Blogging Lull
Published on Jul 11, 2008 at 2:03 pm.
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I know that I haven’t been posting for a while. This summer has been extra busy. And, on top of that, a ton of extra stuff just dropped on me a bit over a week ago, and I will be tied up with that for the rest of the month. So, don’t expect a whole lot of blog posts this month. I am still here, but I am completely swamped. I’ll try to get an occasional post in, though.
-Astroprof
Carnival of Space #61
Published on Jul 3, 2008 at 11:41 am.
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I have had an awful lot going on this summer, and this week has been no exception, so there has not been much time for blogging. So, to get your fix on the astronomy and space side of the blogosphere, you might want to check out the Carnival of Space. The 61st edition of the carnival is being hosted this week at Mang’s Bat Page. If you run out of things to read there, then here is an archive of past editions of the Carnival of Space.
Any of you who write space related blog postings might consider submitting them to the Carnival of Space to get more exposure. To do so, just email a link to your submission to carnivalofspace@gmail.com.
-Astroprof
Tunguska, one century later
Published on Jun 30, 2008 at 4:10 pm.
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On the morning of June 30, 1908, people throughout the world were minding their own business. Then, a great fireball streaked across the sky over a remote part of Asia. Soon after a titanic explosion rocked Siberia. The explosion was heard for great distances, and it was even detected by its overpressure at sites around the world as the pressure wave circled the globe more than once. Debris in the atmosphere turned days and nights into twilight across the northern hemisphere for weeks afterwards.
The remote location delayed word reaching scientists in major cities. The remote location also meant that travel to the site was an expedition rather than just a trip. Considerable planning was needed, as well as gathering of supplies. Before scientific expeditions could make it into the area, the world fell into war. The Great War, now known as World War I, pretty much kept everyone occupied for a number of years. After the war, Russia was deep into the throes of revolution. So, it was over two decades before scientists made it into the area. What they saw shook the world. An entire forest was devastated by the explosion.
Right away, speculation began to run rampant. Nobody had seen something like this. But, just a few years later, that changed. During World War II, weapons scientists began to realize that for very large bombs, the overpressure can do more damage than the immediate explosive fireball. And, to maximize the coverage of that overpressure, the bomb should be detonated above the ground. The blast damage from the atomic bombs dropped on Japan at the end of the war displayed similarities to the Tungaska blast pattern, only the Tunguska blast was much, much larger. Soon, a favored hypothesis was that the blast was caused by some sort of air burst. But, an air burst of what?
The Solar System is a shooting gallery. There are a lot of things flying around out there. Most of these things are small, so when they run into Earth, they simply appear as meteors (shooting stars). A few, though, survive passage through Earth’s atmosphere to strike the ground. These are meteorites. The larger the meteorite, the bigger the explosion when it hits the ground, and the bigger the crater. Earth has plenty of craters.
But, the Tunguska event shows signs of an atmospheric explosion, not a crater. There have been numerous attempts to find a crater, but so far all have been fruitless. At present, there are still a few claims that have yet to be evaluated by the scientific community, but most today feel that there is no crater. So, how could something so big hit Earth and not leave a crater? Well, it obviously had to be something that did not make it to the ground. So, what could that be?
One of the early contenders was that it may have been something that would not survive the high temperature and pressure of passing through the Earth’s atmosphere. A comet was suggested as fitting the bill. After all, comets are icy bodies, so they would tend to vaporize during entry into Earth’s atmosphere. Of course, it would have to be a smaller body than most comets, so perhaps it was a piece of a comet that had broken off. An likely parent body was even postulated: Encke’s Comet. Comet Encke was known to shed pieces now and then. And, Encke’s Comet comes close to Earth. In fact, in June, Earth is quite near the comet’s orbit, passing through a swarm of debris shed by the comet. This debris gives us the Beta Taurid Meteors, which peak in late June and early July. Furthermore, the bodies approach Earth from the daylight side of the planet, not unlike the object that created the blast. However, this hypothesis has gradually fallen into disfavor.
The top hypothesis today is that a stony asteroid was the progenitor of the Tunguska blast. But, how can a huge chunk of rock not make it through the atmosphere? Well, remember the asteroid Itokawa. That is an example of a rocky asteroid that is not a solid chunk of rock. It is at best a pile of rubble. Such a body would hit the atmosphere moving at dozens of kilometers per second and shatter into billions and billions of pieces from the shock of the sudden deceleration. Those pieces would separate, the debris would pancake, and the air in front of the body would be compressed and heated into a great fireball. The energy released by the ensuing explosion would be huge. For a body of only tens of meters across, the resulting explosion could easily be equivalent to that of a hydrogen bomb. Even for a fairly solid rock, the stress of hitting the atmosphere would be an awful lot to stand. For rocky bodies, the tiny ones burn up. The small ones make it to the ground as meteorites. The medium sized ones blow up in the atmosphere. The large ones, miles across, would probably make it to the ground. Favoring the asteroid hypothesis is dust found at the impact site consistent with the composition of asteroids.
So, both the asteroid and comet hypotheses are still alive, but the scientific community is leaning heavily towards an asteroid, based on the evidence currently available. Also, Earth crossing asteroids in that size range are very common. Comets or comet fragments of the right size are quite rare by comparison.
But, just how big was the explosion? For many years, I had heard estimates of about the equivalent of 25 megatons of TNT. But, in recent years, the estimate had dropped considerably, with about 12 MT being about average. There have been suggestions that the trees of the area were easier to knock over than had been thought, and so the blast damage was overestimated. I have heard estimates of blast strength as low as about 3 MT. That is about what you’d expect from a hydrogen bomb. This happened a century ago. But, there is a reason to try to nail down the size of the blast beyond simple curiosity. The smaller the blast, the smaller the body that caused it. And, there are a lot more small bodies flying around than large ones. So, knowing the size of the body responsible for the blast gives us an idea of how likely it is happen again anytime soon.
Estimating the size of the body causing the explosion is difficult. For one thing, we don’t know how big the blast really was. For another, we don’t know how fast the impacting body was moving. Recently, we’ve been able to compute the atmospheric effects far better, and that suggests a much smaller body may have been responsible than had generally been assumed. If so, then the risk of another Tunguska event goes up. The smallest bodies that I’ve seen proposed are believed to strike Earth perhaps once every couple hundred years. Now, that doesn’t mean that we are safe for another hundred years. Ask the people in Iowa that have had their second hundred year flood in under two decades. A hundred year flood simply means a 1% chance of flooding each year. A once every couple hundred year chance of impact really means is that there is a 0.5% chance of impact each year. Of course, I’ve heard other estimates that were far more comforting, such as a chance of impact once every thousand years or so (about a 0.1% per year).
Over the years, of course, there have been far wilder suggestions of what caused the impact, ranging from a miniature black hole to a chunk of antimatter. And, there have been suggestions of non-natural causes, too, such as a crashing flying saucer or a scientific experiment gone awry. But, the simplest and far most likely scenario is of an impact by an asteroid or comet.
-Astroprof
Images courtesy Wikimedia Commons
Albedo
Published on Jun 27, 2008 at 12:06 pm.
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The word of the day: Albedo.
When you look up information about planets, one of the bits of data given is the albedo of the planet. Albedo is one of the vocabulary words that introductory astronomy students have to learn. According to the textbook that we are using, Mars has an albedo of 0.15, Jupiter has an albedo of 0.44, and Venus has an albedo of 0.59. So, what is albedo? What do these numbers mean?
Put very simply, albedo is a measure of the reflectivity of a body. You compute the albedo by dividing the amount of reflected light by the amount of incident light. So, an albedo of 0.25 means that a body reflects 25% of the light that shines on it. Unless otherwise stated, albedo normally refers to visual light. Rocky bodies, such as Mercury or the Moon, have low albedos. They are gray, and they absorb more light than they reflect. Icy bodies, such as Pluto, reflect a lot of light, so their albedos are high. Venus is covered in clouds that are very reflective, so it has an albedo greater than 0.5. That means that it reflects more light than it absorbs.
We talk about the albedo of planets, comet nuclei, moons, and asteroids. Another related term is absolute magnitude. In astronomy, magnitude is a measure of how bright an object appears. My stellar astronomy students know the term absolute magnitude as being how bright a star would appear if it were located at a distance of 10 parsecs (32.6 light years) away from us. It is a way of differentiating how bright an object really is from how bright it appears. Such a thing would also be useful for planets, asteroids, and comets. It is a way to directly compare them to one another. A larger body will appear brighter, simply because it reflects more light because of its size. A smaller one, though, could appear just as bright if it were more reflective and had a higher albedo. So, we can define something analogous to stellar absolute magnitude for objects within the solar system. Unfortunately, to the consternation of hosts of astronomy students, the term used is the same term: absolute magnitude. Of course, when we are talking about the absolute magnitude of a planet or asteroid, we definitely do not mean how bright it appears if it were a distance of ten parsecs away. Instead, this planetary absolute magnitude is basically how bright a body would appear as seen from the Sun if it were at a distance of 1 AU from the Sun (that is the distance that Earth is from the Sun). The absolute magnitude of a body in this system can be computed using the equation:
The H stands for the absolute magnitude (to avoid confusing it further with the stellar absolute magnitude, usually referred to as M in equations). D is the diameter of the body in kilometers. Naturally, for irregularly shaped bodies, it would be the effective average diameter. A in this equation is the albedo. There are other factors, of course, that I am not considering. Some substances reflect light differently at different angles of incidence. And, of course, some objects reflect different colors of light differently. But, this is a pretty good approximation. It is as far as we get in the introductory classes.
- Astroprof
Update
Published on Jun 26, 2008 at 12:01 pm.
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For my regular readers, I thought that I’d just post an update. Yes, I am still alive. I had a ton of things going on in the last week that basically took over and kept me from having time to compose any blog entries. There has certainly been lots to write about, though, what with the discoveries going on at Mars and elsewhere. I’ll try to get back into the swing of things, though, and get back to regular entries.
-Astroprof
Phoenix Flash Memory Problem
Published on Jun 19, 2008 at 6:49 pm.
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Filed under Mars.
According to a JPL press release, the Phoenix lander on Mars recently had an issue with its flash memory. You can read more about the incident on Emily Lakdawalla’s blog posting about it. Flash memory is a nonvolatile memory such as that used in memory sticks. The RAM on board the spacecraft loses data when it is powered down for the night. But, the flash memory holds onto the data. Flash memory is very durable, so it is normally a safe way to store data. However, Tuesday night, Phoenix lost some data when it powered down. Apparently what happened was that there was not enough memory to hold onto all of the data that was trying to be stored. The spacecraft stores not just science data, but also information about spacecraft operations, too. Maintaining the health of the spacecraft obviously has high priority. After all, if you lose the spacecraft, then you lose all future science data. Normally, there isn’t all that much spacecraft data that needs to be stored, so there is plenty of room on the flash memory for science information. Apparently, though, something happened Tuesday to cause the spacecraft to store a great deal more spacecraft data than normal. That didn’t leave room for all of the science data. So, when Phoenix powered down, the science data was lost.
The data that was lost was, for the most part, not all that critical. It was mostly images. And, of course the cameras can take more pictures, so that is not a big deal. Apparently, the robotic arm did do a bit of digging, and not all of the images taken of the ground before the digging got transmitted to Earth, so that information is lost, but that is likely not a major deal. What is worrisome, though, is why the spacecraft had so much housekeeping data that it displaced science data. The Phoenix team is working on that, though. For now, the plan is to make sure that the science data is transmitted back to Earth, if at all possible, before powering down the spacecraft. In the meantime, they are trying to make sure that this won’t happen again, even if they don’t transmit all the data back home.
When I heard about a problem with Phoenix’s flash memory, it reminded me of a far more serious problem that the Mars rover Spirit had with its flash memory. Nearly four and a half years ago, Spirit was having a problem where it rebooted several times per day. That problem was solved by reformatting the flash memory. The problem with Phoenix sounds different. Hopefully, they’ll quickly figure out the problem and be able to correct it. They don’t seem too worried about the situation, though, so that is good.
-Astroprof
Image courtesy NASA, JPL
Jupiter is back
Published on Jun 17, 2008 at 1:39 pm.
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Filed under astronomy, skywatching.
Of course, Jupiter didn’t leave the Solar System, become invisible, or any such thing. But, we haven’t seen it for a while in the evening skies. And, observers who have stayed up late or got up before dawn have been seeing Jupiter. It just hasn’t been visible at sunset in quite a few months.
Since ancient times, people have looked at the sky and observed that some “stars” appeared to wander from constellation to constellation. These wandering “stars” were called planets. Jupiter is one of these planets. It takes about 12 years to move around the sky. But, people have also observed that night after night, the entire sky shows a subtle shift to the west when observed at the same time of night, about 1 degree per day. This is due to Earth’s motion around the Sun.
Since Earth moves around the Sun faster than Jupiter, it is no wonder that we left it behind. It was prominent in the sky last summer. But, Earth moved on. Eventually, by winter, the planet was hidden behind the Sun. (For my readers in the southern hemisphere, reverse the terms “summer” and “winter”!) But, you might wonder, “Where is Jupiter?” You might say to yourself, “I saw it about sunset last year at this time. Where is it now?” After all, it’s been a year, and Earth is back to where it started. So, what has happened to Jupiter? Well, quite simply, it moved.
Remember, planets move around the Sun. Jupiter is a planet that is farther from the Sun than Earth. So, that means that once in a while, Earth gets between Jupiter and the Sun. When that happens, Jupiter is said to be in opposition. That means that it appears opposite the Sun as seen from Earth. Jupiter was last at opposition on June 6, 2007. On that date, Jupiter rose at about sunset and set about sunrise. It was up all evening. It was visible as soon as the sky got dark enough to see it during twilight.
But, one year later was June 5 (Remember that this year was leap year, so one orbit of the Earth was 365.26 days later, and that was June 5). But, Jupiter was nowhere to be seen. You had to wait a bit over two hours after sunset for Jupiter to rise. What happened? Well, remember, Jupiter moves, too. Since it takes Jupiter almost 12 years (11.86 years, actually) to orbit the Sun, in the year that it took Earth to go around the Sun, Jupiter had moved about 1/12 of the way around its orbit, so it was no longer opposite the Sun from Earth in early June.
One orbit of Earth around the Sun did not result in another opposition for Jupiter (or any other planet). So, Earth must move a bit farther around its orbit for it to line up again so that Earth is between the planet and the Sun. Thus, the next opposition of Jupiter will not occur until July 9, 2008. So, oppositions of Jupiter occur at intervals of just over 13 months. This time interval, from opposition to opposition, is called the synodic period of a planet. Jupiter’s synodic period is about 399 days.
Since Jupiter will be rising shortly after sunset in early July, and it will be progressively rising earlier as the month wears along, then it will be a favorite target for amateur astronomers all month long, and for the next several months, too. July will be particularly good, though, because we will be closest to Jupiter at that time, so it will look slightly larger in the telescope than normal. But, because Jupiter is so far from us to start with, this effect is far less noticeable than it is with Mars.
But, you don’t have to wait until July to see Jupiter. You can still see it now. You’ll just have to stay up a bit later. For my part of Texas, Jupiter is currently rising just after 10pm. At the top of this posting is a view of what the sky would look like at almost 11:30 at night on June 19 facing to the southeast. I picked that night, two nights from now, because the Moon will be very close to Jupiter then. It will be just a little to the west of Jupiter. The following night, it will be just a little to the east of Jupiter. That will make it appear a little down and to the left of Jupiter as seen from Texas. In fact, on June 20, if you are in the right part of the world, the Moon will be passing Jupiter at a distance of only about 4 times the Moon’s apparent diameter. But, that happens just after dawn on that date, as seen from here. Observers in the Pacific, Australia, or Asia will see that happen, though.
-Astroprof
Skyview produced using Stellarium software
