Orbiting too close to the Sun for conventional telescopes to risk a glance and for ordinary spacecraft to approach, hot and rocky Mercury remains the least explored of the terrestrial planet family that includes Venus, Earth, and Mars. But thanks to a pioneering APL spacecraft, Mercury’s elusive reputation is about to change.

Launched on August 3, 2004, the APL-built MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is slated to start orbiting the planet in March 2011. While NASA’s missions to Venus and Mars have produced exciting data and theories about processes that formed the inner planets, scientists expect Mercury to yield its own fascinating story.

MESSENGER’s mission: complete a detailed exploration of the inner solar system and help us understand the forces that shaped planets like our own.

Start of a Journey. Soon after MESSENGER’s predawn liftoff from Cape Canaveral Air Force Station, Florida, computer screens in the MESSENGER Mission Operations Center at APL lit up with electronic word that the spacecraft had come out of a launch-induced tumble, deployed its two solar panels, and was operating normally.

The celebrations had to be short; the critical task of operating a first-of-its-kind planetary explorer was at hand. But the moment was a significant milestone for the most ambitious and challenging NASA mission that APL has ever led.

MESSENGER faces a long cosmic road: more than 6 years, nearly 5 billion miles, 15 loops around the Sun, and multiple flybys of Earth, Venus, and Mercury before beginning a yearlong study of the innermost planet. In a way it’s the project’s second journey, coming after a five-year odyssey in which APL and its determined partners set out to push the limits of engineering and technology and pull off a space science first.

Technical Innovation. Only one spacecraft has visited Mercury; NASA’s Mariner 10 sailed past it three times in 1974 and 1975 and gathered detailed data on less than half the surface. An orbiter mission was the next logical step, but high technical risks and potential price tags ($1 billion in some studies) cooled NASA to the idea of sending a probe back to the hot planet.

APL began shaping concepts for a Mercury orbiter in the mid-1990s —incorporating new technologies, tougher materials, and creative mission designs that gradually made the mission more feasible. In 1999 NASA selected MESSENGER as the seventh in its Discovery Program of lower-cost, scientifically focused exploration projects, and gave APL the formal nod to build the spacecraft in 2001.

The technical challenges of working in Mercury’s orbit are intense. The planet’s average distance from the Sun is only 36 million miles, about two-thirds closer than Earth. MESSENGER could face the intensity of 11 suns while orbiting the dayside of Mercury, then endure a steep temperature drop while it passes over the nightside. (Mercury experiences the solar system’s largest swing in surface temperatures, from highs above 840°F to lows below –350°F.)

The double threat of searing sunlight and roasting heat bouncing off the planet below made it necessary to build a tough spacecraft. But MESSENGER also had 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 had to be compact enough to lift off on a conventional and cost-effective rocket.

The MESSENGER team tackled each of these challenges. A ceramic-fabric sunshade and a mission design that limits time over Mercury’s hottest regions will protect MESSENGER without expensive cooling systems. The spacecraft’s graphite-epoxy composite structure—strong, lightweight, heat tolerant, and the first of its kind for a deep-space satellite—is integrated with a low-mass propulsion system that efficiently stores and distributes the propellant that accounted for 55% of MESSENGER’s launch weight.

On the way, an innovative trajectory will swing MESSENGER past Earth, Venus, and Mercury several times, using the tug of the planets’ gravity to adjust the spacecraft’s path and position it to orbit Mercury. This “economy route” takes longer but requires less fuel than a straight shot to the planet, ultimately lowering costs and creating more room on the spacecraft for science instruments and systems.

Tackling Challenges. The MESSENGER team faced programmatic challenges too. Cost-capped Discovery missions demand tight schedules and budgets as well as conservative staffing requirements. APL aimed to limit costs by completing spacecraft fabrication, integration, and testing within 32 months, but when the launch date was moved twice to allow for additional testing, the overall program budget went up.

So, fortunately, did the team’s resolve. Late deliveries of some subsystems bumped the testing schedule off track; a later discovery that several spacecraft electronics boards needed reworking added additional schedule pressure. The second launch-date change (from May to August 2004) extended the mission lifetime by 21 months, potentially exposing MESSENGER to even more punishing heat at Mercury.

The response was what APL sponsors have come to expect, especially from an institution building its 61st spacecraft. Team members worked around the clock to install the subsystems when they arrived, and to test and re-install the revamped electronics boards. The thermal team reacted quickly to its new challenge by designing additional blanketing and further insulating the spacecraft’s most sensitive components.

MESSENGER was on the launch pad and ready to go by the end of July. After thunderstorms wiped out the first launch opportunity on August 2, MESSENGER took off aboard its Delta II the next morning, lighting the dark sky with a brilliant trail of fire and smoke.

Project leaders say the work that went into designing and building MESSENGER is paying off beautifully, and with a successful run of early operations behind it, the team is ready to guide the spacecraft through the inner solar system and put it on target to begin orbiting Mercury in 2011. APL and its science team partners excitedly await that date.

Team Effort. All told, organizations in 24 states and six countries played a role in MESSENGER’s development—including about 800 people at APL alone.

Dr. Solomon of the Carnegie Institution of Washington leads its Principal Investigator. APL manages the mission for NASA and designed, built, and operates the spacecraft. MESSENGER’s science instruments were built by APL, NASA Goddard Space Flight Center, the University of Michigan, and the University of Colorado. Composite Optics and GenCorp Aerojet provided its structure and propulsion system. MESSENGER’s science team draws expertise from 13 institutions across the country.

Made in the Shade

MESSENGER incorporates proven technologies where it can—and also relies on several critical innovations:

• The sunshade is a thin, high-tech parasol made of Nextel ceramic cloth with several inner layers of Kapton plastic insulation. While MESSENGER's Sun-facing side could heat to nearly 700°F during the Mercury orbit, the shade will keep the spacecraft operating near room temperature.
• Two singe-sided solar panels supply MESSENGER's electric power. Each custom-developed panel has two rows of mirrors for every row of solar cells; the small mirrors reflect the Sun's energy and keep the panel cooler. The panels also rotate, tilting their surfaces just enough to get the required sunlight while staying at safe operating temperatures.
• MESSENGER's circularly polarized array antennas—the first on a deep-space mission—"point" electronically and allow MESSENGER to return data without relying on a deployable gimbaled antenna.

Why Mercury?

Of the terrestrial planets—Mercury, Venus, Earth, and Mars—Mercury is an extreme. It's the smallest and densest, it has the oldest surface and the largest daily variations in surface temperatures, and it's the least explored. Understanding Mercury will tell us more about how Earth formed and evolved.

Carrying seven scientific instrument—five built mainly at APL—MESSENGER will provide the first images of the entire planet. The mission will also collect detailed information on the composition and structure of Mercury's crust, its geologic history, the nature of its thin atmosphere and active magnetosphere, and the makeup of its core and polar materials.

MESSENGER's science team is shaping its investigation around several questions, including: Why is Mercury mostly made of iron? Why is it the only inner planet besides Earth with a global magnetic field? How can the planet closest to the Sun, with daytime temperatures near 840°F, have what appears to be ice in its polar craters?

Leading the Way

Since the launch of the successful NEAR (Near Earth Asteroid Rendezvous) mission— on an asteroid—NASA has awarded three planetary missions to APL: CONTOUR (Comet Nucleus Tour), MESSENGER, and the New Horizons Pluto-Kuiper Belt mission. MESSENGER is the 61st since we entered the space business 45 years ago; it capitalizes on the Laboratory’s experience and inherits subsystem designs from missions such as NEAR and Ionosphere, Mesosphere Energetics and Dynamics), an Earth-orbiter currently studying the upper reaches of our atmosphere.