraft’s orientation during normal cruise and orbital operations.

Communications

Communications

MESSENGER’s X-band coherent communications system included 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 – developed by APL and located with the fanbeam antennas on the front and back of the spacecraft – were the main link for sending science data to Earth. For better reliability the high-temperature environment antennas were fixed; they “pointed” electronically across a 45° field without moving parts, and during normal operations at least one of the two antennas pointed at Earth.

High-gain antennas sent radio signals through a narrower, more concentrated beam than low-gain antennas and were 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, were used for lower-rate transmissions such as operating commands, status data, or emergency communications. MESSENGER’s downlink rate ranged from 9.9 bits per second to 104 kilobits per second; operators could send commands at 7.8 to 500 bits per second. Transmission rates varied according to spacecraft distance and ground-station antenna size.

Command and Data Handling

Command and Data Handling

MESSENGER’s “brain” was its Integrated Electronics Module (IEM), a space- and weight-saving device that combined the spacecraft’s core avionics into a single box. The spacecraft carried a pair of identical IEMs for backup purposes; both housed a 25-megahertz (MHz) main processor and 10-MHz fault protection processor. All four were radiation-hardened RAD6000 processors, based on predecessors of the PowerPC chip found in some models of home computers. The computers, slow by current home-computer standards, were 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 were turned on at all times and protected the spacecraft by turning off components and/or switching to backup components when necessary. The main processor ran 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 included a solid-state data recorder, power converters, and the interfaces between the processors and MESSENGER’s instruments and systems.

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

For data, MESSENGER carried two solid-state recorders (one backup) able to store up to 1 gigabyte each. Its main processor collected, compressed, and stored images and other data from MESSENGER’s instruments onto the recorder; the software sorted the data into files similar to how files are stored on a PC. The main processor selected the files with highest priority to transmit to Earth, or mission operators could download data files in any order the team chose.

Antenna signal strength (and downlink rate) varied with spacecraft-Earth distance and ground-station antenna size. While orbiting Mercury MESSENGER stored most of its data when it was farther from Earth, typically sending only information on its condition and the highest-priority images and measurements during regular eight-hour contacts through NASA’s Deep Space Network. The spacecraft sent most of the recorded data when Mercury’s path around the Sun brought it closer to Earth.

Guidance and Control

Guidance and Control

MESSENGER was well protected against the heat. It always knew its orientation relative to Mercury, Earth, and the Sun and was “smart” enough to keep its sunshade pointed at the Sun. Attitude determination – knowing in which direction MESSENGER was facing – was performed using star-tracking cameras, digital Sun sensors, and an Inertial Measurement Unit (IMU, which contained gyroscopes and accelerometers). Attitude control for the 3-axis stabilized craft was accomplished using four internal reaction wheels and, when necessary, MESSENGER’s small thrusters.

The IMU accurately determined the spacecraft’s rotation rate, and MESSENGER tracked its own orientation by checking the location of stars and the Sun. Star-tracking cameras on MESSENGER’s top deck stored a complete map of the heavens; 10 times a second, one of the cameras took a wide-angle picture of space, compared the locations of stars to its onboard map, and then calculated the spacecraft’s orientation. The Guidance and Control software also automatically rotated the spacecraft and solar panels to the desired Sun-relative orientation, ensuring that the panels produced sufficient power while maintaining safe temperatures.

Five Sun sensors backed up the star trackers, continuously measuring MESSENGER’s angle to the Sun. If the flight software detected that the Sun was “moving” out of a designated safe zone it could initiate an automatic turn to ensure that the shade faced the Sun. Ground controllers could then analyze the situation while the spacecraft turned its antennas to Earth and await instructions – an operating condition known as “safe” mode.

Instruments

MESSENGER carried 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 - imaged all of Mercury for the first time, as well as gathered 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.

Overview of the Instruments

Overview of the Instruments

The instruments included the Mercury Dual Imaging System (MDIS), the Gamma-Ray and Neutron Spectrometer (GRNS), the X-Ray Spectrometer (XRS), the Magnetometer (MAG), the Mercury Laser Altimeter (MLA), the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), and the Energetic Particle and Plasma Spectrometer (EPPS). The instruments communicated to the spacecraft through fully redundant Data Processing Units (DPUs). The instruments are labeled in the picture below.

The process of selecting the scientific instrumentation for a mission is typically a balance between answering as many science questions as possible and fitting within the available mission resources for mass, power, mechanical accommodation, schedule, and cost. In the case of MESSENGER, the mass and mechanical accommodation issues were very significant constraints. Payload mass was limited to 50 kilograms (110 pounds) because of the propellant mass needed for orbit insertion. The instrument mechanical accommodation was difficult because of the unique thermal constraints faced during the mission; instruments had to be mounted where Mercury would be in view but the Sun would not, and they had to be maintained within an acceptable temperature range in a very harsh environment. In each case the mass included mounting hardware and thermal control component and power was the nominal average power consumption per orbit; actual values vared with instrument operational mode.

Mercury Dual Imaging System (MDIS)

Mercury Dual Imaging System (MDIS)

Mass: 8.0 kilograms (17.6 pounds)
Power: 7.6 watts
Development: Johns Hopkins University Applied Physics Laboratory

This instrument consisted of two cameras that mapped landforms, tracked variations in surface spectra and gathered topographic information.

The multi-spectral MDIS had wide- and narrow-angle cameras (the “WAC” and “NAC,” respectively) – 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 pivoted, giving it the ability to capture images from a wide area without having to re-point the spacecraft and allowed it to follow the stars and other optical navigation guides.

The wide-angle camera had a 10.5° by 10.5° field of view and could observe Mercury through 11 different filters and monochrome across the wavelength range 395 to 1,040 nanometers (visible through near-infrared light). Multi-spectral imaging helped scientists investigate the diversity of rock types that formed Mercury’s surface. The narrow-angle camera could take black-and-white images at high resolution through its 1.5° by 1.5° field of view, allowing extremely detailed analysis of features as small as 18 meters (about 60 feet) across.

For additional information about and examples of MDIS images, visit these pages:

Gamma-Ray and Neutron Spectrometer (GRNS)

Gamma-Ray and Neutron Spectrometer (GRNS)

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

Gamma-Ray Spectrometer (GRS)
Mass: 9.2 kilograms (20.3 pounds)
Power: 16.5 watts
Development: Johns Hopkins University Applied Physics Laboratory, Patriot Engineering, Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory

GRS measured gamma rays emitted by the nuclei of atoms on Mercury’s surface that are struck by cosmic rays. Each element has a signature emission, and the instrument looked for geologically important elements such as hydrogen, magnesium, silicon, oxygen, iron, titanium, sodium, and calcium. It also detected naturally radioactive elements such as potassium, thorium, and uranium.

Neutron Spectrometer (NS)
Mass: 3.9 kilograms (8.6 pounds)
Power: 6.0 watts
Development: Johns Hopkins University Applied Physics Laboratory, Patriot Engineering, Los Alamos National Laboratory

NS mapped 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 light atom (like hydrogen), it loses energy and is detected as a slow (or thermal) neutron. Scientists looked 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 (XRS)

X-Ray Spectrometer (XRS)

Mass: 3.4 kilograms (7.5 pounds)
Power: 6.9 watts
Development: Johns Hopkins University Applied Physics Laboratory

XRS mapped the elements in the top millimeter of Mercury’s crust using three gas-filled detectors (Mercury X-Ray Unit, or MXU) pointing at the planet, one silicon solid-state detector pointing at the Sun (Solar Assembly for X-rays, or SAX), and the associated electronics (Main Electronics for X-rays, or MEX). The planet-pointing detectors measured fluorescence, the X-ray emissions coming from Mercury’s surface after solar X-rays hit the planet. The Sun-pointing detector tracked the X-rays bombarding the planet.

XRS detected emissions from elements in the 1-10 kiloelectron-volt (keV) range – specifically, magnesium, aluminum, silicon, sulfur, calcium, titanium, and iron. Two detectors had thin absorption filters that helped distinguish among the lower-energy X-ray lines of magnesium, aluminum, and silicon.

Beryllium-copper honeycomb collimators gave XRS a 12° field of view, which was narrow enough to eliminate X-rays from the star background even when MESSENGER was at its farthest orbital distance from Mercury. The small, thermally protected, solar-flux monitor was mounted on MESSENGER’s sunshade.

Magnetometer (MAG)

Magnetometer (MAG)

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

MAG characterized Mercury’s magnetic field in detail. The three-axis, ring-core fluxgate detector helped scientists determine the field’s precise strength and how it varies with position and altitude. Obtaining this information was a critical step toward determining the source of Mercury’s magnetic field.

The MAG sensor was mounted on a 3.6-meter (nearly 12-foot long) boom that kept it away from the spacecraft’s own magnetic field. The sensor also had its own sunshade to protect it from the Sun when the spacecraft was tilted to allow for viewing by the other instruments. While in orbit at Mercury the instrument collected magnetic field samples at 50-millisecond to one-second intervals; the rapid sampling took place near Mercury’s magnetosphere boundaries.

Mercury Laser Altimeter (MLA)

Mercury Laser Altimeter (MLA)

Mass: 7.4 kilograms (16.3 pounds)
Power: 16.4 watts
Development: NASA Goddard Space Flight Center

MLA mapped Mercury’s landforms and other surface characteristics using an infrared laser transmitter and a receiver that measured the round-trip time of individual laser pulses to determine the planet's topography. The data was also used to track the planet’s slight, forced libration – a wobble about its spin axis – which told researchers about the state of Mercury’s core.

MLA data combined with Radio Science Doppler ranging was used to map the planet’s gravitational field. MLA viewed the planet from up to 1500 kilometers (930 miles) away with an accuracy of 30 centimeters (about one foot). The laser’s transmitter, operating at a wavelength of 1,064 nanometers, delivered eight pulses per second. The receiver consisted of four sapphire lenses mounted on beryllium structures, a photon-counting detector, a time-interval unit, and processing electronics.

Mercury Atmospheric and Surface Composition Spectrometer (MASCS)

Mercury Atmospheric and Surface Composition Spectrometer (MASCS)

Mass: 3.1 kilograms (6.8 pounds)
Power: 6.7 watts
Development: Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder

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

The Ultraviolet and Visible Spectrometer (UVVS) determined the composition and structure of Mercury’s exosphere – the extremely low-density atmosphere – and study its neutral gas emissions. It also searched for and measured ionized atmospheric species. Together these measurements helped researchers understand the processes that generated and maintained 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 had 25-kilometer altitude resolution at the planet’s limb.

Perched atop the ultraviolet spectrometer, the Visible and Infrared Spectrograph (VIRS) measured 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 was 3 kilometers at Mercury’s surface.

MESSENGER Visible and Infrared Spectrograph (VIRS) Data Users¹ Workshop 2014

Energetic Particle and Plasma Spectrometer (EPPS)

Energetic Particle and Plasma Spectrometer (EPPS)

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

EPPS measured the composition, distribution, and energy of charged particles (electrons and various ions) in and around Mercury’s magnetosphere using an Energetic Particle Spectrometer (EPS) and a Fast Imaging Plasma Spectrometer (FIPS). Both were equipped with time-of-flight and energy-measurement technologies to determine simultaneously particle velocities and elemental species.

From its vantage point near the top deck of the spacecraft, EPS observed ions and electrons accelerated in the magnetosphere. EPS had a 160° by 12° field of view for measuring the energy spectra and pitch-angle distribution of these ions and electrons. Mounted on the side of the spacecraft, FIPS observed 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 provided nearly full hemispheric coverage.

Radio Science (RS)

Radio Science (RS)

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

The MESSENGER science and operations teams communicated with the MESSENGER spacecraft using the large ground-based antennas of the Deep Space Network (DSN). While sending commands and receiving data, the DSN antennas used sensitive receivers to measure the slight changes in frequency caused by MESSENGER’s motion in Mercury’s gravity field. The largest DSN antennas were 70 meters (230 feet) in diameter. These antennas had about as much surface area as a football field.