Star Struck

Getting to Pluto

What were you doing on January 19, 2006? If you were in any way connected to the New Horizons mission to Pluto, you were at the Kennedy Space Center in Florida to watch it being launched on its long journey. I chose this video for the audible reactions of the onlookers as much as anything. A rocket launch is a thrilling event!

When New Horizons began its journey, Pluto had not yet been demoted to “dwarf planet” status. Still, for most people, it marks the outer boundary of our solar system, the farthest outpost on the way to the stars. Getting there requires a very fast ship.

Getting anywhere in space is always a trade-off between how fast you can get there and how big a payload you can transport. A limiting factor for both is how big a velocity boost you give your mission at the outset. Once most interplanetary missions have achieved orbit around the Earth, they use very powerful chemical rocket engines that burn for a matter of minutes but change the spacecraft’s velocity significantly. (There are exceptions; see this article about the Dawn spacecraft.) They then coast for months or years on the way to their destination. The resulting trajectory is a long and curving one, as planets and spacecraft all orbit the sun. For example, this is the path taken by the Curiosity rover spacecraft to Mars:


So how do you get to Pluto?

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Posted in Solar System, Spacecraft

Bright Objects

In case you were wondering, the two very bright objects visible in the sky shortly after sunset are Venus (in the west) and Jupiter (in the southeast). After the sun and the moon, these are the two brightest natural celestial objects. If you see a star-like object even brighter than Venus that is slowly moving across the sky (and not blinking—those objects are called airplanes), it is probably the International Space Station (ISS). In its current and final configuration with widespread solar cells it outshines Venus, its brightness surpassed only by the sun and the moon. Here is the sky view at 8 pm on Thursday, March 26th from Lynchburg, looking to the south.  You’ll notice the bonus planet of Mars below Venus; look for its characteristic orange-red color.

Capture

Speaking of the ISS, there will be an especially nice viewing opportunity (for the Lynchburg area) this Sunday night, March 29th. The station will appear in the northwest at about 6:28 pm, pass almost directly overhead, and fade from view in the southeast at 6:34 pm. The sun will still be out but the station should be bright enough to see, especially if you know when and where to look.
As for the weather: I can’t help you there!

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Posted in Planets, Sky Phenomena

Earth Calling Dawn

On March 6, the Dawn spacecraft was captured by the gravity of Ceres which is, depending on your preferences, either the largest asteroid of the solar system, or one of many dwarf planets. On its approach, Dawn’s cameras took these images of the two hemispheres from a distance of 28,000 miles.

These are by far our best views ever of this object, but no new ones have been released since then. What’s going on? Is NASA hiding evidence of alien life?  Although I have to rank this speculation alongside the moon-hoax conspiracy theories in its utter lack of plausibility, I wondered myself about the lack of new data. So I did a little digging, and the explanation is both more prosaic and more interesting.

Here is a diagram of the path taken by the spacecraft on its approach, looking down on Ceres from above its north pole. The sun is out of the image to the left.

dawn-trajectory-polar
The white circles are at one-day intervals, and the closer together they are, the more slowly the spacecraft is traveling. Today, on March 18, Dawn is near the apex of this path, moving only at 37 miles per hour relative to Ceres, getting ready to fall back toward its target picking up speed as it does so. You can see from this that it is now farther from the dwarf planet than it was on its approach.

Dawn has a remarkable rocket engine on board, an ion propulsion system that can operate for months at a time.

Its thrust is almost nothing (equivalent to the force of a sheet of paper on your hand), but operating over long periods of time allows it to move the spacecraft around the solar system, albeit quite slowly. If the spacecraft’s engine were shut down just as it entered orbit, it would loop around Ceres in a long elliptical path. Instead, it continues thrusting to put Dawn into a close circular orbit with an altitude of 8400 miles. And that orbit is a polar one, which allows it to image the entire surface. This shows the same orbital approach, this time from the side. North is up and the sun is again out of the image to the left.

dawn-trajectory-side
So…no NASA conspiracy. No aliens. In a week or two we should be getting some spectacular closeups!

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Posted in Solar System, Spacecraft

Time on the Clock

If you are as I am, among those who think the people responsible for Daylight Saving Time, while not necessarily evil, are at least deluded…then let’s talk about clocks and time.
One of the arguments I hear when I engage in my semi-annual rant against DST is this: “It’s all just arbitrary anyway. What does it matter?” Granted, hours and clocks are human inventions, but they are inextricably tied to astronomical cycles. A day is the time that our planet takes to turn once on its axis. A year is the time Earth takes to travel once around the sun. Noon is the middle of the day, at least when we are on standard time. Sort of. Let’s take a closer look.
For those of us in the northern mid-latitudes (Lynchburg is at 37.4° N), the sun is nearly always in the southern part of the sky. It travels from east to west during the day, from our left to our right as we face the south. When it reaches its highest altitude, it is directly south. This is local noon.

local noon

But here’s the thing. Shift your position just a few miles east or west and that time will be different. At Lynchburg’s latitude, go a little over 50 miles east or west, and there will be a four minute difference in the time of local noon.
In times before long-distance travel and communication, this didn’t really present a problem. Each settlement kept its own time by a sundial, and no one really concerned themselves with such fine divisions of time as whether it was 9:32:28–roughly half past nine o’clock was close enough. It was the advent of railroads, first in Britain and then in the U.S., that brought about the need for some standard of time. Even so, it was not until 1918 that the current system of time zones was set by law in the U.S. Clock time is the same throughout a time zone, and each time zone is centered on a longitude where solar time is the same as that zone’s clock time.
The Eastern Time Zone in which Lynchburg lies is centered at 75 degrees west longitude, five time zones west of the prime meridian, the 0° longitude line that runs north and south through Greenwich, England.

timezones

Lynchburg’s longitude is a little more than 79° west, so our local noon is a few minutes later than 12:00 (or 1:00 when DST is in effect). We’re not that far from the center of the Eastern Time Zone, so the difference is not large. At the boundary between one time zone and another, the difference can jump from clock time being significantly earlier than solar time to being significantly later.
Political considerations obviously play a part in where time zone boundaries are set. In the U.S., Indiana probably holds the record for the most confusing divisions.  In China before 1949 and the Communist takeover, China had a very sensible five time zones. From 1949 forward, the entire country observes a single standard time, although in the far western provinces there are unofficial variations. For China’s easternmost province, this leads to the ridiculous clock time for sunrise on July 1 of 3 am! This map beautifully illustrates worldwide offsets between solar time and clock time.

Our clock time is geared as closely as is practical to solar time without having multiple different clock times for nearby locations. Except, of course, when we observe Daylight Saving Time. I’ll close with a story that is almost certainly not true, but ought to be. Supposedly a Native American chief who first heard of DST remarked “Only the white man’s government would be so stupid as to cut a foot off the top of a blanket, sew it onto the bottom, and think they have a longer blanket.”

 

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Posted in Sky Phenomena

Dwarf Planets and New Planets, Oh My!

Just about the time that one NASA spacecraft is closing in on Ceres (formerly known as an asteroid, now semi-officially designated as a dwarf planet) and another nears its encounter with Pluto, a group of astronomers has suggested that there may be two or more planets at the outer reaches of our solar system. Didn’t we settle this when we kicked Pluto out of the family of planets?

To keep it short: planet is a word with no universally agreed-upon scientific definition. A very good summary of the subject is here for those who want to dig deeper. But let’s talk about Ceres and the possible “extra” planets.

Ceres and Dawn

dawn approaching Ceres

The Dawn spacecraft is amazing for a number of reasons:
• It uses an ion propulsion engine to navigate its way around the inner solar system.
• It was the first spacecraft to orbit Vesta, the second-largest asteroid and the fourth discovered.

• It will be (in March 2015) the first spacecraft to orbit Ceres, and therefore the first spacecraft to orbit two different asteroids. Here is the latest (January 13th) series of images from Dawn as it approaches Ceres. These will rapidly improve as Ceres draws nearer!

I hesitate to predict what we will find once we are there. Such predictions nearly always turn out to be wrong! Stay tuned.

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Posted in Planets, Spacecraft

First Steps to Deep Space

Tomorrow (December 4th) morning at 7:05 am EST, the first uncrewed test flight of NASA’s new human spacecraft will lift off from the Kennedy Space Center in Florida.

For those of us who remember the Apollo moon flights of the ‘60s and ‘70s, the Orion capsule is very familiar.

It is indeed a return to basics for America’s crewed space missions. The space shuttle was a remarkable machine, but it brought to mind the classic comment about a camel being a horse that was designed by a committee. Space flight is of course inherently dangerous. The position of the shuttle orbiter, strapped to the side of an enormous fuel tank and solid rocket boosters rather than at the top of the rocket with an escape system—well, we’ve all seen how this contributed to the loss of two shuttles and their crews.
So what is different about the Orion capsule? For starters, it is bigger. The Apollo command module squeezed three astronauts into a volume of 218 cubic feet (imagine a cube of 6 x 6 x 6 feet). Orion will accommodate four crew members in a roomier cabin of 316 cubic feet. Its attached service module is also designed for longer-duration missions, and for excursions into deep space—beyond the moon’s orbit. On such missions to an asteroid or to Mars, crew members would spend their time in an attached habitat, using the Orion to get to space and back to Earth.
Like the Apollo spacecraft however, Orion has to withstand the stresses of atmospheric re-entry at speeds far above those of a vehicle re-entering from low Earth orbit. A Soyuz returning from the International Space Station hits the atmosphere at roughly 17,000 miles per hour. Coming back from a lunar or a deep space mission means velocities up to 25,000 miles per hour at the top of Earth’s atmosphere. A craft designed to return from such missions has to be sturdy.
And of course the flight controls and computers on board are fifty years more advanced than on Apollo. The computer that landed men on the moon was far less capable than the phone you carry in your pocket.
This test flight is designed mostly to test how well Orion withstands that fiery re-entry. A Delta IV Heavy booster will take the capsule to an altitude of 3600 miles; re-entry speed should be around 20,000 mph. This high-altitude flight will also take it through the inner Van Allen radiation belt, testing the radiation shielding for the craft’s electronics (and ultimately for its human crew.)

Capture
NASA is a government agency, subject to the shifting political winds blowing out of Washington. So I will refrain from making any predictions about future missions and limit myself to this: it’s about time, and I will be watching! You can do the same here.

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Posted in human spaceflight, Spacecraft

October 8 2014 Lunar Eclipse

Just a quick post to show you this great eclipse photo from my friend Mike Overacker, shot with a Canon 6D and a 600 mm lens at f/5.6.  The small dot to the left and slightly above the Moon is the planet Uranus.  Thanks for sharing, Mike!

OverackerEclipse01

Posted in Uncategorized

A Month For Eclipses

October brings both a lunar and a solar eclipse to viewers in North America. The lunar eclipse will be in the early morning hours of Wednesday, October 8th when the moon is in its full phase.  Two weeks later when the moon is new, we can see a partial solar eclipse in the late afternoon of October 23rd.

Lunar Eclipse

Eclipses, both lunar and solar, occur when the Earth, the sun, and the moon all fall on the same straight line. In a lunar eclipse, the Earth lies between the sun and the moon, and its shadow falls on the moon.   DIAGRAM IS NOT TO SCALE!

For observers on the east coast of the U.S., the full moon will be setting in the west as the eclipse begins at 5:15 am EDT. This is when you will first notice the Earth’s shadow beginning to take a “bite” out of the fully illuminated moon, which will then be roughly 25° above the horizon in Lynchburg.  When totality begins (the moon is completely within the Earth’s shadow) at 6:25 am EDT, the moon will only be about 10° above the horizon.  The end of totality and the setting of the moon set are at the same time: 7:24 am EDT.  Here is how to estimate angular distance using your outstretched arm:

Does the moon go completely dark during totality? If the Earth were an airless rock, it would.  But the Earth’s atmosphere refracts (bends) light so that some of it falls on the moon’s surface even in a total lunar eclipse.  Red light is the most highly refracted, and the term “blood moon” references this.  The actual color can range from rather bright orange to a deep red and depends both on the exact Earth-moon geometry and on whatever might be in the Earth’s atmosphere during the eclipse.  Volcanic eruptions can make for rather dark moons during totality.

The farther west you are, the higher the moon will be in your sky. Folks on the west coast of the U.S. will need to get up earlier, but they will be able to see the moon much higher in the sky during totality.

Solar Eclipse

Two weeks later the sun, moon, and Earth line up to create a solar eclipse.  Again, the diagram is not to scale.

This will be a partial (not total) solar eclipse, with only part of the sun’s surface blocked out by the moon. And just as the moon was setting for east coast observers two weeks ago, so will the sun be setting for this eclipse.  For Lynchburg, maximum eclipse and sunset are at about the same time: 6:20 pm and 6:30 pm EDT respectively.  Again, folks farther west have a better view.

It cannot be stated often enough: DO NOT LOOK DIRECTLY AT THE SUN! Only filters specifically designed for solar viewing (most definitely NOT ordinary sunglasses) should be used.  Far better is an indirect projection device you can easily make yourself with a gum wrapper and a white sheet of paper.

Let’s hope Lynchburg’s notorious weather is kind to us!

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Posted in Sky Phenomena

How Do They Make Those Telescopes So Big?

Telescopes gather light, and that light conveys information about distant and dim objects.  The more light the instrument gathers, the more information is conveyed.  And the bigger the telescope, the more light you can gather.

If you are of a certain age and have any awareness of astronomy, you probably remember that for most of your lifetime, the largest telescope in the world was the 200-inch (5-meter) reflector at Mount Palomar in California.  The casting, grinding, and polishing of this mirror was an impressive technological feat, unsurpassed until the Soviet installation of a 6-meter scope in 1975.  Today 8-meter mirrors are almost routine, and construction has begun on instruments with apertures of almost 40 meters!  Here is a diagram of the world’s largest telescopes, including some that are not yet actually built.

How do they make them so big?

The Hale telescope on Mount Palomar represents the limit of a particular mirror-creation technology, not of ultimate mirror size.  It was first cast in 1934 at the Corning Glass Works as a giant cylinder of the then new Pyrex glass, and that first casting was unsuccessful.  The cracked disc is on display at the Corning Museum of Glass in upstate New York.

The original cylinder blank was ground to produce a curved surface that would focus the light falling on it, then polished to make the surface as smooth as possible.  A thin coating of reflective aluminum was then vapor-deposited on that curved surface to make a mirror.

This cylinder was thick and had a huge thermal mass.  What that means is that the glass takes a long time to cool off or warm up.  This is not good when you need to eliminate air currents created by the mirror being at a different temperature from the air around it.  While the Hale telescope is able to compensate for this, the Soviet 6-meter instrument has been plagued by poor seeing that is at least partially due to its huge thermal mass.  This visualization of the convection currents created by a candle can give you some idea of the degraded view afforded by a mirror that can’t easily reach thermal equilibrium with its environment.

The mirrors had to be thick to maintain their shape against gravity or wind.  But beginning in the 1980s, thin mirrors were supported on their back sides by actuators that could maintain the proper curvature.  A thinner mirror not only has less thermal mass, it requires a less massive support structure too.  Another innovation that greatly reduces the time needed to create the curved mirror surface is spin casting, as explained in this video.

And finally, the really large mirrors are segmented—they are actually made up of hexagonal pieces whose overall curvature creates a single focal point.

Recently a mountaintop in Chile was leveled to provide a site for the European Extremely Large Telescope.  This image shows the size of the segmented mirror.

And this is an artist’s conception of the final installation scheduled for completion in 2022.

Posted in Observatory and Telescopes

Hold the Nobels For Now

Oops. Maybe.

In March, a team of astronomers working with the BICEP2 radio telescope at the South Pole announced an exciting discovery, claiming to have discovered patterns in the cosmic microwave background that would exist if the universe underwent an enormous expansion almost immediately after the Big Bang. This “inflation” is a well-established feature of modern cosmology, and seems to be consistent with all the observational data, but direct evidence for its validity had not yet been seen. If the BICEP2 team’s claims were true, their results provided that direct evidence. But I was relieved to find that in my original post, I had included the obligatory scientist’s word of caution that the claim needed to be confirmed and reviewed. It looks like we’ll need to wait for results from other experiments before we can say that we have that evidence for inflation.

This doesn’t mean that inflation is wrong. It just means that the BICEP2 team’s evidence for it is flawed.

I’m going to leave out an enormous amount of detail and scientific jargon to give the version with which you can impress your friends, neighbors, and casual acquaintances at your next social gathering. The claimed detection was of particular polarization patterns in the cosmic microwave background. (For explanations of these terms, go again to that March Star Struck post.) The problem is that there are other sources of polarization. The BICEP2 team used some preliminary data from another team’s work that hadn’t yet determined how much of the polarization seen came from each of two well-known sources. One of those sources is our own galaxy, whose dust polarizes microwave radiation as well.

The bottom line? We don’t know how much of the polarization announced in March is from the cosmic background radiation (CMB), and how much from our own Milky Way galaxy. The announced CMB polarization may (and may not) be overstated.

There are other teams working in this area, and we may have more definitive results by the end of the year. In the meantime, I hope no one from the BICEP2 team has bought non-refundable tickets to Stockholm.

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Posted in Cosmology