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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Events: March 2011 – a special month for messages from Mercury, and it’s about time!

Mercury: As Seen from Our Backyards – and from Orbit

We don’t get a good look at Mercury often – and neither does NASA. But that all changes – for us and for NASA – this month.

Why don’t we get a good look at Mercury? Because this smallest of planets is just too darned close to the Sun, so it seldom gets high above the horizon, and when it shows up at all, it’s usually in twilight. Actually, in any given year we always have several chances to see it as one month it peeks above the eastern horizon just before dawn, then a couple of months later it gives us another opportunity when it is just above the western horizon right after sunset. March will be the best – and most convenient – chance for us to see it this year. In fact, we have the unusual situation of Jupiter pointing the way!


NASA hasn’t been to Mercury for 37 years! (The only NASA spacecraft to visit Mercury was Mariner 10 in 1974 and 1975.) In these days of space probes and planet landers, that’s a real long time between visits. But this month the MESSENGER spacecraft – which swooped in for a close look at Mercury while passing it in 2008 and 2009, hopes to complete its mission by becoming the first spacecraft to go into orbit around Mercury. NASA hopes it will be sending images and scientific data from orbit for the next year. Orbit insertion – after a flight of nearly seven years and nearly five billion miles – comes in mid-March just as we’re getting our best chance to see the elusive planet with our naked eye.

More details on the NASA mission in a moment, including a neat little spacecraft model you can build, but first let’s look at what we can see with our naked eye or when aided by ordinary binoculars.


Finding Mercury: Timing is everything

Lots of sky watchers never see Mercury, but it’s actually quite easy to see. You just have to look in the right place at the right time. And this year in March brilliant Jupiter is going to make it extra easy because Mercury stages a Jupiter flyby in the middle of the month. (Though Mercury and Jupiter will appear close together no matter where on Earth you’re seeing them from, this is generally a Northern Hemisphere event. The farther south you go, the earlier they set, and for much of the Southern Hemisphere they are either too near the horizon or already set by the time the sun sets.)

So here is when and where to get a good look at Mercury:

  • Of course you first need an unobstructed western horizon and very clear skies.
  • The charts are for about 30 minutes after sunset as seen from mid-northern latitudes – but generally good for anywhere in the Northern Hemisphere.
  • Each chart has a line on it indicating 10 degrees above the horizon – 10 degrees is what is covered by your first when held at arm’s length.
  • If you can find Jupiter, the brightest “star” low in the west, then you should have no trouble finding Mercury, which will be significantly dimmer, but still bright.
  • Binoculars will help, and the charts show a typical binocular field of view with Jupiter centered. (Caution: Be sure to wait until about 10 minutes after sunset before pointing binoculars in that general direction. Seeing the Sun with binoculars will damage your eyes!)
  • Pick a date from the charts below, click on the chart for a larger version, and plan to observe starting about 20 minutes after sunset and looking due west.
  • You may spot Mercury before and after these dates – these are just the easiest times because of the proximity to Jupiter.

March 11, 2011

March 12, 2011

March 13, 2011

March 14, 2011

March 15, 2011

March 16, 2011

March 17, 2011

March 18, 2011

March 19, 2011

March 20, 2011

March 21, 2011

Since ancient times Mercury has been known as “fleeting” and for good reason – it just doesn’t stay in one place for long, so look fast, because it will soon be gone again. This makes perfect sense when you consider that Mercury is the closest planet to the Sun and thus has the shortest orbit. It’s 88-day orbit means it goes around the Sun nearly four times while we’re making one trip. It is traveling at more than 107,000 miles an hour, while we’re poking along at just 66,000 miles an hour.

This shows the positions of Mercury and Earth on March 11, 2011. (Prepared from Solar System Live Orrey available on Web.)

I think of Mercury as something of a jokester. Because it is so close to the Sun, it’s always going to be near the Sun in our sky. That means it’s frequently drowned out by the light of the Sun. Even when it’s shining quite brightly, it may be doing so in bright twilight and thus be difficult to see. For example, as it gets higher in our sky this month, it should be easier to see because we’re looking through less atmosphere, and, of course, it’s farther away from the Sun so the background sky is darker. But, as it gets higher it’s also moving to a position more between us and the Sun and thus from our perspective we see less and less of the lit portion until it eventually becomes a crescent – so it actually gets dimmer. It is brightest at the start of the month and is a still a brilliant -1.3 on March 11. To put that in perspective, Sirius, the brightest star in our sky, is -1.5. But when Mercury is that bright it also is relatively low and thus in more twilight. In just 10 days it drops almost a full magnitude, to -0.5 – but it’s higher, so I suspect it’s likely to seem almost as bright. By the end of the month, though, it is magnitude 1.7 and has dropped to about 7 degrees above the horizon 30 minutes after sunset. Given the right conditions, this will make a nice target in a small telescope because it will show as a tiny crescent – but it will be much harder to find because of the twilight and dimness of the planet.


Meanwhile, Back at NASA, MESSENGER Is on an Historical Mission

None of this matters much to the folks at NASA. Their biggest challenge comes on March 17, 2011. They have high hopes, of course, but this is a complex mission, and while it has already returned significant images and new information, orbiting Mercury is the goal. Here’s how NASA put it in a recent press release.

“Although we feel that the preparations to date – and those scheduled for the next month – have been well thought-out, that the decisions made to define the specific activities were sound, and that the level of review and rehearsal has been more than adequate, we recognize the extraordinary complexity and unique nature of this endeavor,” says APL’s Peter Bedini, MESSENGER’s project manager. “But at this point, four weeks out, we are well positioned for success. The spacecraft is healthy, continues to operate nominally, and is on course to be at the right place at the right time at 8:45 P.M. ET on the evening of March 17.”

Of course they’ve had to solve tons of problems all along the way, not the least of which was how to protect delicate scientific instruments from the incredible heat generated by being so close to the Sun. The Sun is up to 11 times brighter at Mercury than here on Earth, and Mercury’s surface temperatures can reach about 840 degrees Fahrenheit. So the key to keeping MESSENGER cool (around average room temperature), is to have it carry its own beach umbrella – a sun shade made of heat-resistant ceramic cloth.

I remain in awe that the scientists and engineers can pull off a mission of this complexity. MESSENGER was launched August 3, 2004. It has reached speeds in excess of 140,000 miles an hour. (The Space Shuttle pokes along at 18,000 miles an hour when in orbit, and the fastest military jet is far slower than a Shuttle.) The straight line distance between Earth and Mercury can be as little as 60 million miles – yet the MESSENGER odyssey has taken it around the Sun and past Earth in 2005, past Venus in 2006 for a close flyby and a second Venus flyby in 2007, followed by Mercury flybys in January and October of 2008 and another in September of 2009. These close approaches to the planets can cause the spacecraft to speed up or slow down – and, in fact, it is the slowing down that has been part of the reason for the recent flybys of Mercury. In the 2009 event the speed of the spacecraft was cut from 12,000 mph to 6,000 mph and that was a critical to putting it into position for going into orbit. (I found this animation of the entire mission a bit hard to follow – I had to pause it at several stages – but that is simply because the mission is indeed lengthy and complex.)

An image from a Mercury flyby in January 2008 showing portions of the planet never imaged before. (Click for a much larger version.)

MESSENGER will still need its on-board rocket motors for the final maneuvers and, in fact, will use most of its stored fuel for this last effort. Here’s how NASA describes its plans for the night of March 17:

At 8:45 p.m. EDT, MESSENGER — having pointed its largest thruster very close to the direction of travel — will fire that thruster for nearly 14 minutes, with other thrusters firing for an additional minute, slowing the spacecraft by 862 meters per second (1,929 miles per hour) and consuming 31% of the propellant that the spacecraft carried at launch. Less than 9.5% of the usable propellant at the start of the mission will remain after completing the orbit insertion maneuver, but the spacecraft will still have plenty of propellant for future orbit correction maneuvers.
The orbit insertion will place the spacecraft into an initial orbit about Mercury that has a 200 kilometer (124 mile) minimum altitude and a period of 12 hours. At the time of orbit insertion, MESSENGER will be 46.14 million kilometers (28.67 million miles) from the Sun and 155.06 million kilometers (96.35 million miles) from Earth.

There’s a wonderful animation of the orbit insertion here, but this is a large file, so you need patience for the full movie to download.

NASA artist conception of MESSENGER spacecraft. All the alphabet soup refers to different instrument packages.

MESSENGER is a far cry from the Buck Roger’s style space ships we used to see in the movies and comic books. It’s a boxy little craft with instrument packages stuck on here and there and two wing-like solar panels for battery power, plus the sun shield. All of this in a squat box measuring roughly the size of a desk – about four feet by four and a half feet, by six feet. The thermal shade is about four feet by eight feet and semi-cylindrical. It weighs in at about 2,300 pounds and more than half of that is fuel.

NASA provides a neat little model you can make of MESSENGER by simply printing out the directions and doing a little cutting and folding. You will find complete step-by-step photos with download links to the model here.

What can we learn?

As the mission began, NASA outlined its goals. Some of these have already been achieved to one degree or another in the Mercury flyby by MESSENGER. But all can benefit from more exploration.The flyby gave a quick look – and an opportunity to image most of the planet. But being on station for a year is much different than taking a snapshot. Still, here are the basic scientific objectives as NASA outlined them:

Mercury’s density implies that a metal-rich core occupies at least 60% of the planet’s mass, a figure twice as great as for Earth. MESSENGER will acquire compositional and mineralogical information to distinguish among the current theories for why Mercury is so dense.

Before the MESSENGER mission, only 45% of the surface of Mercury had been photographed by a spacecraft! Using its full suite of instruments, MESSENGER will investigate the geologic history of Mercury in great detail, including the portions of the planet never seen by Mariner 10.

Mercury has a global internal magnetic field, as does Earth, but Mars and Venus do not. By characterizing Mercury’s magnetic field, MESSENGER will help answer the question of why the inner planets differ in their magnetic histories.

Through a combination of measurements of Mercury’s gravity field and observations by the laser altimeter, MESSENGER will determine the size of Mercury’s core and verify that Mercury’s outer core is molten.

At Mercury’s poles, some crater interiors have permanently shadowed areas that contain highly reflective material at radar wavelengths. Could this material be ice, even though Mercury is the closest planet to the Sun? MESSENGER will find out.

MESSENGER will measure the composition of Mercury’s thin exosphere, providing insights into the processes that are responsible for its existence.

Undoubtedly, there will be surprises. In fact, there already have been. For example, in the flybys, what looks like ice was discovered on parts of the planet. Keeping in mind that typical temperatures on Mercury would make a pizza oven seem comfortably warm, that’s astounding. Here’s NASA’s take on the ice:

So how is it that the planet closest to the Sun has temperatures that possibly can sustain ice? Simply, Mercury’s axis of rotation is such that sections of the planet, the deep floors and walls of craters near its poles, are always shaded. In these frigid areas of Mercury, temperatures can dip to minus 350 degrees Fahrenheit. Radar pictures from Earth show material in the craters that resembles ice, but its identity is one of Mercury’s mysteries that NASA hopes to solve.

Now this is not a place I want to spend my next vacation! I mean 840 degrees in the Sun and -350 degrees in the shade! Sure helps put our seasons into perspective – and MESSENGER gives us a lot to ponder as we watch this beautiful little “evening star” climb up our western sky and pass mighty Jupiter this month. The ancients marveled at Mercury, their “messenger of the gods.” But the more we learn about it through our modern MESSENGER, the more awesome it becomes.


Spring Equinox and Zodiacal Light

Don’t forget the equinox – a great time to note where east and west really are on your horizon, for the sun rises and sets due east and due west that day and for all practical purposes a few days before and after the equinox.

This year the equinox arrives in the Northern Hemisphere at 7:21 EDT on March 20th.

Oh darn! Yes, that is “daylight time.” For most of the U.S. and Canada “daylight time” began at 2 am, March 13. I say “darn” because they keep making the Daylight Time period longer – starting it sooner and ending it later in the year – and that makes it difficult for little kids to get out and observe when it’s really dark – an hour and a half after sunset. Frequently, by the time it’s dark, it’s too late for them. And, of course, they can observe in the winter when it gets dark early, but frequently our winter nights are too cold for them. Makes it difficult to share the beauty of the night sky and the awe of the universe with our younger citizens!

Beauty — like the delicate glow that reaches up from the western horizon on a March evening and is known as the Zodiacal Light. I wrote more about it last month. However, March is the best time to see it, and you need an area as free of light pollution as you can get. We’re talking about something about as faint as the Milky Way, so your eyes must be dark adapted and you don’t want any competition from the Moon. For March, 2011, you could look the first few days of the month, or wait until the last two weeks of March or the first few days of April. You want to start looking about an hour and 20 minutes after local sunset.


The Moon and Other Planets This Month

  • New Moon, March 2
  • First quarter, March 12
  • Full Moon, March 19
  • Last quarter, March 26.

The crescent Moon makes a nice paring with Venus on the morning of March 1 and joins it once more on the 31st – sort of lunar bookends for those early morning star gazers. Also, it is near Jupiter on March 5th and 6th.

We bid farewell to Jupiter this month, which should be obvious if you checked out the maps showing it and Mercury earlier in this post. But as it sets in the west, Saturn is rising in the east, and it’s a special treat for the users of small telescopes, though they’ll have to stay up late for the best views. (When using a telescope it’s best to look at stars and planets when they’re high in the sky.) Don’t expect to see the rings in binoculars, however; they’re not powerful enough. But any small telescope will show them.

As the month begins Saturn rises about two hours after sunset, which means you’ll get a good look at it about a fist or so above the horizon in the southeast three hours after the Sun sets. By the end of the month it’s rising as the Sun sets – but, of course, on the other side of the sky. The full Moon gets fairly close to it on the 19th. The bright star in the same general vicinity is Spica, a guidepost star we’ll leave for another month.

Spica is about as close to first magnitude as you can get and very blue. Saturn will be at its brightest in March at about magnitude .4, so you should see it as brighter than Spica with a distinct yellow hue. (For more on star colors, how to see them, and what they tell us, see this post.)

Venus is still a lovely “morning star” and stays with us all month, though it rises only about two hours before the Sun, so it is best seen in twilight, low in the southeast. You can’t miss it though. Nothing outshines it except the Moon and the Sun.


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