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    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 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.


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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