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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.

Look east! In August 2011 – kick back, lie back, look up and enjoy our home galaxy!

This is the month to meet your neighbors – a few billion of them at least!

In August we break our pattern of focusing on bright stars and instead focus on that ancient stream of stars known as the Milky Way – our own galaxy. This means observing a bit later than normal, and if you live within urban or suburban light pollution, going to where you have really dark skies. This does not mean you have to move to – or visit – Arizona. I live in one of the worst light pollution regions of the US, and I can see the Milky Way from my back yard – and see it even better if I take a 12-minute drive to a nearby wildlife sanctuary. But I do have significantly darker skies than people just a mile or two from me. You need a clear moonless night and your eyes need to be well dark adapted. Then you want to look up for a wide, faint “cloud” with a  roughly north-to-south orientation.

I've reduced the brightness and contrast on this image in an attempt to approximate what can be seen from an area with light to moderate light pollution. Still, a photograph always shows more - but it just can't capture the magic of being there. In this case the photographer also caught a Perseid meteor. As you can see, the heart of the Milky Way is nicely framed by the bright Summer Triangle stars of Vega, Deneb, and Altair. Click image for larger version.

Seeing the Milky Way is worth the special effort. It is one of the most beautiful and awe-inspiring astronomical sights, and your naked eye is the best way to take it all in, though binoculars will provide a special treat as well.  In what follows, we’ll focus on where you should be to observe the Milky Way, when you should look. and finally,  where in the sky you should look.

1. Where you should be

Sadly, most people today are routinely denied this sight because of light pollution, but don’t despair! While the darker your skies are, the better, like me you may find that pretty dark skies are just a short drive away. There is an international guide to light pollution and here’s what it shows for light pollution in and around “Driftway Observatory,” my backyard.

On this map of light pollution for southeastern New England, Driftway Observatory is right in the center on the border of an orange/yellow area. Obviously black is the best. Blue is darned good. Green and yellow are desirable. Orange means getting poor; red and white are quite terrible. You should look for at least a yellow area - but to the south of a heavily light-polluted city if possible.

You can get a map  for any region of the world. The simplest path is to go here. Scroll down, to the thumbnail maps and choose a region of the world that suits you and download the map for that region. Another path is limited to observers in the United States, Canada, and Mexico. For them there are “Clear Sky Charts” – astronomical viewing weather forecasts – for hundreds of locations. You can find a location near you by starting here.  Underneath your regional Clear Sky Chart you will see a short list of “Nifty links.” The last one takes you to a light pollution map for that region. It may be helpful to know your latitude and longitude first, so If you don’t know what it is, you can find it here. All of this is useful information for any sky observer to have, so if you track down a Clear Sky Clock for your region,f or example, bookmark it.

Here’s how to make sense of the light pollution maps in terms of seeing the Milky Way.

Red – “Milky Way at best very faint at zenith.”

Orange – “Milky Way washed out at zenith and invisible at horizon.”

Yellow – “Some dark lanes in Milky Way but no bulge into Ophiuchus. Washed out Milky Way visible near horizon.”

Green – “Milky Way shows much dark lane structure with beginnings of faint bulge into Ophiuchus.”

If you can get into the blue, grey, or black areas – enjoy! I envy you 😉

One critical point though: Pay attention to where there are cities. They will create light domes that will wash out at least areas fairly low in the sky. In my situation I have two small cities, Fall River to the northwest and New Bedford to the northeast. Both have populations of around 100,000 and both create light domes in those regions of the sky. Fortunately, the northern sky isn’t important for seeing the Milky Way, especially in August. But if you have a large city – or shopping mall, or anything that might create a light dome – it is better to look for an area south of it. In August in mid-northern latitudes the  Milky  Way is best from right overhead on down to the southern horizon. That’s why my best view is from a wildlife sanctuary just a few miles away and right on the north shore of  Buzzards Bay and the ocean. It means when I’m looking at the southern Milky Way – towards the very center of our galaxy – I’m seeing it over a huge expanse of water where light pollution is the least.

2. When to look

Begin looking early on a moonless, August evening and ideally, when the skies are crystal clear – frequently this comes right after a cold front passes. Although the Milky Way can be seen many months of the year, one of the best times to see it is in August, about two hours after sunset. In 2010 your best views will come between August 1st and 15th – after that the Moon will offer more and more interference each night for the next two weeks.  However, by the 31st, you should get in a solid hour of Milky Way treat before the waning, gibbous Moon rises. If you miss it in the first two weeks of August, try again the first two weeks of September – this guide will still be useful, though everything will have moved higher and to the west a bit.

I say two hours after  sunset because it takes that long in mid-northern latitudes for it to get fully dark at this time of year, and you need full darkness. (You can find out the local time Astronomical Twilight ends – when it is fully dark – by going to this Web site. From the drop-down menu you’ll find there, choose “astronomical twilight.”) However, you can certainly start looking earlier. This is something where beach chairs or lounges are nice, and maybe even a blanket.  You can start about an hour after sunset when the brightest stars are visible. This will help you get your bearings and you can dark adapt as the skies get darker.

Finally, you need to protect your eyes from white lights. It takes 10-15 minutes for your eyes to become about 50 percent dark adapted. At that point your color vision is as good as it will get, but your sensitivity to dim light will continue to increase. In another 15 minutes or so you will reach about 90 percent dark adaption. The remaining 10 percent can take as long as four hours.  So I consider that after half an hour my eyes are about as good as I can expect them to be.  During all this time and beyond you should avoid looking at white light. You can use a red light to check a chart if you like, but keep it dim and use it sparingly. If you’re in a location where automobiles drive by, don’t look at them – close your eyes and turn away.

Where to look

When you set up your blanket or lounge chair, do your best to align it on a north-south axis with your head to the north and feet to the south. You may want to favor the east just a bit.

What you want to find as you start out is the familiar guidepost stars of the Summer TriangleVega, Deneb, and Altair. These were new guidepost stars in May, June, and July. If you are just starting this journey in August,they are still easy to pick out from our chart.  As the sky in the east starts to darken they will be the first stars visible, 30-45 minutes after sunset.

Click image for a larger view. (Derived from a Starry Nights Pro screen shot.)

You can download a printer friendly version of this chart here.

The brightest – and highest – of the three will be Vega, which will be approaching a point overhead. There are roughly two fists (24 degrees) between Vega and Deneb and nearly four fists (39 degrees) between  Deneb and Altair, so the Triangle is huge.

These three Summer Triangle stars roughly bracket the Milky Way – that is Vega is near the western border, Altair the eastern border, and Deneb is about at midstream.  But you need to wait, of course, for it to get darker before you can see the Milky Way.   The boundaries of the Milky Way, as with any stream, are not sharp and regular. It tends to meander a bit with little pools of light and some deep, dark areas as well.

As the skies darken and your eyes continue to dark adapt, you should try to find three distinctive asterisms that will anchor both ends of the Milky Way, plus the middle.  If you have found Deneb, then you have the first star in the Northern Cross. In fact, you may want to see this as a stick figure of the constellation Cygnus the Swan.  In that case, Deneb marks its tail; the bar of the cross, its wings, and its long neck stretch out to the south as if it were flying down the Milky Way. To the north you should locate the “W” of Cassiopeia described in detail in our “Look North” post this month. And to the south, find the “Teapot,” which we described in more detail last month. Here’s a chart showing the whole sweep of that section of sky.

Click image for larger view. (derived from Starry Nights Pro screens hot.)

You can download a printer friendly version of this chart here.

Now, if it is about two hours after sunset and if you are in a location away from light pollution and, of course, are enjoying one of those crystal clear nights with dark-adapted eyes, then you also should be seeing the Milky Way. It only takes time and patience for you to trace it out – to see areas that are brighter than others – as well as some dark patches that don’t mean the absence of stars, but the presence of obscuring dust. But don’t think of the dust as getting in the way – think of it as star stuff – for what you are seeing in many sections of the Milky Way are the parts of our galaxy where new stars are being born. Relax and explore with your binoculars – start to absorb the majesty of millions – no billions – of stars!  If conditions are right – and you have a dark sky – it will look to the naked eye like faint clouds that get brighter as your eye traces them out from north to south.

And what is it you are seeing and why does it appear this way to you? That’s the important question. And this is where you have to do some mental gymnastics.

Think of our galaxy as a large pizza pie with extra cheese and goodies heaped in the center.  Now put yourself away from that center – perhaps one-half of the way towards one edge and buried down at the level of the crust. That’s a pretty good simulation of our galaxy and our place in it. You really need to get outside it – we can only do this in our imaginations – and look at it from that perspective. If we could get outside it, here’s approximately what we would see:

Two view of our Galacy, the Milky Way. The one on the left is from  aposition above it, the one on the right shopws you the galaxy edge-on.  This is a screen shot from the wonderful, free software, "Where is  M13."

The image on the left is how we think our galaxy would look if we could get above it and look down on it – like a big pinwheel of stars.  And what if you could see it edge on? Well, that’s the picture on the right. (This is a screen shot  from a wonderful – and free – software program called “Where is M13” that helps you understand where various objects really are in relation to us and the rest of the galaxy.)

OK – focus on the edge-on image – and note how really thin most of the galaxy is. It is about 100,000 light years across, but on average just 1,000 light years thick.

plane_view_MW

Now imagine yourself on a small dot (the Earth) rotating around that small dot in our image – the Sun. Do you see a lot of stars when you look “up” – that is, look in the direction of the words  “The Sun.”

No – in fact, if you look down, you don’t see many stars either – or for that matter, if you look in just about any direction there are relatively few stars visible to you. Why? Because the disc is just 1,000 light years thick, and most of the time you’re looking right through it the short way.  But  look along the plane of the galaxy – say  directly to the right or left – and what a difference!

Looking to the left you see many stars – in fact, a thin river of stars. Looking this direction, you’re looking through about 20,000 light years of star-filled space. We are looking along the plane, generally towards the outer rim, when we look at the W of Cassiopeia. Look along the plane to the right, and you see even more stars in a much wider river. Now you’re looking through about 30,000 light years of star-filled space and then right at the star-rich, galaxy core. And this, in a general way, is what we are doing when we look toward the Teapot of Sagittarius. That’s why the Milky Way is so much brighter and denser in that direction.

Not too difficult to understand – but this is only a rough sketch. As recently as 2008 scientists came up with a much different perspective of our galaxy than we had had up until then. Prior to the latest study, we thought the galaxy was a spiral with a bulge in the center and four main arms. Now they see it as a barred spiral – that is, the bulge in the center looks more like a bar that spills into two – not four – main spiral arms. There are other smaller arms in the spiral, and it all gets quite complex.

The problem, of course, is there is no way we can get outside our galaxy and look in. The distances are incredibly vast. Even if we could send a space probe at the speed of light, it would be thousands of years before it got outside our galaxy, took some pictures of us, and sent those pictures back. So we have to try to decide what the galaxy really looks like from the outside by studying it from the inside. Imagine, for a moment, being inside your body and trying to figure out what you look  like by what you can see from the inside, and you get an idea of the problem. Fortunately we can see other galaxies, and in later months we’ll be looking at one that looks a lot like what we think ours would look like if we could only get outside it and look back.

Meanwhile, relax – look up – and dream of all  the wonders that are out there and sending their messages back to you in the form of millions of tireless photons that have traveled thousands of years to reach your eyes and ping your brain on this dreamy August evening.  Harvest some of those photons by surfing the Milky Way with your binoculars. You will notice that in some areas it is quite dense and you may even discover some tiny, tight clusters of new stars – or a globular cluster of old stars, or even a little hazy patch where new stars are being born.  You need a telescope to see these well, but you can just discern some of them with binoculars, and with telescope or binoculars, what you really need to see with is your mind’s eye. Knowing what you are looking at is what brings this faint cloud alive and turns it into the awesome collection of billions of stars – and more billions of planets –  that it is.

Look North In August 2011 – All hail the Queen! (OK – the “W”)

Click image for larger view. (Derived from Starry Nights Pro screen shot.)

For printer friendly chart, download this.

The easily recognizable “W” of Cassiopeia (kass ee oh pee’ uh), the Queen, is well up in the northeast early on an August evening. Find it and you have a good starting point for tracing the Milky Way on south through Deneb.

When the “W” circles to a point high overhead, it will look like an “M,” of course, but that’s just part of the fun. Some people also see this asterism as forming the chair – or throne – for Cassiopeia. I like it because along with the Big Dipper, it nicely brackets the north celestial pole and provides another rough guide for finding Polaris. As the “W” rises, the Dipper plunges until it may be too close to the horizon for many to see. Both the stars of the Dipper and the stars of the “W” are 28 degrees from Polaris – roughly three fists.  When the Dipper gets on the horizon, the “W”  turns into an “M” directly above Polaris, so just measure three fists down from this “M” and you should be in the right region for finding the North Star.

Normally I do not find constellations or their associated myths too useful. Cassiopeia is an exception. Knowing the myth connected with this constellation will help you remember several important neighbors, and though we’ll meet these in the next two months, I’ll give you a “heads up” now and repeat the story when we meet the others. It goes like this:

Cepheus (King of Ethiopia)  and Cassiopeia (Queen of Ethiopia) have a beautiful daughter, Andromeda. Cassiopeia bragged so much about Andromeda’s beauty, that the sea nymphs got angry and convinced Poseidon to send a sea monster to ravage Ethiopia’s coast. To appease the monster, Cepheus and Cassiopeia  chained the poor child (Andromeda)  to a rock. But don’t worry. Perseus is nearby and comes to the rescue of the beautiful maiden, and they ride off into the sunset on Pegasus, Perseus’ flying horse! These five constellations – Cepheus, Cassiopeia, Andromeda, Perseus, and Pegasus – are all close to one another in the sky and all are visible in the fall, so we will meet them soon.

One of the bright stars of Cassiopeia is also a special aid to finding your way around the heavens, but in a more modern sense. It is part of an asterism known as the “Three Guides.”  These three bright stars are all very close to the Zero Hour Right Ascension circle in the equatorial coordinate system – the system that is roughly the celestial equivalent of latitude and longitude and is commonly used to give a permanent address to stars and other celestial objects. These three bright stars mark a great circle that goes through both celestial poles and the equinoxes and is known by the imminently forgettable name of  “equinoctial colure.”

Click image for larger view. (See note at end of post for source of this drawing.)

We’ll meet the other two stars in this asterism next month, but for now, simply take note of Beta Cassiopeia. It’s marked on our chart and is the bright star at that end of the “W” that is highest in the sky this month. Remember that this star is very near the “0” hour  circle, which you can visualize by drawing an imaginary line from Polaris through Beta Cassiopeia and eventually the south celestial pole. This line will cross the ecliptic at the equinoxes.  Of course, this helps only if you are familiar with the equatorial coordinate system! If that means nothing to you, then don’t clutter your mind with this right now.

The source for the drawing showing the equinoctial colure can be found here.

Events August 2011: Join ‘Dawn’ and visit the brightest asteroid, Vesta!

NASA simulation of Dawn spacecraft arriving to orbit Vesta.

Latest News: Dawn’s “Smooth Move”  – with the latest pictureYour  Mission to Vesta

As August begins, the mission of Dawn, a US spacecraft launched four years ago,  is at its first peak, as the spacecraft knuckles down to a year-long orbit of the brightest asteroid, Vesta. We’ll be treated this month to lots of close-up images – and some serious science data reflecting on the very beginnings of our solar system – but why not visit Vesta yourself?  Using ordinary binoculars it is easily seen as a star-like object in our southeastern skies around midnight. Recording  the asteroid’s looping path for the next three months will help build your  star-finding skills and improve your knowledge of the night sky.  Meanwhile, Dawn will continue to orbit Vesta right through to next July when it breaks away and heads for its second target, the dwarf planet Ceres, which Dawn is due to reach in the winter of 2015. (Its mission started in September of 2007.)

Here’s a time lapse movie simulation showing Vesta at one-day intervals from August 1 to early November 2011.  It traces Vesta’s path against the background stars of Capricornus – what I call the “Arrowhead”  asterism – and shows how the asteroid changes, growing dimmer as it appears to stop and reverse direction.  Look closely at the video – Vesta is the little dot that moves!  Click the full screen option in the lower right corner under the video – that will give you a much better view.  And don’t worry, there are more detailed charts and instructions to help you find Vesta – this just gives you a general idea of where it is , and how much it moves over three months.

Of course, some of the more knowledgeable night sky watchers may wonder why I’m not featuring the wonderful Perseid meteor shower this month. The reason is simple: The  nearly full Moon is going to eat up all but the very brightest meteors. Still, if you happen to be enjoying the Moon on the nights of August 11 or 12th, don’t be surprised if a few bright Perseids burn their way through the glare! But this is not the year to get a good view of this favorite shower. We do have Saturn riding off into the sunset as Jupiter slowly makes its way from the morning sky to more convenient evening viewing hours – and Mars is well placed well before dawn. You’ll find details on observing these planets here.

Your Mission to Vesta

Got your 3D glasses handy? Even if not, this close-up from Dawn cameras gives you a good idea of what this piece of a planet wannabee looks like! At 330 miles it's the largest asteroid, now that Ceres, nearly twice its size, is classified as a dwarf planet.

The real astral satisfaction this month comes from tracking down Vesta.  A nice project is to print out your own chart and record Vesta’s position on several different nights as observing conditions permit. How fast it moves from night to night tells you something about how relatively close it is to us, and that loop it makes (see video) marks the fact that the Earth overtakes and passes it, since our orbit is shorter and we’re moving faster. When we first see Vesta it is in retrograde (east to west) motion against the background of stars.  But in early September it will nearly come to  a halt before resuming its normal eastward path among the stars and heading back very close to the point we saw it on August 1. In the process, it will have two very close encounters with a 4th magnitude star, Psi (Ψ) Capricorni.

Here are several charts showing Vesta’s path. It is actually bright enough this month to see with the naked eye. But for that you would need both good eyesight and skies pretty much free of light pollution. For most of us, though, it will be an easy target using binoculars, and the best time to look will be around midnight when it’s about as high as it gets just east of south.  Digging one little star-like dot out of a dim section of sky is always a fun challenge.  Think of being the first to discover that this dim star moved! Oh the excitement that must have caused back in 1807!

First, the big view!

OK – let’s start with a finder chart showing the general region. There’s just one bright guidepost star in this area of sky, Fomalhaut, and you can use that as a starting point. The easiest asterism to see is the Teapot in the southwest. The “Arrowhead” asterism – the constellation Capricornus – holds Vesta, but consists mainly of fourth and fifth magnitude stars – visible, but dim, much like the fainter stars of the Little Dipper. I’ve included the general positions of Ceres (which we’ll examine in detail next month when it’s brighter and higher in the sky) and Neptune, which hasn’t moved much from where you may have found it in July.  (See detailed Neptune charts here.)

This general region of the sky contains only one really bright star - Fomalhaut. Besides our target this month, Vesta, it also contains Neptune - the main subject of last month's events column, and the dwarf planet Ceres, which will be a focus in September when it reaches its brightest stage. Click image for much larger version. (Created from Starry Nights Pro screen shot.)

Zooming in on the “Arrowhead”  and Vesta, here’s what we can expect.  Click on the  image for a much larger version. Note – numbers in parentheses are magnitudes with the period omitted to avoid confusion with stars. Thus Vesta’s magnitude on August 1 is 5.7. The green circle represents a seven degree field of view, typical for low power binoculars. (Similar charts will be published next month for September and October.)

Vesta's path in August. Note - numbers in parentheses are magnitudes with the period omitted to avoid confusion with stars. Thus Vesta's magnitude on August 1 is 5.7. The green circle represents a seven degree field of view, typical for low power binoculars. The magnitude of key stars are given to help you distinguish them from Vesta. Click image a for much larger version. Vesta should be easiest to find at the end of the month when for three nights it is very close to Psi Capricorni, a 4th magnitude star. The August 6th position is noted because that's when Vesta is at "opposition" - rising in the east as the sun sets in the west - and at its brightest.

Printer-friendly version of this chart

This same chart can be used to trace the path of Vesta during September and October as well. That’s because Vesta continues moving west for only about one degree past Psi (Ψ) Capricorni. By the middle of September it will appear to slow down, stay in one place for a few days, then start back-tracking quite close to the path it has been following.  So click here to download a printer-friendly (black on white) version of this chart that you can use to record your own observations of Vesta.

 Vesta and other asteroids

This composite image shows the comparative sizes of nine asteroids. Vesta dwarfs all other small bodies in this image. Asteroid Vesta also is considered a protoplanet because it's a large body that almost became a planet and has a diameter of approximately 330 miles (530 kilometers). Caption and image from: NASA/JPL-Caltech/JAXA/ESA (Click image for larger version.)

Asteroids are pieces of  a planet that never formed. As such they are examples of the raw materials form which other planets have formed.

Asteroids are concentrated in the Asteroid Belt between Mars and Jupiter – but they can actually be found throughout the area inhabited by the planets.

Asteroids tend to be quite porous, making it difficult to estimate their density and thus their gravity. In fact, scientists could not predict exactly when the Dawn spacecraft would go into orbit around Vesta because they were unsure of Vesta’s gravitational pull – though Dawn should give them much more precise information on Vesta’s gravity now that it is in orbit.

Ida and Dactyl - NASA photo.

More than 100 asteroids have moons! The first was discovered on August 28, 1993, when the Galileo spacecraft flew within 1500 miles of the asteroid 243 Ida.  The tiny moon, less than a mile across, was dubbed Dactyl.

The first asteroid discovered was Ceres, now considered a dwarf planet.  It was discovered in 1801 by Sicilian astronomer Giuseppe Piazza during a search for a planet believed to be between Mars and Jupiter.  The planet was never found and Ceres, though almost twice as large as Vesta, was judged to be too small to be a planet.  The extra bulk of Ceres allowed it to form as something close to a sphere – a distinction that makes it a dwarf planet. However, it is not large enough to have gobbled up  other small objects in its neighborhood as a true planet would do. The best known dwarf planet is, of course, Pluto.

Though Vesta is smaller than Ceres,  it is closer to us and has a more reflective surface, so it appears significantly brighter.

In binoculars and most telescopes asteroids appear as dots, just like stars. It takes the Hubble Space telescope – or better yet,  a spacecraft  – to really see what they look like.

During the early years of the solar system, asteroids appear to have engaged in a game of celestial bumper cars. The result is there are many, many more small ones than large ones – and some appear to be just loosely bonded rubble heaps.

Such asteroid collisions still can happen, and just last year the Hubble Space Telescope captured pictures of what certainly seems to be such a collision. See the images and read about it here.

Asteroids are similar to comets, but unlike comets they do not give off gas and dust as they get near the Sun.

The two small moons of Mars are believed to be captured asteroids, as are many of the irregular moons of the Gas Giants – the four outer planets.

For many years asteroids were known simply as small planets in France and Germany (petit or kleine), though in England they were called “Minor Planets.” Only in America were they called asteroids, the term now adopted by international agreement.

Oh – some will tell you Vesta is not the brightest asteroid and technically they are right. There is one that will come quite close to Earth – when first discovered they thought it might actually hit us – in 2029. On that close approach it will be third magnitude – easily visible to the naked eye.  But since this is a brief event, and we regularly see Vesta, Vesta is still the brightest for all practical concerns.

And yes, asteroids colliding with the Earth are still a scary prospect, though in the past decade they have done a wonderful job of locating  – and ruling out  – most of the ones that might prove a threat. But there’s a lot of space and a lot of chunks of rock, so the search goes on.

The Dawn Mission to Vesta

Stepping onto my soapbox: I wish we would pat ourselves on the back more often and with more vigor. The Dawn Mission  to Vesta and Ceres  is an incredible accomplishment of science and technology resulting from a long and successful collaboration between government, private industry, and academe. It is absolutely fantastic that we can invent robots such as this, build them, and ship them off on a mission nearly a decade long to find and orbit what are really two very small specks of dust in the vast empty space of our solar system.  It deserves a lot more attention than the media gives it and all of us should take a lot more pride in its success!

Whew! OK – down off my soapbox. If you really want a feel for the complete Dawn mission, watch this NASA video – though it’s a bit long, so be patient.

http://video.google.com/googleplayer.swf?docid=-5412000236766165719&hl=en&fs=true

And here’s how NASA sums up the mission:

The top level question that the mission addresses is the role of size and water in determining the evolution of the planets. Ceres and Vesta are the right two bodies with which to address this question, as they are the most massive of the protoplanets, baby planets whose growth was interrupted by the formation of Jupiter. Ceres is very primitive and wet while Vesta is evolved and dry. The instrumentation to be flown is complete, flight-proven and similar to that used for Mercury, Mars, the Moon, Eros and comets. The science team consists of leading experts in the investigation of the rocky and icy planets using proven measurement and analysis techniques.

Vesta shape model overlaid with a false texture maps

Dawn has the potential for making many paradigm-shifting discoveries. Ceres may have active hydrological processes leading to seasonal polar caps of water frost, altering our understanding of the interior of these bodies. Vesta may have rocks more strongly magnetized than on Mars, altering our ideas of how and when dynamos arise with important lessons for Mars, Earth and Mercury. Ceres may have a thin, permanent atmosphere distinguishing it from the other minor planets.

The three principal scientific drivers for the mission are first that it captures the earliest moments in the origin of the solar system enabling us to understand the conditions under which these objects formed. Second, Dawn determines the nature of the building blocks from which the terrestrial planets formed, improving our understanding of this formation. Finally, it contrasts the formation and evolution of two small planets that followed very different evolutionary paths so that we understand what controls that evolution.

This mission is very timely. Its journey in time to understand the conditions at the formation of the solar system provides context for the understanding of the observation of extra solar-planetary systems. It provides data on the role of size and water in planetary evolution and forms a bridge between the exploration of the rocky inner solar system and the icy outer solar system. Finally, it completes the first order exploration of the inner solar system, addresses NASA’s goal of understanding the origin and evolution of the solar system and complements ongoing investigations of Mercury, Earth and Mars.

What impresses me is the lengthy journey – a mission that takes off in September 2007, reaches Vesta in July of 2011, and should reach Ceres in February of 2015. Just hitting the target is amazing – but hitting the target and having the space craft work after all that travel in an extremely hostile environment is really incredible.  Here’s a NASA simulation of the  Dawn trajectory. Click image for larger view.

How does it accomplish this? There is a gravity assist from Mars, but a lot of Vesta’s motion comes from an onboard rocket engine – very unusual. Huge solar panels and the use of a fantastic new type of rocket – the Ion Engine – are the keys. NASA describes it this way:

Ion Engines are the most exciting new rocket propulsion system since the Chinese invented the rocket about a thousand years ago.

Most rocket engines use chemical reactions for power. They combine various gases and liquids to form chemical explosions which push the rocket through space. Chemical rocket engines tend to be powerful but have a short lifetime.

Ion Engines use electric fields instead of chemical reactions. Ion Engines tend to be much less powerful, but they are so efficient, they can last for years before running out of fuel.

To learn more about the Ion Engine, go here.

And I have to admit, I learn best when I can get my hands on something, so while NASA considers model building a “kid’s” activity, I feel it’s a great way to develop a sense of what this mighty little spacecraft is all about. NASA provides directions and plans for a paper model. All you have to do is download, print, cut, and fold. To give it a try, go here.

We started this post by describing how to look out to Vesta. So let’s end it by looking back. It helps put it all in perspective. Here’s a NASA simulation of how the Earth and Sun would look, were you riding on the Dawn spacecraft as it orbits Vesta. (Click image for much larger version.)

Goodbye Saturn, Good Evening Jupiter, Good Morning Mars!

Goodbye Saturn, Good Evening Jupiter, Good Morning Mars! – that about sums up the planets’ parade for this month. Saturn, sadly, is getting lower in the west each evening, and by the end of the month really will be too low for decent views in a small telescope, though you should be able to find it easily enough if you have a clear western horizon.

About an hour after sunset you should see a pair of almost identical "stars" near the western horizon - but only the one on the left (to the south) is a real star. Its name is Spica and it is icy blue in color. The other "star" is Saturn and should look a bit yellowish, though when objects are this close to the horizon colors are tricky. At the beginning of August they will be nearly two fists above the horizon and a little more than one fist apart. By the end of the month they both will be much closer to the horizon. Click on image for larger version. (Prepared from Starry Nights Pro screen shot.)

Jupiter actually rises before midnight at the start of the month and earlier each night as the month progresses – so technically it is now in the evening sky – but again, for small telescope users you’ll have to wait for the early morning hours to get a really good view.

At the start of the month Jupiter is by far the brightest "star" rising in the east and well placed by about 2 am. By the middle of the month it will look like this about 1 am and by the end of the month Jupiter will be close to this location by about midnight. Click image for larger view. (Prepared from Starry Nights Pro screen shot.)

And Mars is definitely an early morning object in the eastern sky and so distant its disc is tiny and really has little appeal for the small telescope user, but for the naked eye viewer it’s fun to watch it zip across Gemini, the Twins.

Mars is playing hard-to-get in August. At the start of the month it's reasonably placed at 4 am in a field that contains several stars brighter than it. It will be about the same brightness as Castor. As the month progresses the stars will rise significantly, but since Mars is traveling eastward quite quickly against the background of stars it takes a step "downward" as the sky background appears to move upward. So while Mars gets higher by the end of the month, it's only by a few degrees. Compare its color to the bright stars Aldebaran and Betelgeuse - all three should have a red tint. Click image for larger view. (Prepare from Starry Nights Pro screen shot.

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