Mission to Mercury

If landing on Mars looks tricky, imagine what NASA faces trying to slingshot Messenger to the planet closest to the sun

By Fred Guterl|Wednesday, April 21, 2004

It’s 2:30 in the afternoon, on a blustery winter day in Holtsville, New York, and Frank Melillo’s housebound beagle, Princess, has been barking incessantly for an hour and a half. That’s how long Melillo has been standing in the shadow of his apartment building, his windbreaker zipped up to his Adam’s apple, trying to get a bead on the planet Mercury with his Celestron 8 telescope. Except for a few clouds, the sky is blue and bright, and it seems an odd time for an amateur astronomer to be planet hunting. But Mercury, the planet closest to the sun, shows its face at odd times. So for the umpteenth time, Melillo stretches his right hand up toward the southern sky and sights the sun with his pinkie finger. Then he picks out wispy cloud details beyond the tip of his thumb—about 20 degrees, more or less, to the east. Since the clouds he’s using for reference are moving pretty quickly in the wind, he hurriedly trains his telescope on the slice of sky between the building and a white pine tree. “It should be right there,” he says.

Getting Mercury in the crosshairs of a telescope is as problematic for professional astronomers as it is for amateurs like Melillo. It’s easy to sight planets farther than Earth from the sun—Mars, Jupiter, Saturn—because they sit smack in the middle of the night sky. But to see Mercury and Venus, one must look more toward the sun. Venus is often bright enough and far enough from the sun to stand out in the evening or morning sky, but Mercury is notoriously elusive and dangerous to sight with a telescope that could easily fry a retina if too much sunlight gets in. Mercury has never been photographed by the Hubble Space Telescope, because stray light from the sun could ruin the instrument’s electronics. There are only 30 or 40 days a year when Mercury isn’t too close to the sun to be seen by any means, and that doesn’t count days lost to bad weather. For a planetary scientist applying a year in advance for time on the best telescopes, Mercury can be frustrating in the extreme. That’s why Melillo and other amateurs often snag the best Mercury photographs. “I tell Frank to let me see every image he gets,” says Ann Sprague, a planetary scientist at the Lunar and Planetary Laboratory at the University of Arizona in Tucson. “I would be foolish not to look at them.”

Mercury is not just hard to view from Earth. A major challenge facing NASA’s scrappy new Messenger probe, scheduled for blastoff from Cape Canaveral next month, is to compensate for the sun’s huge gravitational pull. Rocketing to Mercury is a bit like running down a hill and then trying to stop suddenly. NASA hopes to use Venus as a brake, taking advantage of its gravitational field to keep Messenger from careering past Mercury and into orbit around the sun. If all goes well, earthbound scientists will have an orbiter around the inside planet by July 2009.

The timing couldn’t be better. As astronomers home in on finding an Earth-like planet in another solar system, they need to find out more about Mercury so they can better understand the precise signature of a solar system like ours. So far we’ve detected only huge planets in other solar systems, most orbiting very close to their suns. Understanding our own solar system better will allow us to infer the existence of smaller planets in other systems. Many people assume that Mars is the destination of choice for finding extraterrestrial life. But Mercury may hold the secrets that count.

At twilight on a good day, Mercury briefly appears as a flickering dot on the horizon. Sky watchers, first with the naked eye and later with telescopes and radar and other gear, have assembled strange facts about the planet hiding in the sun’s blaze. Fact one: Mercury’s orbit is highly elliptical, so it passes about 29 million miles from the sun on its closest approach, or perihelion, and swings out to about 44 million miles at its farthest point, or aphelion. Fact two: Mercury completes its orbit around the sun in only 88 Earth days. Fact three: It takes 58.6 Earth days, exactly two-thirds of an orbit, for Mercury to revolve once on its axis. The combination of those motions means that one Mercurial day—sunrise to sunrise—requires three full rotations and two orbits, or 176 Earth days. Mercury and Venus are the only known worlds where the day is longer than the year.

Mercury’s elliptical orbit complicates things even further. In their recent book, Exploring Mercury (Praxis Publishing, 2003), astronomers Sprague and Robert Strom spell out what the planet’s odd behavior would mean to a visitor. Let’s say you’re standing at the planet’s equator at dawn when Mercury is farthest from the sun. The temperature would be 300 degrees Fahrenheit below zero, rising to a balmy 80 degrees by midmorning, 22 Earth days later. By noon (day 44), Mercury would be at its perihelion, the sun would have swelled from twice the size it appears from Earth to three times as big, and Mercury’s temperature would be well on its way to the day’s high of 800°F. Although the sun would be 11 times brighter than it is on the clearest Earth day, the Mercurial sky, devoid of any atmosphere to speak of, would be as black as night.

That’s just the beginning of the weirdness. Because of the two-thirds synchrony between Mercury’s rotation period and its orbit, the sun does some pretty odd tricks in the sky. For instance, if your day happened to start when Mercury was at its aphelion, the sun would rise in the east, hover for a while, loop about the sky, and set, finally, in the west. So much for sundials.

Accounts of Mercury’s eccentricities have ricocheted through the centuries. Chronicles of Mercury sightings show up in Mesopotamia (November 15 in 265 B.C.) and China (June 9 in A.D. 118), and linguistic evidence in Germanic languages links the planet with the deity Woden, also known in Scandinavian languages as Odin. Astronomers Galileo Galilei of Italy and Thomas Harriot of England trained their telescopes on Mercury in 1609. It wasn’t until the end of the last century, though, that astronomers began to realize that Mercury wasn’t just erratic in behavior but in substance too. Earth-based observations revealed that Mercury is peculiarly dense in composition, leading astronomers to ask: Are we missing something fundamental about the formation of the solar system?

The question was compelling, but the idea of slowing down a probe enough to send it careering down the sun’s gravity pit so it could get a glimpse of Mercury seemed far-fetched to the NASA brass in the 1960s. A few engineers, though, came up with the crazy idea of using a gravitational pull from Venus to reduce the speed of a probe without using excessive amounts of fuel. Mariner 10, launched in November 1973, swung by Venus before making three passes by Mercury, getting about as close as Earth is to the moon. The probe sent back more data than giddy planetary scientists had ever had before, but the thrill of the mission came less from what it told them about Mercury than from the inkling it gave them of what they had yet to learn.

Instead of solving mysteries, Mariner 10 deepened them. As it flew by Mercury, it ran smack into a wave of charged particles from the solar wind that had apparently been deflected by a powerful magnetic field. How could a tiny planet that is just 40 percent larger than Earth’s moon possess a magnetic field that strong? The moon’s field withered long ago, and so did Mars’s. Planetary scientists believe that these small celestial bodies had cooled down and frozen solid in the 4.5 billion years since the solar system was born. Previous astronomical observations suggested that Mercury contains more metal in proportion to its volume than Earth does but did not indicate whether all this metal was distributed evenly or concentrated in a huge iron core. The long-held assumption was that it takes a planet the size of Earth to insulate a molten iron core, whose currents and eddies act as a dynamo to generate a magnetic field. A huge, molten iron core would explain Mercury’s apparent density and its magnetic field. But what would explain the iron core?

The shocking discovery that Mercury has a strong magnetic field led to three working hypotheses. First, Mercury might have once been a bigger planet that somehow got its outer layers stripped off. Earth, like the other terrestrial planets, consists of about one-third iron core by mass; its outer two-thirds is made of a lighter, nonmetallic shell of silicates. If Mercury started out the same way, somehow it would have had to get rid of about three-quarters of its silicate outer shell to arrive at the planet’s current density. What event could cause such a downsizing? Did the sun have a temper tantrum, throwing off enough heat and radiation to make today’s sunspots and coronal mass ejections look like hiccups by comparison? Such a radiation bath would have vaporized most of Mercury’s crust, and over the eons the solar wind would have blown the remnants back out into the solar system.

Another possibility is that Mercury was once a bigger planet and lost its outer shell not through an encounter with the sun but in a collision with an asteroid or another planet. Back in the early years of the solar system, planetoids formed by the accretion of dust and gas, and increasingly these objects began bumping into one another. Big collisions were fairly commonplace; a similar collision almost surely created our moon. Eventually the collisions led to a war of relative giants, like sumo wrestlers knocking each other out of the orbital ring. The destruction of Mercury’s outer shell would have meant a collision with another planet of similar size, the remains of which would have plunged into the sun.

There’s a third, more intriguing possibility. Back when the solar system was a swirling disk of dust and gas (the solar nebula), the material closer to the sun—about Mercury’s distance—was composed of stuff that was different from the material of which Venus, Earth, and Mars were made. Perhaps this stuff was richer in iron and other metals. As the nebula cooled, silicate material and metallic material tended to condense into small particles. Because the rocky silicate particles would have been much lighter than the metal ones, they would have slowed down more as they moved through the dust and gas of the still-forming solar system, forming two separate rings. The lighter, innermost ring may have been consumed by the sun, leaving behind the ring of metallic material, which had more time to cool into larger bodies that resisted the slowing effect of the gas and dust. Eventually, that ring formed into Mercury.

This last scenario puts Mercury squarely in the center of some of the most important questions in astronomy. If correct, it upsets current thinking on how our solar system formed and, by extension, how other life-supporting solar systems might have formed. One way to address the issue would be to observe the process occurring in other, younger planetary systems, but that would require making observations that planetary scientists haven’t yet been able to manage. Another way is to look in our own backyard, as it were, and get to the bottom, once and for all, of the riddle of Mercury.

“Even though we didn’t set out to link anything we’re doing at Mercury to other planetary systems, that field has been advancing so quickly that Mercury has achieved another interesting importance,” says Sean Solomon, principal investigator for the Messenger mission. “Mercury is our outpost on the innermost remnant of the solar nebula, the material out of which all the planets form.”

Shooting Mariner 10 past Mercury was one thing. Getting the orbiter to slow down enough to orbit the planet involves a challenge of a much higher order. The Messenger mission calls for a Delta 2 rocket to send the probe off in the same direction as Earth’s orbit and have it spiral inward toward the sun. Like a car merging onto a highway, Messenger must first accelerate in order to get in sync with Mercury as it speeds around the sun. But before it reaches Mercury, the probe will overtake Venus three times, killing some momentum each time. Messenger will then do two flybys of Mercury and on the third will fire its rockets and slow virtually to a halt relative to the planet, allowing the planet’s gravity to grab it.

In keeping with the “better, faster, cheaper” credo NASA adopted in the early 1990s, the budget for Messenger is a $350 million shoestring, including ground control for the five years it will take to get there. That called for creative engineering. The intense solar radiation the probe will encounter made it necessary to hide all the electronics behind a shielding blanket, which means all the instruments had to be crammed into a relatively small footprint. Because the craft is made to fly as close as 124 miles from Mercury’s surface, the engineers also had to deal with the considerable amount of heat that would be reflected up from the planet’s surface to the probe’s instrument-laden underbelly. Pipes—equipped with diodes that allow heat to travel in one direction only—will ferry heat from the electronics out to radiation panels.

More radical is Messenger’s frame, made out of a carbon-fiber composite rather than the traditional aluminum. Composite was chosen to keep weight to a minimum. The frame weighs 174 pounds, about 25 to 30 percent lighter than if it had been made of aluminum. Engineers quickly found that such a fundamental change had a ripple effect throughout the design. For instance, each instrument and electronics box must be connected to an electrical ground, which in most cases merely requires it to be wired to an aluminum frame. Because composite doesn’t conduct electricity, engineers installed sheets of copper foil throughout the probe. “It’s one of those obvious, basic things you don’t think about until you actually start building the thing,” says Ted Hartka, the mission’s lead mechanical engineer, who supervised assembly of the probe at the Applied Physics Laboratory in Laurel, Maryland.

Heat dissipation was a drawback of composite too. An aluminum frame easily conducts heat. Carbon fibers also conduct heat, but in most composites the fibers are essentially sitting in a resin glue, which insulates them. Hartka tried using a specially designed composite with a coal-tar base, and though the material was a good heat conductor, it turned out to be too brittle. In the end, Hartka and his colleagues compromised. They used a strong, stiff carbon composite for the frame and applied a coat of the coal-tar-based material to areas where heat dissipation is a potential problem.

Members of the engineering team are confident that things will go smoothly from here on out, but Messenger has been delayed from its original launch date last month because of several faulty parts and late delivery of a few key components from a third-party manufacturer. That also tipped the project over its budget by roughly 2 percent. If a May launch date cannot be held, the next window is a 15-day period in July and August. Moreover, the engineers know that a thousand things could go wrong once Messenger reaches Mercury. “This is not a routine business,” says Ralph McNutt, a project scientist. “You’re dealing with things going to very high speeds. If you break something, it breaks in a hurry. It’s very, very unforgiving.”

No one is more aware of how tough—and surprising—the planet can be than the mission’s principal scientific investigator. For millennia Mercury has resisted scrutiny, and Solomon is braced for the possibility that Messenger will yield still more questions. “We may find something that’s not identical to any of our predictions,” he says. Which could be the best possible outcome.


The best global view of the heavily cratered surface of Mercury—a mosaic of more than 140 images snapped by Mariner 10 in March 1974—reveals expansive plains that may have been created by volcanic activity. Because of Mercury’s proximity to the sun, Earth-based telescopes don’t offer a clear look at the planet. Even Mariner 10 managed to photograph only about half the surface.


Mercury Facts

  • Diameter: 3,010 miles
  • Distance from sun: 28.6 to 43.4 million miles
  • Moons: 0
  • Length of year: 88 Earth days
  • Orbit inclination: 7 degrees
  • Length of one day (sunrise to sunrise): 176 Earth days
  • Tilt of axis: 0 degrees
  • Volume: 5.6% of Earth’s
  • Atmosphere:
    • 32% K
    • 25% Na
    • 15% O+O2
    • 7% Ar
    • 6% He
    • 5% N
    • 4% CO2
    • 3% H2O
    • 3% H2
  • Diameter of largest crater: “Beethoven,” 400 miles
  • Minimum distance from Earth: 48 million miles 
  • Brightness of sun: 5x to 11x brighter than when seen from Earth 
  • Orbital speed: 23 to 35 miles per second

Does Mercury Have Water?

One of the enduring myths about Mercury is that it’s a lot like our moon, just hotter. It’s true that Mercury is only 40 percent larger than the moon, and at a glance its cratered surface does seem moonlike. But Mercury is a much different place. Messenger’s mission will be to expound on the differences.

Mercury’s cratered surface is unique in the solar system. The strong gravitational pull created by the high density of the planet means that when a meteor strikes the surface, the dust and vapor it spews in all directions, called ejecta, doesn’t travel far. And yet, some of Mercury’s craters have streaks of ejecta comparable in length to those found on the moon. The likely reason: By the time asteroids and comets strike Mercury, they’re going so fast toward the sun that their energy rocks the planet. Messenger will record these features when it photographs the 55 percent of Mercury’s surface its predecessor, Mariner 10, missed.

One of Messenger’s key tasks is to search for evidence regarding its iron core. Mercury is known to have a slight wobble, or variability, in its spin, and precise measurements of the speed of the planet’s spin may indicate whether it has a liquid or a solid center. Messengerwill also be looking for the presence of sulfur on the surface. If Mercury’s core contains liquid iron, it needs some added ingredient that acts like antifreeze; sulfur is the leading candidate. Scientists hypothesize that sulfur from the core could have made it to the surface through volcanoes or could have been deposited by meteorites. In either case, sulfur atoms in the Mercurial atmosphere would emit detectable ultraviolet light.

A close-up examination of surface geologic features may yield clues about the early days of the solar system. Mercury appears to have many craters, like the moon, but also many smooth, flat plains. Are these the result of crater impacts, which may have spewed lava in all directions? Or are they lava flows from volcanic activity that had nothing to do with the impact craters?

And then there’s the matter of water at the poles. Radar readings from Earth show something shiny and reflective—something icelike. Because Mercury’s axis of rotation doesn’t tilt at all (which means there’s no summer or winter) some deep craters at the poles may be in perpetual shadow, and that could protect any ice patches from the heat of the sun. Aside from the astounding notion that the solar system’s hottest planet could have ice at the poles, the find would yield a great deal of data about the action of comets as the solar system’s water-ferrying bucket brigade. Comets are thought to have deposited water on Earth over billions of years; perhaps they have done the same on Mercury. Or perhaps the coating is not ice but a large deposit of sulfur. —F. G.

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