Emily LakdawallaMar 14, 2011

365 Days of Astronomy Podcast: A MESSENGER to Mercury

I've got another 365 Days of Astronomy podcast airing today, this one an overview of the MESSENGER mission with particular attention to what's been learned in the three Mercury flybys, and what's going to happen when it enters orbit only a little more than three days from now! You can listen to my MESSENGER podcast at the 365 Days of Astronomy website or read the transcript below. There was also another 365 days podcast about MESSENGER, an interview with Louise Prockter by Bob Hirshon of the AAAS, on Friday. For more details on what to expect later this week, see the orbit insertion page at the MESSENGER website, where they've just posted the official press kit. If you've never read a mission press kit before, they are incredibly informative, and I highly recommend the download. One important fact to take note of is that the first image to be taken by MESSENGER from orbit will not come until two weeks after a successful orbit insertion, on March 29.

I will, of course, be reporting everything I hear about the spacecraft as it flies toward its date with Mercury on Thursday. There's a press briefing tomorrow and live NASA TV and webcast coverage planned for Thursday evening (my time; orbit insertion is at 17:45 PDT / 00:45 March 18 UTC). Stay tuned.

MESSENGER entering orbit at Mercury
MESSENGER entering orbit at Mercury Image: NASA / JHUAPL

Here's the podcast transcript:

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 happened before I was born, in 1974. Another reason I'm so impatient is that it's taken MESSENGER more than six years to get from Earth to Mercury.

Why has it taken so long? Mercury is among the closest planets to Earth; only Venus and Mars get closer. 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 it's 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 it couldn't go into orbit. It took some genius trajectory designers to figure out a route to orbit for MESSENGER, 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 Venus twice, and Mercury three times, on its way to enter orbit this week.

Not only is it hard to get to Mercury orbit, it's also hard to operate a spacecraft that'll suffer 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 one-year orbital 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 of NASA's Deep Space Network radio dishes 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 that this covers 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 almost 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. Still, with careful planning and operation of its two cameras, one color wide-angle and one monochrome narrow-angle camera, 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 get 3D data using a laser altimeter. This is really the part of the orbital mission that I'm most excited about. A laser altimeter 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. The first laser altimeter to be sent to another planet was Mars Orbiter Laser Altimeter or MOLA, which was on Mars Global Surveyor. 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 unseen until MESSENGER.

So 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 Mercury. 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.

Map of Mercury after MESSENGER’s third flyby
Map of Mercury after MESSENGER’s third flyby Following MESSENGER's final Mercury flyby before entering orbit, the map coverage of Mercury is nearly complete. Mariner 10 mapped about 45% of the planet (green outline). MESSENGER covered another 20% on its first flyby (blue outline). The second flyby nailed 25% more (red outline). The most recent flyby filled in another 5%, including the last missing piece of the equator and mid-latitudes. Now only 5% of the planet remains unmapped, most of it poleward of 60° north and south latitude.Image: NASA / JHUAPL / CIW

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 over a long stretch of Mercury's history, both before and after most of its impact craters formed.

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 them, I'll post them at planetary dot org slash blog. This has been Emily Lakdawalla from the Planetary Society. Thank you for listening!

First simultaneous measurement of sodium and calcium in Mercury’s exosphere
First simultaneous measurement of sodium and calcium in Mercury’s exosphere

In the upper part of this figure, two histograms represent typical observations in the tail region of Mercury's exosphere from calcium (left) and sodium (right) atoms. Known as "spectral lines," these emissions have been scaled to approximately the same peak level for ease of comparison; however, the sodium emission is much brighter than that of calcium. Each emission occurs at a unique wavelength, with that of sodium in the yellow part of the visible spectrum and that of calcium in the blue part. The sodium emission is actually two very closely spaced emissions that are usually termed the D lines of sodium. The peaks of the two emissions are just separated (indicated by the D2 and D1 labels) in the figure. These are the same emissions that produce the yellow glow in sodium vapor lamps often used in street lighting.

The middle image of this figure shows the spatial distribution of sodium emission, which extends away from the planet in the anti-sunward direction. In the image, north is up and the Sun is to the left. The sodium emission shows two broad peaks that are located close to the planet to the north and south, and there is less emission near the equatorial region. The bottom image shows the spatial distribution of calcium emission. In contrast to the sodium emission, the calcium emission is mostly symmetric about the equatorial region and less bright near polar regions. The spatial variations between the calcium and sodium distributions indicate that the processes controlling these two species are likely different.


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