2008 BT18 Passing Earth
Published on Jul 13, 2008 at 1:14 pm.
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Tomorrow, July 14, asteroid 2008 BT18 will pass Earth at a distance of only 0.0151 AU (less than six times the distance between the Earth and the Moon). Asteroids pass Earth all the time. It is actually pretty sobering to realize just how many of those things are going by. When I first began teaching astronomy, asteroids were sort of a footnote to the class. Once you had covered all the important things, like planets, you talked about the miscellaneous other things. Many books even titled the chapter on asteroids “Miscellaneous Other Solar System Objects” or “Solar System Debris.” The implication was that these bodies were not important. But, even back then, we knew that asteroids occasionally run into Earth. Craters exist all over the planet. Granted, they are often hard to find because they are covered by erosion and tectonic processes, but they are here. Astronomers knew of a few dozen asteroids whose orbits come near Earth. But, then technology became available to permit extensive surveys looking for these bodies. We now know of nearly a thousand such bodies that come close enough to be categorized as potentially hazardous, and more are being discovered all of the time. We also now realize that asteroids are an important part of the Solar System. Many are remnants of the objects that were coming together to form planets as the Solar System was forming. There are likely millions of the things out there. And, impacts are far more frequent than we had first suspected. So, when I teach my planetary astronomy class, I now move the discussion of asteroids up to before I cover the planets in detail.
The asteroid 2008 BT18 was discovered January 31, 2008, but the LINEAR program. 2008 BT18 is one of those potentially hazardous objects. Tomorrow it passes Earth, but at a very safe distance. In fact, for as far as we can reliably compute its orbit, it will continue to miss Earth. In fact, it may never run into Earth. But, it comes close enough that it needs to be watched. Above is an orbital diagram, courtesy JPL, showing the current position of the body. As with many Earth crossing asteroids, its orbit is not just near Earth. In fact, the semimajor axis of its orbit is 2.22 AU, out in the asteroid belt. But, it has a very elliptical orbit, ranging from about 0.89 AU out to almost 3.55 AU. Interestingly, the orbit of 2008 BT18 has a semimajor axis that is very close to one of the asteroid belt’s Kirkwood gaps. That may have something to do with its large eccentricity.
If that were all that there was to this asteroid, it would still be interesting to write about. However, there is more! It is close enough to Earth already that the giant radar at the Arecibo observatory is able to monitor the object. What researchers have found is that 2008 BT18 is not one asteroid, but two! A story at Spaceweather.com labels 2008 BT18 as a binary asteroid. However, the Arecibo radar image, shown here, seems to show the secondary as just a dot. Wouldn’t that mean that it is more like a moon than a binary asteroid? Well, that is hard to say just yet. Getting the size right on radar images is tough. But reports are that the primary (larger object) is about 600 meters across, and the secondary (smaller object) is about 200 meters acrossSo, we’ll have to wait a bit to see if 2008 BT18 is really a binary asteroid, or an asteroid with a moon. But, what is the difference? In either case, these bodies, as they orbit the Sun, will orbit each other around the center of mass point between them. If that center of mass point is located within the larger body, then the smaller one is a moon. If the center of mass point is located outside of the primary body, then they form a binary asteroid. The sticking point, though, is that if the orbit of the secondary is elliptical enough, then sometimes the center of mass point will be inside the primary, and sometimes outside of it. Then, what do we call these things? Also, as you can see from the radar image, the primary is not spherical. That is pretty typical of asteroids. Only the largest ones would be expected to be spherical. But, that might mean that the center of mass is sometimes inside the primary and sometimes outside of it depending upon the primary’s orientation! Now, I don’t know anything about the orbit of the secondary, so I am just listing the possibilities. But, this does illustrate how these things can get complicated.
Binary asteroids, or asteroids with moons, are not all that uncommon. In fact, a lot of asteroids seem to be double lobed, perhaps being a binary asteroid in which the two bodies have drifted together. We also have known that asteroids can have moons since August of 1993, when the Galileo spacecraft flew past the asteroid 243 Ida, discovering a moon as seen in the image below. That moon eventually was named Dactyl (243 Ida I).
We are still learning about asteroids. So, you can see why I now cover them early in the semester rather than at the end as a footnote to the Solar System. And, I seem to be winning converts to my way of thinking. After explaining what I do to other astronomy faculty, several have decided that my approach is a good idea and they are restructuring their courses to follow the same approach.
-Astroprof
Images courtesy JPL, Arecibo
A Long, Quiet Solar Minimum
Published on Jul 11, 2008 at 3:21 pm.
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My previous post was about how I’ve not been blogging much lately. The Sun’s been pretty quiet lately, too. In fact, there has been very little to look at on the Sun for a while. It has now been quite a long time since I’ve taken my students out to look at the Sun during the astronomy classes. The image above, courtesy of SOHO, is not just an orange circle. It is actually an image of the Sun. I know, it doesn’t look all that exciting. It’s not totally featureless, but it is far less impressive than images a few years ago. As I said, there hasn’t been much to look at for some time. For the last couple of years, there have frequently been no sunspots to observe. The lack of sunspots is a symptom of not much magnetic activity going on. Of course, for a lot of people, that is good, because lack of magnetic activity means that there is a low probability of a big solar flare and the accompanying radiation and geomagnetic storms. That means that satellites, power grids, communications, etc. don’t have to worry about disruptions due to solar activity.
But, is there something wrong with the Sun? Why are there no sunspots? A recent news story called attention to the fact that there have been no sunspots for a while. To discuss that, though, we need to first realize that the Sun does not have constant activity. For hundreds of years, astronomers have observed that sometimes there are more sunspots than at other times. Eventually, it became apparent that the Sun tends to build up activity to where it has a lot of spots, and then activity wanes until there are few, or even none. The Sun stays quiet like that for a while, and then activity picks up, and eventually the Sun’s face is routinely covered in spots again. The time that it takes for the Sun to go through this cycle is about 11 years, give or take a year. The Sun does not follow a set schedule, so some cycles are a little quicker than others, and some last a little longer. Below, courtesy of NASA, is a graphic showing solar sunspot activity for the last four centuries.
You notice that every decade, or so, Solar activity drops off. Sometimes it drops to near zero and hangs that way for a year or so. But, during an extended period of time in the late 17th Century, there were practically no sunspots at all for several decades. During this period of time, sunspots were the exception, rather than the rule. In fact, it was a big deal if an astronomer observed a sunspot then. We call this period of time the Maunder Minimum. But, sunspots are a symptom, not a cause, of solar activity. So, few sunspots means that the Sun itself is being pretty quiet. The Maunder Minimum also corresponds to a period of time when Earth’s climate was a bit screwy. In fact, in much of Europe (where we have the best weather records from the period), there was a general cooling — a period known as the Little Ice Age. The MSU press release that I cited above raises the specter that another Maunder Minimum event may be on the horizon.
However, let’s not be too quick to rush out and buy parkas. After all, the Sun is at solar minimum right now. It is supposed to be at a lull of sunspot activity. And, as I wrote a few months ago, the next sunspot cycle is already showing signs of getting started. It just takes a while. Today, an article at Science@NASA suggests that there is nothing at all wrong with the Sun. It may be that the current sunspot minimum is lasting a shade longer than average, but so what? An average is an average. By definition, there are times that are going to be longer and shorter than the average! What would be really be indicative of something being amiss would be if this minimum were lasting longer than at any time since the Maunder Minimum. But, that is not the case. There have been other minima lasting this long during the 20th Century. So far, the current minimum, though a little longer than average, is less than one standard deviation from the average length. You won’t need to start getting antsy until it is about two standard deviations late. Whenever anything is even slightly out of dead average, it seems that you always have somebody coming along claiming catastrophe is about to strike.
Interestingly, I read a novel a few years ago by Roger Zelazny and Thomas Thomas entitled Flare about the end of a Maunder Minimum type of event. It was quite an interesting read, and I recommend it. In the book, a Maunder Minimum started right at the beginning of the 21st Century, during which time humans moved forward in space exploration and colonization. Suddenly, sunspot activity began again, and all hell breaks loose. According to the book, the lack of activity from the Sun counteracted global warming due to greenhouse gases. Sadly, even if all the people who are convinced that the Sun is indeed about to go into a Maunder Minimum (which I doubt), evidence seems to be mounting that the current level of greenhouse gas emissions would more than make up for any cooling that might result. And, it is not even clear if events like the Little Ice Age are global or regional. So, just sitting around hoping that the current solar activity lull leads to an extended period of inactivity is a poor plan for dealing with global warming. And, indications are that this may be just the lull before the storm. Some solar researchers seem to be calling for an unusually active solar cycle about to happen.
-Astroprof
Images courtesy SOHO, NASA
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
