Date: March 14, 2011

Title: A MESSENGER to Mercury


Podcaster: Emily Lakdawalla

Organization: The Planetary Society Blog:

Description: On March 18th (17th for some of us) the MESSENGER spacecraft enters orbit at Mercury, bringing a five-plus-year journey to a close and becoming the smallest planet’s first orbiter. I’ll explain why it took so long for MESSENGER to get there, what MESSENGER saw during its three flybys, and what scientists hope to achieve during MESSENGER’s one-year nominal mission.

Bio: Emily Lakdawalla is a planetary geologist and writer who works for the world’s largest space interest group, The Planetary Society, as its blogger, web writer, and contributor to the weekly Planetary Radio podcast. She is also a contributing editor for Sky & Telescope magazine. She lives in Los Angeles with a 3-year-old who can list all the planets for you, a new baby who has yet to learn their names, and a husband who likes to pretend he doesn’t know anything about space.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Chris Barber. To my wonderful wife on our wedding anniversary. Thank you for past 12 years and the many more to come. I love you more than all the stars in the sky. Your loving husband, Chris.


A MESSENGER to Mercury

The day is finally here. On March 18, the MESSENGER spacecraft will finally go into orbit around Mercury. I say “finally” for two reasons. For one, this will be the first time that Mercury has had an orbiter, despite the fact that the initial reconnaissance was performed before I was born, in 1974. For another, it’s taken MESSENGER more than six years to get from Earth to Mercury.

Why has it taken so long? Mercury is, at times, among the closest planets to Earth; only Venus and Mars get closer to us. But it sits very, very deep in the Sun’s gravity well, and Mercury is the smallest planet, with only about 5% the mass of Earth. It’s hard to shed the angular momentum that an Earth-launched spacecraft starts with, it’s hard to fight the Sun’s gravity, and hard to match orbits closely enough to Mercury’s to allow the little planet’s gravity to grab the fast-moving spacecraft from the clutches of the Sun. So there’s only been one previous Mercury mission, Mariner 10, which did three flybys in 1974 and 1975, but couldn’t go into orbit. It took some genius trajectory designers to figure out a route to orbit, a route that involved a grand total of six gravity-assist flybys. On its long journey, MESSENGER launched from Earth and then passed it again once, then past Venus twice, and Mercury three times, on its way to enter orbit at Mercury.

It’s also hard to operate a spacecraft that lies three times closer to the Sun, and consequently suffers solar radiation that’s ten times more intense than what we feel here at Earth. MESSENGER has all its delicate instruments tucked underneath a reflective sunshade, which protects it well enough, but making sure the spacecraft always keeps its shade between itself and the Sun as it quickly orbits a small planet and continuously turns to focus its cameras on its surface is also hard to do.

So it’s taken some ingenuity to get MESSENGER to the point that it can enter Mercury orbit. The long delay between Mariner and MESSENGER has had one great effect: the technology on MESSENGER is far superior, even though MESSENGER is a product of the Discovery program, which produces NASA’s cheapest missions. MESSENGER will cost us about $450 million end to end, a bit more if its mission gets extended.

What will MESSENGER do at Mercury? Before it does anything else, it has to get into orbit; and as Japan’s Akatsuki showed us last fall, a successful orbit insertion can never be taken for granted. According to the spacecraft’s clock, it’ll be just a little past midnight on March 18 when it begins to fire its main engine, blasting through a third of its total fuel budget in just fifteen minutes. Five massive radio dishes from NASA’s Deep Space Network will be tuned in to the faint signals from the spacecraft, watching for the crucial shift in the frequency of its radio signal that will indicate that the engines are doing what they need to.

Once it’s in orbit, MESSENGER is planned to operate for one Earth year. Mercury orbits the Sun so much faster than Earth, so the mission will actually last four Mercury years. But Mercury rotates extremely slowly; one Mercury solar day from sunrise to sunrise lasts exactly two Mercury years, so the MESSENGER mission will see only two Mercury solar days.

MESSENGER’s orbit will be a polar one, which allows it to sweep from north to south and back over the planet to map every part of its surface. But unlike the nearly circular orbits that most Mars and Moon orbiters enjoy, MESSENGER’s will be very elliptical. Spacecraft in elliptical orbits move fast while close to the planet at one end of the orbit, and move slowly while far from the planet at the other end. When MESSENGER is over the north pole, it will be very close to the surface, just a few hundred kilometers up. Over the south pole, it’ll be as much as 15,000 kilometers away. The elliptical orbit means that MESSENGER will get a very good map of Mercury’s magnetic field and ultra-thin atmosphere and get good measurements of how those interact with the fields and particles that stream out of the Sun. But it’s not as ideal for photo mapping. But with careful operation of its two cameras, one wide-angle and one narrow-angle, MESSENGER should be able to map 96% of the planet at an average resolution of about 200 meters per pixel; it should get more than 80 percent of the planet twice, which allows 3D views of the surface.

MESSENGER will also be able to get 3D views using a laser altimeter. This is really the part of the orbital mission I’m most excited about. The first laser altimeter to be sent to another planet was Mars Orbiter Laser Altimeter or MOLA, which went to Mars with Mars Global Surveyor. It works by beaming a set of laser pulses at the surface and measuring how long it takes the light to reflect back, using a very precise clock. MOLA’s maps of Mars revolutionized our understanding of Martian geology. Right now there’s another laser altimeter operating at the Moon, the Lunar Orbiter Laser Altimeter also known as LOLA, and it, too, is helping geologists understand what’s built the lunar landscape, particularly in places that never see sunlight near the north and south poles. It takes time to put these data sets together, so the best results from MESSENGER’s Mercury Laser Altimeter won’t begin to come out until after it’s completed its first mapping cycle.

Even without entering orbit at Mercury, MESSENGER has already vastly improved our understanding of the smallest planet. MESSENGER’s predecessor, Mariner 10, had a capable instrument suite including cameras, spectrometers, and various detectors that could sense magnetic fields and charged particles. But its cameras only wound up mapping 45% of the planet. That’s because Mariner 10 flew past Mercury at the same position on Mercury’s orbit each time. And because Mercury is the only planet in a spin-orbit resonance with the Sun, the same face of Mercury was sunlit for each of Mariner 10’s flybys. So half of the planet remained terra incognita.

As far as first photographs of the surface goes, MESSENGER has already finished the work that Mariner started, and then some. In its first flyby, it mapped nearly half of the planet. And in the second flyby, it saw almost exactly the opposite face of Mercury in sunlight. After the third flyby, MESSENGER had mapped nearly 90% of the planet, and moreover, its maps were in color, something Mariner couldn’t do. Only the two poles and a narrow sliver of longitude remain unseen by spacecraft. The orbital mission maps will be better for comparative geology, because they’ll cover the whole planet at similar lighting angles, but some of the flyby images of some parts of Mercury are actually more detailed than anything MESSENGER will get from its orbit. This is because of the elliptical orbit, which goes high over the south pole; some of MESSENGER’s flyby images of southern regions will be better than the orbital mission data.

Scientists have made lots of discoveries from the flyby data. For instance, we now know that Mercury had active volcanoes. Some of these volcanoes shot gassy lava explosively into the airless skies, while others poured floods of lava out onto the cratered plains. The volcanoes were active from before the end of the heavy crater bombardment suffered by Mercury, Earth, the Moon and Mars, to well after the bombardment was over.

Speaking of craters, lots of Mercury’s craters seem to punch through one type of rock that covers the surface to something that has a different composition. We don’t know what those different compositions are yet, but they show up as blues and oranges and reds in heavily processed versions of MESSENGER’s images.

MESSENGER has also studied Mercury’s tail, mapping magnesium, calcium, and sodium. Yes, Mercury has a comet-like tail, because unlike the other three terrestrial planets it lacks an atmosphere to keep high-energy particles from slamming into its surface and sputtering off atoms. So, like the tail of a comet, Mercury’s tail is made of the same stuff that makes up the surface of the planet.

All in all, it’s been a very productive mission even before it goes in to orbit. So what’s left to do in the orbital mission? Plenty. The overarching theme of MESSENGER’s scientific investigations is to understand what the smallest, densest, and oldest of the planets has to tell us about how all the planets in the solar system formed and evolved. The MESSENGER science team has identified six questions that the spacecraft will be investigating during its one-year mission.

Why is Mercury so dense? What’s its geologic history? What makes its magnetic field? How is its huge core put together? What’s the weird stuff at Mercury’s poles, visible from Earth-based radio telescopes? And what’s in its atmosphere?

At first glance, Mercury looks a lot like the Moon — a gray, heavily cratered world that’s been geologically dead for some time. But the MESSENGER flybys have already shown us a world whose history and present are very different from the Moon’s. If all goes well on March 18, MESSENGER will be positioned to deliver brand-new views of an extreme world at the inner edge of the solar system, and I can’t wait for the results. Whenever I see results, I’ll post them at planetary dot org slash blog. This has been Emily Lakdawalla for the Planetary Society Blog. Thank you for listening!

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