It’s very easy to overlook modern photographs of Saturn, like the one above, as something you could see from a telescope based on Earth. In fact, Cassini, a probe launched by NASA almost 12 years ago is responsible for most of the beautiful images, like the one above made in September. Although it left Earth’s orbit in October 1997, the Cassini probe didn’t enter Saturn’s orbit until July 2004.
CICLOPS, the nerdy name NASA scientists have given to the Cassini imaging team, has a collection of images from the probe since 2004.
Neil Young, Harvest Moon, Harvest Moon (1972)
Europa / Io / tidal heating / Saturn / aurora / Shepherd Satellites / Titan / Uranus / Neptune / ice giants / Bode’s law
October 5, 2009
A student posed a great question today: How fast is Jupiter’s equator rotating? If Jupiter rotates once every 9.9 hours and its equator has a radius of about 71,500 km (44,428 miles). For one rotation, an object at the equator would have to travel the circumference of Jupiter: 2*pi*radius. So an object at the equator would be moving at about 45,000 km/hour (28,000 mph).
The escape velocity of the Earth is 11.186 km/s, or about 40,000 km/hour (about 25,000 miles/hour). So on Earth, something moving as fast as an object at the cloud top of Jupiter would fly out of Earth’s gravity into space. Jupiter is much more massive, and its escape velocity is therefor much bigger (it is harder to escape from Jupiter’s gravity). Fortunately, Jupiter’s escape velocity is about 214,000 km/hour (13,300 miles/hour), so that cloud-top object is still bound to Jupiter. Well, not so much fortunately, because anything at Jupiter’s cloud top would be cold, devoid of oxygen, and would have to deal with lightning. Whatever.
Another great question dealt with what causes Jupiter’s magnetic field. Jupiter has no liquid iron core, but we think it probably has a liquid metallic hydrogen layer under extreme pressure. The motion of that liquid is probably what causes Jupiter’s magnetic field, but there are still plenty of uncertainties. It may be that we are completely wrong, and that in 30 years we will completely replace this theory (but Jupiter will probably still be a planet in 30 years!). But it’s the best theory we’ve got.
One great way we know there is a magnetic field on Jupiter is to see aurorae in Jupiter’s atmosphere near its poles. The picture above, from Hubble, shows such an aurora on Jupiter.
Aimee Mann, Satellite, Bachelor No. 2 (2000)
Santana, Europa, Moonflower (1977)
tags: Martian meteorites / life on Mars? / Jovian planets / Jupiter radiates more energy than it receives / Great Red (or Burnt Orange) Spot / Jupiter’s belts / lightning on Jupiter / outer, middle, and inner moons of Jupiter / Callisto / Ganymede
September 30, 2009
Cultural reference: Cosmos was a 13-part series that aired on PBS starting in 1980 presented by Carl Sagan, who was like Neil deGrasse Tyson, only a little more pretentious. The series is quite slow-paced, but is an excellent introduction to astronomy, especially its place in humanity; it’s great for a late-night wind-down.
You can currently view the entire 13-part series online (albeit with commercials) at hulu.com/cosmos.
September 28, 2009
The above image, and many shown in class today, were taken from the MESSENGER probe, which was launched by NASA in 2004 to study Mercury. The MESSENGER probe is currently scheduled to make its last fly-by of Mercury on Sept. 29, 2009 (next Tuesday!), and will come to rest in orbit around Mercury in March 2011. You can find some great images released by NASA from this probe, which has already done two fly-bys of Mercury, here:
The Black Keys, 240 Years Before Your Time, The Big Come Up (2002)
Red Hot Chili Peppers, Subway to Venus, Mother’s Milk (1989)
Memphis Slim, Mother Earth, Memphis Slim at the Gate of the Horn (1959)
tags: Mercury / tidal locking / rays from craters / Discovery scarp / Venus’ slow, retrograde rotation on its axis / Venus’ atmospheric circulation / Venus’ thick CO2 atmosphere / sulfuric acid clouds / Russian Venera 13 and 14 probes / corona
September 22, 2009
Sometimes astronomy is as much about admiring some of our beautiful views as it is about getting the theory straight.
Fresh off a recent repair mission, the Hubble telescope has been taking sharper than ever photographs of deep space objects. A new batch of Hubble images was released last week, and got a lot of much-deserved press. (You can find out where the Hubble space telescope is at any given moment here.)
The above image was one such recently released image, of Planetary Nebula NGC 6302. We’ll talk more as we go along about planetary nebulae, which are the second-to-last stages of stellar evolution for most low-mass stars like our sun. They usually make for great astronomical photographs. NASA has a good writeup that accompanied this image.
Daily, NASA astronomers update an Astronomy Picture of the Day, and they are worth regularly checking on. Tuesday’s image, of The Crescent Nebula, was especially memorizing.
Cannonball Adderley, Star Eyes, The Quintet Plus (1961)
September 17, 2009
Q (e-mail): I had a question about Kepler’s 2nd law. I don’t understand the “sweeping” part. What exactly does it mean when it says, “planets sweeps out equal areas in equal times”?
The gist of Kepler’s 2nd law is that objects in an elliptical orbit speed up when they are closest to the object they are orbiting and slow down when they are furthest from the object they are orbiting. The above image illustrates this — click on it for a full-size image.
A great animation illustrating this concept can be found on the course Web site. You will need the login and password information provided in class (it can also be found in the “Course Documents” page of this class’s section on Blackboard). Once you get that, you can view the animation here:
http://www.as.utexas.edu … 04_Kepler2AreaTInt.swf
There are several great animations in the animation section of the course Web site. Especially useful is one demonstrating the retrograde motion of Mars, which does not require a password:
September 16, 2009
Lunar mare are the large dark patches we see on the moon, mostly visible on the side facing Earth. The maria were formed by volcanic eruptions on the moon, some perhaps triggered by impact events (as in a large asteroid impacting the moon). Perhaps the most famous mare on the moon is the Sea of Tranquility (or Mare Tranquillitatis) — it is pictured above. This was the landing site for Apollo 11, the first manned landing on the moon.
The Sea of Tranquility is relatively easy to spot in a full moon: It’s the main part of hair in the woman in the moon, or the head of the rabbit in the moon. A good outline of many of the moon’s mare can be found here.
Sometimes it’s hard to keep the meanings of synodic periods and sidereal periods straight. In order to remember, I always put them in alphabetical order: sidereal comes before synodic, which helps me remember that sideral periods are shorter. This is always true when something orbits in the same direction as it rotates (like the Earth around the sun or the moon around the Earth).
That doesn’t help much with the explanation of why there are different sidereal and synodic periods. A great animation illustrating this concept with the moon can be found at:
Radiohead, Sail to the Moon, Hail to the Thief (2003)
Creedence Clearwater Revival, Bad Moon Rising, Green River (1969)
The Police, Walking on the Moon, Reggatta de Blanc (1979)
tags: man/woman/rabbit in the moon / maria / fossil magnetic field / lunar cratering / far side of the moon / giant impact (“impact trigger”) theory for moon formation
September 16, 2009
UT’s football stadium is built from the ground up, but many stadiums in the northeast are built like bowls dug into the ground, especially Michigan Stadium in Ann Arbor, which held more than 112,000 spectators in 2003 (the current NCAA record).
Now imagine turning the Meteor Crater near Flagstaff, Arizona into a football stadium. That’s what travel writer Henry Fountain did for The New York Times earlier this year, estimating some 2 million people could encircle a football field in the middle. (The above photo helps get the scale of the crater, and was taken by John Burcham for The New York Times).
All from a meteor that was only about 50 meters (or half a football field) across!
Since the Moon is in synchronous orbit, the same half always faces the Earth (giving us what we call the dark side of the moon). However, because the Moon’s orbit isn’t circular, we see more than 50% (about 59%) of the surface of the moon in a given lunar cycle. This effect is called libration, and can best be visualized by a time-lapse animation of the Moon’s surface throughout a lunar cycle: such a theoretical animation was shared by Wikipedia user Tom Ruen and is uploaded with this post. It’s pretty entertaining to watch.
tags: meteors / erosion / radioactive heating / Moon / synchronous rotation / libration / tides / spring and neap tides / Roche limit
September 11, 2009
Q (E-mail): I was thinking about the notion of the Moon gradually shifting away from the Earth. I was wondering if, theoretically speaking, it is possible for the Moon to have a ‘planet status?’ Can the Moon begin to orbit the Sun if it gets far enough out of the Earth’s orbit? Also, would the lack of a moon orbiting the Earth cause any difference in the rate at which the Earth orbits?
In short, no, the Moon will probably never orbit the sun instead of the Earth. The only reasonable way to take the moon out of Earth’s orbit would be to have a massive body (like a rogue planet that’s somehow left its own orbit around the sun) fly very close to the Moon, slingshotting the Moon out of its Earth orbit. Even if that were to happen, odds are the Moon would fly out of the solar system entirely.
As we talked about in class, the slow drift of the Moon away from Earth is caused by the Moon’s tidal drag causing a decrease in the Earth’s angular momentum (angular momentum is a measure of the rotational energy of a system). That energy expands the Moon’s orbit, pushing it further away from the Earth at roughly 3 cm a year. The Moon’s tides will have less influence the further out it orbits, so this effect eventually diminish, stop, and reverse itself.
Even so, think about the practical limits to this: The sun will become a red giant star (we’ll talk about this more later in the semester) in about 7 billion years. When it does, the Earth and Moon are toast. Even if the Moon were to push away from the Earth constantly at 3 cm a year (which it won’t), the Moon’s orbit would only be one-and-a-half times bigger after 7 billion years. That wouldn’t get it nearly close enough to the sun to start orbiting the sun itself.
The lack of a moon would not have a large affect on Earth’s orbit, but it would change Earth’s rate of rotation, since having no moon would mean less decrease in the Earth’s rotational period. (This rotational slowdown caused by the Moon is why we have leap seconds.)
Cool thought experiment. How about this: How will the Moon’s increasing distance away from the Earth affect eclipses? If you use some numbers from your textbook (or do some clever Googling), can you figure out when the last solar eclipse will be if the Moon keeps receding at 3 cm each year?
September 11, 2009
(Props to The Onion: “Spider-Man Mask Spices Up Blind Date”)
Q: Which elements are used to radioactively date old rocks on Earth?
In class we had a tough time being certain of an answer, but the Internet to the rescue. Wikipedia (which has fairly reliable articles on scientific topics) claims that rocks are often radioactively dated using the decay of Potassium (into Argon) or Uranium (into Lead).
Potassium-40 has a half-life of 1.25 billion years. Uranium-235 has a half-life of about 704 million years. That naturally occurring type of uranium can be used to make nukes, so that isotope of potassium is probably the more commonly used source to date rocks and thus set a lower limit to the age of Earth.
The last word goes to the U.S. Geological Survey, which lists several isotopes for radiometric dating, including uranium, potassium, rubidium, thorium, and some element called samarium.
September 8, 2009