MESSENGER

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Scientific instruments

Included two CCD cameras, a narrow-angle camera (NAC) and a wide-angle camera (WAC) mounted to a pivoting platform. The camera system provided a complete map of the surface of Mercury at a resolution of 250 meters/pixel (820 ft/pixel), and images of regions of geologic interest at 20–50 meters/pixel (66–164 ft/pixel). Color imaging was possible only with the narrow-band filter wheel attached to the wide-angle camera.

Objectives:

Flyby Phase:

Acquisition of near-global coverage at ≈500 meters/pixel (1,600 ft/pixel).

Multispectral mapping at ≈2 kilometers/pixel (1.2 mi/pixel).

Orbital Phase:

A nadir-looking monochrome global photomosaic at moderate solar incidence angles (55°–75°) and 250 meters/pixel (820 ft/pixel) or better sampling resolution.

A 25°-off-nadir mosaic to complement the nadir-looking mosaic for global stereo mapping.

Completion of the multispectral mapping begun during the flybys.

High-resolution (20–50 meters/pixel (66–164 ft/pixel)) image strips across features representative of major geologic units and structures.

Principal investigator: Scott Murchie / Johns Hopkins University

Measured gamma-ray emissions from the surface of Mercury to determine the planet's composition by detecting certain elements (oxygen, silicon, sulfur, iron, hydrogen, potassium, thorium, uranium) to a depth of 10 cm.

Objectives:

Provide surface abundances of major elements.

Provide surface abundances of Fe, Si, and K, infer alkali depletion from K abundances, and provide abundance limits on H (water ice) and S (if present) at the poles.

Map surface element abundances where possible, and otherwise provide surface-averaged abundances or establish upper limits.

Principal investigator: William Boynton / University of Arizona

Determined the hydrogen mineral composition to a depth of 40 cm by detecting low-energy neutrons resulting from the collision of cosmic rays with the minerals.

Objectives:

Establish and map the abundance of hydrogen over most of the northern hemisphere of Mercury.

Investigate the possible presence of water ice within and near permanently shaded craters near the north pole.

Provide secondary evidence to aid in interpreting GRS measured gamma-ray line strengths in terms of elemental abundances.

Outline surface domains at the base of both northern and southern cusps of the magnetosphere where the solar wind can implant hydrogen in surface material.

Principal investigator: William Boynton / University of Arizona

Mapped mineral composition within the top millimeter of the surface on Mercury by detecting X-ray spectral lines from magnesium, aluminum, sulphur, calcium, titanium, and iron, in the 1–10 keV range.

Objectives:

Determine the history of the formation of Mercury

Characterize the composition of surface elements by measuring the X-ray emissions induced by the incident solar flux.

Principal investigator: George Ho / APL

Measured the magnetic field around Mercury in detail to determine the strength and average position of the field.

Objectives:

Investigate the structure of Mercury's magnetic field and its interaction with the solar wind.

Characterize the geometry and time variability of the magnetospheric field.

Detect wave-particle interactions with the magnetosphere.

Observe magnetotail dynamics, including phenomena possibly analogous to substorms in the Earth's magnetosphere.

Characterize the magnetopause structure and dynamics.

Characterize field-aligned currents that link the planet with the magnetosphere.

Principal investigator: Mario Acuna / NASA Goddard Space Flight Center

Provided detailed information regarding the height of landforms on the surface of Mercury by detecting the light of an infrared laser as the light bounced off the surface.

Objectives:

Provide a high-precision topographic map of the high northern latitude regions.

Measure the long-wavelength topographic features at mid-to-low northern latitudes.

Determine topographic profiles across major geologic features in the northern hemisphere.

Detect and quantify the planet's forced physical librations by tracking the motion of large-scale topographic features as a function of time.

Measure the surface reflectivity of Mercury at the MLA operating wavelength of 1,064 nanometers.

Principal investigator: David Smith / GSFC

Determined the characteristics of the tenuous atmosphere surrounding Mercury by measuring ultraviolet light emissions, and ascertained the prevalence of iron and titanium minerals on the surface by measuring the reflectance of infrared light.

Objectives:

Characterize the composition, structure, and temporal behavior of the exosphere.

Investigate the processes that generate and maintain the exosphere.

Determine the relationship between exospheric and surface composition.

Search for polar deposits of volatile material, and determine how are the accumulation of these deposits are related to exospheric processes.

Principal investigator: William McClintock / University of Colorado

Measured the charged particles in the magnetosphere around Mercury using an energetic particle spectrometer (EPS) and the charged particles that come from the surface using a fast imaging plasma spectrometer (FIPS).

Objectives:

Determine the structure of the planet's magnetic field.

Characterize exosphere neutrals and accelerated magnetospheric ions.

Determine the composition of the radar-reflective materials at Mercury's poles.

Determine the electrical properties of the crust/atmosphere/environment interface.

Determine characteristics of the dynamics of Mercury's magnetosphere and their relationships to external drivers and their internal conditions.

Measure interplanetary plasma properties in cruise and in Mercury vicinity.

Principal investigator: Barry Mauk / APL

Measured the gravity of Mercury and the state of the planetary core by utilizing the spacecraft's positioning data.

Objectives:

Determine the position of the spacecraft during both the cruise and orbital phases of the mission.

Observe gravitational perturbations from Mercury to investigate the spatial variations of density within the planet's interior, and a time-varying component in Mercury's gravity to quantify the amplitude of Mercury's libration.

Provide precise measurements of the range of the MESSENGER spacecraft to the surface of Mercury for determining proper altitude mapping with the MLA.

Principal investigator: David Smith / NASA Goddard Space Flight Center

Launch and trajectory

The MESSENGER probe was launched on August 3, 2004, at 06:15:56 UTC by NASA from Space Launch Complex 17B at the Cape Canaveral Air Force Station in Florida, aboard a Delta II 7925 launch vehicle. The complete burn sequence lasted 57 minutes bringing the spacecraft into a heliocentric orbit, with a final velocity of 10.68 km/s (6.64 miles/s) and sending the probe into a 7.9 billion-kilometer (4.9 billion mi) trajectory that took 6 years, 7 months and 16 days before its orbital insertion on March 18, 2011.

Traveling to Mercury and entering orbit requires an extremely large velocity change (see delta-v) because Mercury's orbit is deep in the Sun's gravity well. On a direct course from Earth to Mercury, a spacecraft is constantly accelerated as it falls toward the Sun, and will arrive at Mercury with a velocity too high to achieve orbit without excessive use of fuel. For planets with an atmosphere, such as Venus and Mars, spacecraft can minimize their fuel consumption upon arrival by using friction with the atmosphere to enter orbit (aerocapture), or can briefly fire their rocket engines to enter into orbit followed by a reduction of the orbit by aerobraking. However, the tenuous atmosphere of Mercury is far too thin for these maneuvers. Instead, MESSENGER extensively used gravity assist maneuvers at Earth, Venus, and Mercury to reduce the speed relative to Mercury, then used its large rocket engine to enter into an elliptical orbit around the planet. The multi-flyby process greatly reduced the amount of propellant necessary to slow the spacecraft, but at the cost of prolonging the trip by many years and to a total distance of 7.9 billion kilometers (4.9 billion miles).

Several planned thruster firings en route to Mercury were unnecessary, because these fine course adjustments were performed using solar radiation pressure acting on MESSENGER's solar panels. To further minimize the amount of necessary propellant, the spacecraft orbital insertion targeted a highly elliptical orbit around Mercury.

The elongated orbit had two other benefits: It allowed the spacecraft time to cool after the times it was between the hot surface of Mercury and the Sun, and also it allowed the spacecraft to measure the effects of solar wind and the magnetic fields of the planet at various distances while still allowing close-up measurements and photographs of the surface and exosphere.

The spacecraft was originally scheduled to launch during a 12-day window that beginning May 11, 2004. On March 26, 2004, NASA announced the launch would be moved to a later, 15-day launch window beginning July 30, 2004, to allow for further testing of the spacecraft. This change significantly altered the trajectory of the mission and delayed the arrival at Mercury by two years. The original plan called for three fly-by maneuvers past Venus, with Mercury orbit insertion scheduled for 2009. The trajectory was changed to include one Earth flyby, two Venus flybys, and three Mercury flybys before orbit insertion on March 18, 2011.

MESSENGER

MESSENGER

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Mission overview

Mission background

Previous missions

Proposals for the mission

Objectives

Spacecraft design

Attitude control and propulsion

Communications

Power

Computer and software

Scientific instruments

Mission profile

Launch and trajectory

Earth flyby

Two Venus flybys

Three Mercury flybys

Orbital insertion

Primary science

Extended mission

Discovery of water, organic compounds and volcanism

Solar System portrait

View of a total lunar eclipse

End of mission