Cabeus A, not Cabeus
Published on Sep 12, 2009 at 8:05 pm.
5 Comments.
Filed under astronomy, moon.
For the last couple of months, I’ve been so busy that I have not had time to do much blogging. Then, I kept wondering what to start writing about. Recently, the LCROSS program announced their target crater on the Moon: Cabeus A. That gave me an idea for what to post about.
First of all, I know that when it comes time for the impact in a few weeks, the news reporters will likely get the name of the crater wrong. They’ll be calling it crater Cabeus. However, Cabeus is actually a nearby crater. The target crater is Cabeus A, another crater just north of Cabeus. So, I thought that it might be instructive to talk about lunar crater names. The following Lunar Orbiter image shows craters Cabeus and Cabeus A.
Lunar craters, by convention, are named for people who have made contributions to the sciences. The rule is that the craters are to be named for deceased scientists, scholars, artists, and explorers who have made significant or fundamental contributions to their fields. Astronauts or cosmonauts killed in the line of duty also have craters named for them. Craters near the Sea of Moscow, on the far side of the Moon, are named for cosmonauts, and small craters near the crater Apollo are named for American astronauts. The United States Geological Survey maintains a gazetteer of planetary feature names, and the rules of the naming conventions can be found there. These craters are also marked on the following USGS map (click on it to make it bigger).
Crater Cabeus is named for Niccolo Cabeo, an Italian Jesuit priest who was a natural philosopher who lived from 1586 to 1650, during the age of Galileo and Kepler. He taught theology and mathematics at a Jesuit college in Parma. He studied meteorology and physics, making contributions in the study of hydrology and electromagnetism. Cabeo also studied falling objects, showing much like Galileo that different bodies fall alike due to the influence of gravity. Cabeo was aware of Galileo’s studies, but he disputed some of Galileo’s findings. In the realm of electromagnetism, Cabeo studied the effects of static electricity. He also studied magnetism, though his explanation for magnetic phenomena turned out to be quite wrong.
The crater named for Cabeo is near the Moon’s south pole, located at 84.9° S and 35.5° W (-35.5°). The name for this crater was approved in 1935. Crater Cabeus is 98 kilometers across. But, Cabeus was not selected as the LCROSS target crater. The target crater is nearby 48 km diameter Cabeus A, located at 82.2° S and 39.1° W (-39.1°). So, why does this other crater share the name of Cabeus? Answering that is really what inspired me to write this posting.
As it turns out, Cabeus A has lots of company. There are thousands of lettered craters. Technically, these are called satellite craters. The term “lettered crater” is a depreciated term, though it is still in wide use. By early in the Twentieth Century, astronomers had high enough resolution images of the Moon to show far more craters than names available. So, the major or prominent craters got names. The less prominent craters, though, were still of some interest to cartographers and researchers, so they needed some sort of naming system. So, the solution was to name the less prominent features as satellites of the more prominent named features. My first introduction to the lettered naming system was the famous pair of craters Messier and Messier A. These are a famous pair of craters in Mare Fecunditatis. The original use of the lettered feature notation was for any otherwise unnamed depressed feature (crater, rille, etc) near a named feature. The designation of the lettered feature, thus, would be the name of the named feature followed by a letter. Originally, a single letter designation was sufficient for most of the otherwise unnamed features, but high enough resolution images showed far too many things that needed designation than letters of the alphabet, so many of the smaller and less prominent features got two letter designations (such as the crater Copernicus JD). Though the lettered craters are often smaller than the named features, that is not always the case. There are several lettered craters that are shallow or otherwise less prominent than some smaller craters that have names. Cabeus A, itself, is actually a pretty good sized crater. Were it located somewhere more easily seen, it would likely have gotten a full blown name rather than a letter designation.
Cabeus A was recently selected as the target crater for the Lunar Crater Observation and Sensing Satellite (LCROSS). The LCROSS will target Cabeus A with a spent Centaur rocket stage, which will plunge into the crater. When the rocket slams into the crater floor, is should blast stuff out that can be examined from orbit and with telescopes here on Earth. The LCROSS spacecraft will fly through the expanding debris plume, analyzing the content of the debris. Then, the LCROSS craft itself will plunge into a different part of the crater, creating a second debris plume that can be monitored by other spacecraft as well as Earth-based telescopes. It is possible that large amateur astronomer sized telescopes (ten to twelve inch diameter or larger) may be able to see something, too.
So, why Cabeus A? Well, there are several reasons that this is a good target crater. For one thing, there is speculation that there may be ice in the floors of craters at the lunar poles. The ice, if it exists, is believed to come from vapors settling in the polar craters after impacts on the Moon by comets and icy asteroids. The impact of these bodies vaporizes them and clouds of thin water vapor should encircle the Moon for a while. Most of that water is eventually lost to space, but in the far polar regions, the Sun shines at such a shallow angle that portions of the floors of some of the craters may never see direct sunlight. The non-sunlit portions of the Moon are very cold. So, any water vapor getting into the craters might stick to the rocks and soil in these permanently dark zones. The Navy’s Clementine spacecraft was the first to show some indications that ice may be present in these craters, but the data from Clementine is far from conclusive. Successive missions have either failed to show water or have had inconclusive results. LCROSS is the latest attempt to find this frozen water in the lunar polar craters. Cabeus A is close to the lunar South Pole, so it may fit the bill. There are some craters that are farther south and may have more permanently shadowed floors. Unfortunately, the closer a crater is to the pole, the harder it is to see from Earth. Cabeus A is far enough from the pole that it can be seen from Earth, facilitating Earth-based observations of the debris plumes from the impacts, but close enough to the poles that the northern parts of the crater may be permanently shadowed by the crater rim. The image below shows about the perspective that should be visible on the impact date, October 9, 2009.
The impact is scheduled to occur at about 11:30 UT (that will be about 6:30 am Central Daylight Time here in Texas). That is before dawn here in the US on October 9. If the weather is good, I will try to observe the impact, and perhaps even get some pictures.
-Astroprof
Images courtesy NASA, USGS
CoS 117
Published on Aug 24, 2009 at 5:32 pm.
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Filed under blogging.
I have been busy finishing the summer term and getting the fall semester underway. So, I’ve been a bit busy to be blogging. However, plenty of other people around the blogosphere have been hard at work. This week, the 117th edition of the Carnival of Space collects some of the best blog entries in the area of space. This week’s edition of the carnival is being hosted at Simostronomy, so go check it out.
-Astroprof
Perseid Meteors
Published on Aug 11, 2009 at 10:57 am.
3 Comments.
Filed under meteors, observing, skywatching.
Every year, in mid-August, the Earth passes near the orbit of Comet 109P/Swift-Tuttle. The comet, itself, is seldom seen. It was last easily viewed by amateur astronomers in 1992 and 1993. With an orbital period of about 133 years, the comet will next be in the inner Solar System in the year 2126, when it will pass fairly close to Earth (about 15 million miles). But, this comet has been following this same orbit for a very long time. It has made many passes through the Solar System, and each time that it comes near the Sun it sheds a little material. Much of the material shed is in the form of gases. However, some of the material is dust grains or tiny meteoroids. These small bodies then move in orbits similar to that of the parent comet. The orbits are not identical, but similar. Since the orbits are not exactly the same, then over time they become quite separated from the parent comet. Eventually, a whole swarm of this debris is spread out following a path not too dissimilar to the comet’s orbit. Each August, Earth passes through the swarm. When that happens, the debris strikes Earth’s atmosphere and burns up, creating a meteor as seen from an observer on the surface of the planet. These meteors appear to shoot away from the constellation Perseus, so we call them the Perseid Meteors.
The Perseid meteor shower is typically one of the best of the year. It is also very reliable. Some other meteor showers tend to be more active, but not every year or not to every observer. The Perseids are a very old shower. The meteors have had a chance to distribute themselves all around the sky, so the Perseids tend to always put on a good show. Since the Perseids get their name because the meteors appear to radiate away from Perseus, the Perseids put on the best show for observers in the Northern Hemisphere (Perseus is a quite northerly constellation). For observers south of about 30° S, don’t expect to see much. This year, Earth passes through the middle of the debris swarm in the early morning hours of August 12, 2009. That is when you see the most meteors. However, since the Perseid Meteors are such an old meteor shower, the debris field is so spread out that activity starts to pick up a couple weeks before and lasts until a couple weeks after the peak. In fact, you get really good meteor shower activity for a couple days, or so, before and after the peak.
Because the radiant for the Perseids is so far north, the Northern Hemisphere observers get a bonus: the meteors are often seen a bit earlier in the evening. In general, meteor showers tend to be best on the front side of the Earth (much as you get more bugs on the front side of a car as you are driving). From the accompanying diagram, you can see that the “front” side of Earth is from about midnight until about noon.
Midnight and noon, in this context, refer to local midnight and noon, not necessarily midnight and noon according to the clock. Local midnight, for observers here in north central Texas, is about 1:30am, Central Daylight Time. So, from then until dawn would normally be the best time to observe meteor showers. However, since the radiant is so far north, and since the meteors are coming much faster than Earth’s orbital velocity, you’ll start picking up activity as soon as the radiant rises. For us, that will be about 9 or 10 pm (clock time). These meteors skim along and just clip the Earth’s atmosphere. Often you see meteors with long trails when that happens. Typically, though, activity still picks up substantially after 1:30 am and lasts until dawn. That is when you will get the highest level of meteor activity.
What I’ve said is typical for generic Perseid meteors. Remember, this is an old shower, and the debris is pretty much spread out all over the orbit. But, every time the comet comes by, it sheds material, and that fresh material makes a tighter bunch of debris for a very long time until it spreads out. It is very common when watching meteor showers to see activity wax and wane as Earth passes through various densities of debris swarms. This year, Earth is expected to pass through one of the denser parts of the meteoroid debris swarm from about 08:00 to 09:00 UT (that is about 3 to 4 am on August 12 for us in Texas). Unfortunately, the Moon will be very high and bright at that time, making it tough to see the meteors. (Actually, the weather forecast for my location is for clouds all night, so we might not get to see much of anything). The Moon is a waning gibbous right now (a couple days shy of Last Quarter), and it will rise a bit after 11 pm (CDT) for us tonight. So, even without the clouds, this is not a great year to be watching the Perseids. Next year, the Moon will be nearly new, so 2010 will be a very good Perseid observing year.
If you get clear skies at your location, though, then go ahead and go out tonight and look for the Perseid meteors. If you go out before local midnight, then you will see one every few minutes or so. At the peak, you might see one every minute or two (you’d see more if the sky were not so bright from the Moon), and maybe even more if you observe from a dark site.
-Astroprof
A dark spot on Jupiter
Published on Jul 29, 2009 at 11:48 pm.
2 Comments.
Filed under Jupiter, astronomy, observing.
Amateur astronomer Anthony Wesley took an image of Jupiter on July 19 showing a dark spot on the planet. The spot looked remarkably like the sort of impact features left when Comet Shoemaker-Levy 9 hit Jupiter in July 1994. In fact, a lot of people very quickly announced that this was another impact on Jupiter. When I was asked, I hedged a bit. Just because it looks similar, doesn’t mean that it is the same sort of thing! However, soon infrared images of Jupiter showed that the dark spot was also quite warm, as would be expected from an impact. So, now I am willing to accept that this is, indeed, another impact feature.
The impact feature won’t last. Already it is being distorted by the high altitude winds of Jupiter. I don’t want to call it a “scar” as many others are doing, since it is such a transient feature. It no more scarred Jupiter than, say, a mosquito bite scars a human. Likewise, since Jupiter has no solid surface, it is not a “crater” as I’ve also heard both this and the Shoemaker-Levy 9 impact features sometimes described. The impact feature is composed of dust particles and gas. The dark color is likely due to sulfur compounds, if it is dark for the same reasons as the impact spots from Shoemaker-Levy 9.
When an impacting body strikes Jupiter, it doesn’t make it all the way to the center of the planet where there is a solid core. Rather, the object hits the atmosphere and compresses the air in front of it. That tends to slow the incoming body, and it puts stresses on the impacting object. Those stresses can eventually break it apart. The deeper into the atmosphere that the impacting body punches, the thicker the air, so the more the resistance to its motion. Eventually, the impacting body explodes. The stronger it is to start with, then the deeper it makes it into the atmosphere before it breaks apart and explodes.
As the impacting body moves through the atmosphere, though, it has to push the air out of the way. That leaves a sort of tube of hot thin gas behind the impactor as it passes through the atmosphere. Eventually, the pressure surrounding the tube causes the atmospheric gasses to rush back into place, but that takes a few moments. The impacting body typically has reached its terminal depth by the time the tube collapses. Thus, when the explosion happens, it travels up the tube of low pressure air along the entry trail of the impactor. That has the effect of carrying the residue of the explosion up to a point above the upper cloud layers. That was what happened with the Shoemaker-Levy 9 impact. The material dredged up included material from Jupiter’s middle and lower cloud layers, not just the material of the impacting body. The bigger and deeper impacts from the Shoemaker-Levy 9 fragments were the sources of the dark spots then. Studies of the impact sites from Shoemaker-Levy 9 helped planetary scientists understand more about Jupiter’s atmosphere. Hopefully, the same will happen from this impact’s effects.
Jupiter has a very dynamic atmosphere. There are always spots appearing in Jupiter’s clouds from storms. These spots are normally white ovals, but sometimes colored spots can appear. Recently some smaller cousins of the Great Red Spot have made an appearance. Sometimes there are some clear spots that form. Once in a while, though, observers have seen dark spots on Jupiter. Before the Shoemaker-Levy 9 collision, there was all sorts of speculation about what we might see as the effects of the impacts. Speculation ranged all the way from pretty much no visual effect to the formation of a ring system similar to Saturn’s rings. A few of the doomsayers, of course, were going on about Jupiter igniting into a giant explosion since it is made of mostly hydrogen. None of those doomsayers, though, was actually a scientist, so most of us pretty much ignored them. I was at a meeting of the American Astronomical Society in Minneapolis a couple of months before the impact, though, and I heard some planetary scientists proposing that the impact sites might actually be dark in color due the sulfur dredged up from the lower cloud layers. Since nobody had observed an impact event happening, we didn’t really know. After the Shoemaker-Levy 9 impact in 1994, I remember reading speculation that some of the dark spots observed over the last few centuries may have actually been impact feature misidentified as storms. Naturally, there is no way of telling. But, if some of those dark markings were indeed impact events, then that would mean that Jupiter gets hit far more often than most people think. If that is the case, then it would imply that Earth, too, should get hit more often by large bodies than we generally believe. That would be a sobering thought.
But, I should hasten to add that not every dark thing on Jupiter is an impact. On Saturday, July 18, I was doing a star party. Several amateur astronomers came to help. After a while, Jupiter was high enough to view. We noticed that there was a spot on the planet. It was definitely a spot, but the air was very unsteady, and the image was dancing all over the place. It was hard to get a good view of the thing. At first, we speculated that it might be a shadow transit. But, when I looked at my table of Jupiter satellite events, the only shadow transit at about that time had ended an hour earlier. So, we thought that it may be the red spot. If so, then the red spot is darker than it was the last time that I looked. Then, a day or two later, word got out of the dark marking that looked like an impact. That was the same night that we were observing Jupiter! Naturally, I wondered if that was what we were seeing. But, looking at the time that the dark feature was first imaged by Wesley, I can see that it was not the same thing. Even if the impact had already formed the dark marking, it would have been just on the far side of the planet when we were looking. But, looking at a table of Red Spot transits, it makes sense that we were seeing the Great Red Spot. It just didn’t look red. But, with seeing conditions as bad as they were at the time, I wouldn’t trust much of anything that we were seeing that night.
Over the next few weeks, the upper air winds of Jupiter should distort, stretch, and dissipate the particles and aerosols that make up the impact spot. Indeed, that is already happening. Again, that is consistent with this being an impact feature, not a storm. It will be interesting to observe how this changes over time.
-Astroprof
Image courtesy NASA, HST
CoS 113
Published on Jul 27, 2009 at 8:22 am.
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Filed under blogging.
Steinn has the 113th edition of the Carnival of Space over at his Cat Dynamics site, so go check it out. As expected, most of the collected posts deal with the 40th anniversary of the Apollo 11 landing. There’s, of course, a few posts about the impact scar on Jupiter (and that will likely be my next blog entry, too!).
-Astroprof
40 years ago: a big step
Published on Jul 20, 2009 at 6:48 am.
1 Comment.
Filed under history, space exploration.
There’s not much to say that everyone else isn’t already saying all over the internet. Forty years ago, the first humans set foot on another world: the Moon. It may be the nearest other world, but it is nonetheless another world. That is an amazing feat.
In a way, it is hard to imagine that it was only 40 years ago that men set foot on the Moon. In another way, it is hard to imagine that we’ve gone so slow in space exploration in the 40 years since then. Apollo 11 was less than 70 years after the first recorded powered human flight by the Wright brothers. It was only about 30 years after the first rockets were able to reach the edge of the Earth’s atmosphere. It was barely more than a decade after the first artificial satellite was placed into Earth orbit. With that furious rate of technological development, you can understand why many at NASA thought that by the end of the 20th Century we’d have a permanent space station, a Moon base, and a manned mission to Mars. But, then things sort of came to a halt. We’ve made progress, for sure, and it has been important. But, we’ve been going far slower.
Maybe it’s way past time to pick up the pace again.
-Astroprof
Image courtesy NASA
Yes, men really did go to the Moon.
Published on Jul 17, 2009 at 4:31 pm.
6 Comments.
Filed under moon, space exploration.
Like so many other people around the world in 1969, I watched the Apollo 11 landing on the Moon unfold. I watched as the astronauts stepped forth onto the lunar surface. I followed the entire mission on television. So, it was a pretty big shock to me to find that some people didn’t believe that we ever went to the Moon. Making matters worse was a science fiction movie about a faked Mars landing. People often seem to confuse movie fiction with reality, so that fictional work is often pointed to by those who don’t believe that we went to the Moon as proof that it is plausible that NASA faked the missions.
I was young when the astronauts went to the Moon. Later, I went to college and studied physics and astronomy. I have seen the lunar samples brought back from the Moon. I know the science done. I know the results of studies of the samples. They are without a doubt not samples of the Earth. I know of the experiments done by the lunar ranging experiments, where lasers from Earth bounced off of the reflectors left behind. The evidence is clear: the lunar landings are real. Yet, as time passed, even more people seemed to feel that we did not go to the Moon.
Over time, I learned history. I have studied and published several things about the history of space exploration. My research has done nothing to make me doubt the landings. In fact, everything that I have studied makes it even more clear that the landings were real. The more that I learn, the more that I see that the things that the doubters point to don’t make sense. They say that there are no stars in the sky. Of course not. I’ve done astrophotography. The Moon is a favorite target. Though stars are often visible in the telescope next to the Moon, they never showed up on film because the stars are too dim and take so long to show up that the Moon is overexposed. That would be even worse on the surface of the Moon. Non-parallel shadows, yet another favorite of the doubters, also occurs on Earth with uniform lighting an uneven ground. Thus, that does not mean that the landings are faked. What about the flag waving? That, too, is another favorite of the doubters. I wrote about that on my blog some time ago. Wow. I was surprised by the response of those who were unwilling to accept that there was an explantion for what they saw other than what they wanted to hear. Some of the responses that I got were quite rude and unpleasant. Why do the Moon-hoaxers have to be so combative? After all, I don’t go to their web sites and post comments attacking them and calling them stupid.
And, of course, if we really didn’t go to the Moon, wouldn’t our arch rivals of the day, the Soviet Union, have had something to say about it rather than congratulating NASA? Then were monitoring the mission very carefully. They tracked the mission. They monitored communications from the Moon. If there were any hint of an irregularity, wouldn’t they have said something?
But, seeing is believing, as they say. Almost everyone that I meet who claims that the NASA Moon missions were faked, once I start explaining the evidence to the contrary, says that if NASA really did go to the Moon, then why don’t we have pictures of the landers? They keep saying that surely the big telescopes on Earth could see the landers. I try to explain to them the physics of optics: diffraction, resolution, and such. The math shows that no telescope on Earth can resolve anything as small as the landing sites. But, they won’t hear any of that. They insist that they know better. Why doesn’t the Hubble telescope look for the flag on the Moon? Again, I explain that it is no closer than the ground based telescopes. Yes, it is in space, but only just barely. It orbits at the edge of Earth’s atmosphere, technically inside the thermosphere, an outer layer of the atmosphere. It isn’t big enough to see the landers, and besides, the Moon is so bright that it would damage the instruments to look at it. No, they don’t want to hear that. They insist that only if they have pictures of the landers sitting on the Moon would they believe.
Again, they ask why none of the unmanned missions to the Moon have seen the Apollo artifacts. I explain that none of those missions had cameras with sufficient resolution. The smallest thing that they could see is still larger than the landing sites. Again, they don’t want to listen. They watch TV, and they see shows science fiction shows like CSI and they think that the science portrayed there is real and the sort of scientific wizardry shown is real. Granted, much of that is based on real science, but it shows things that are beyond what can actually be done with current technology (hence, I call the show science fiction). I patiently explain that we have simply not put anything in lunar orbit capable of the sort of resolution that would show the landing sites. They don’t want to hear it. Unless they see pictures, they won’t believe it.
Well, now they have to believe. Finally, after years of progress, technology has caught up with what they thought it could do. The latest spacecraft to the Moon, the Lunar Reconnaissance Orbiter actually does have the resolution to see the descent stages left behind by the Apollo missions. And, today they have released images of some of the landing sites. Given that this is the 40th anniversary of the Apollo 11 mission, I picked the image of the Apollo 11 landing site to post here. Click on it to see the picture larger.
At present, the spacecraft is still in a fairly high orbit, so the landers look like small black squares casting shadows. Eventually, the orbiting spacecraft will be put into a lower orbit where it will see even more detail. You can look at more of the images here. So, finally, there are the pictures that the Moon hoax believes have wanted. Will they now finally accept the truth that we went to the Moon? Probably not. There was ample evidence before, so this will not convince them either. They’ll come up with some silly reason not to believe the photos. They have been wanting a spacecraft orbiting the Moon to get pictures. Now the pictures are here. But, they’ll probably say that the pictures are also faked. After all, if they are real, then we really did go to the Moon, and then the Moon hoax believers would be wrong. And, admitting that they are wrong would be too much for them.
-Astroprof
Image credit: NASA, GFSC, ASU
