Dear Friends,
Welcome to the new "listserv" for my space science news e-mails.
1. All five naked-eye planets
Mercury, Venus, Mars, Jupiter & Saturn are visible in the early evening right now. Don’t miss it — go out just after the Sun drops below the horizon, and watch as the sky darkens. You’ll first see Venus in the west, then Jupiter over toward the east. As the sky darkens, you’ll be able to see the other planets. For details on exactly where to look, along with sky maps, go to http://science.nasa.gov/headlines/y2004/19mar_planets.htm?list146819. Be sure to notice that the 5 planets all fall on nearly the same line (or segment of circle) across the sky — demonstrating that all the planets orbit the Sun in approximately the same plane.
For those of you teaching classes, here’s a few questions to ask your students:
- What makes Venus so bright? (Answer: combination of being relatively close to Earth and the fact that it is completely covered by very reflective clouds.)
- Why is Jupiter brighter than Mars, even though Mars is much closer to Earth? (Answer: combination of its much larger size and its much more reflective clouds; fyi, Jupiter radius is about 20 times that of Mars, which means the area of its visible disk is 20^2 = 400 times as large as that of Mars.)
- How can you be sure that the bright object toward the east is Jupiter rather than Venus? (Answer: Because Venus is closer to the Sun than Earth, it must always appear fairly close to the Sun in the sky. Thus, when Venus is visible in the evening, it must be in the west near where the Sun set. When Venus is visible in the pre-dawn sky, it must be in the east near where the Sun will rise.)
- Of the five planets you see, which one is the most difficult to see in general, and why? (Answer: Mercury. Because it is the innermost planet, it is never far from the Sun in the sky. Thus it is usually above the horizon only in the daytime, when the bright sky drowns out its light. We can see it only when it happens to be in an orbital position where it appears far enough from the Sun for us to see it for a short time after sunset (as is the case right now) or before dawn.)
- [for fun:] What object in the sky is far and away the most commonly reported "UFO"? (Answer: Venus is reported frequently as a UFO by people who don’t realize what they are looking at.)
2. Mars Update
The Spirit and Opportunity rovers are still both operating, and have proven to be tremendous successes. They have gathered important new evidence supporting the idea that water has flowed and pooled on Mars in the distant past (probably more than 3 billion years ago). I’ll try to write up a more detailed summary in coming months; meanwhile, here’s recent NASA article about the rover discoveries.
3. Saturn like you’ve never seen it before
The Cassini spacecraft has been traveling through space for nearly seven years en route to Saturn. It is now nearly there, and will enter Saturn orbit on July 1. It is already returning spectacular photos of Saturn. See them at http://ciclops.lpl.arizona.edu/.
4. Is Pluto a planet?
Ask Sedna! … Poor little Pluto has had its planetary status under attack for the past couple decades. When it was first discovered in 1930, astronomers guessed that Pluto was large like other planets of the outer solar system. Over the subsequent decades, however, more refined estimates kept making Pluto smaller. A definitive measurement of Pluto’s size (mass) became possible in 1978, when astronomers discovered that it has a moon, named Charon. Because a planet and moon tug on each other gravitationally, careful observations of Charon’s orbit around Pluto allowed astronomers to determine the masses of both objects. Pluto turns out to be far less massive than any of the other 8 planets — less than 4% the mass of the next smallest planet, Mercury (and only 0.2% the mass of Earth). It is also made mostly of ice, unlike any of the other planets but quite like the objects we call comets. Moreover, Pluto’s region of the solar system is thought to be populated by thousands of other icy comets make up what we call the Kuiper belt. This suggested that Pluto is really a large Kuiper belt comet rather than a true "planet."
Astronomers began detecting Kuiper belt comets in the 1990s. A couple weeks ago, the latest attack on Pluto’s planetary status came with the discovery of an object named Sedna, which is now the largest known comet — except for Pluto. (There’s some debate over whether Sedna is a member of the Kuiper belt or an interloper from the even more distant Oort cloud of comets; see http://science.nasa.gov/headlines/y2004/16mar_sedna.htm.) So should Pluto still be considered a planet? If Pluto had not been accidentally discovered in 1930 and its discovery were made today, it’s likely that we would consider it the largest known Kuiper belt comet rather than a planet. However, historical accidents count for something, so for now many astronomers think that Pluto should keep its planetary status for historical reasons. After all, the "My Very Excellent Mother Just Sent Us Nine Pizzas" mnemonic won’t work too well without Pluto… On the more serious side, some astronomers have proposed a definition in which a planet is any object orbiting the Sun that is large enough to be spherical in shape. Under this definition, Pluto is clearly a planet, as is the largest asteroid (Ceres) and several other objects in our solar system. [Late update: as of late 2004, astronomers are questioning the initial size and mass estimates of Sedna and several other recently discovered Kuiper belt objects; they may be smaller than initially thought.]
For those teaching classes, consider using the following discussion question:
- Most of the comets of the Kuiper belt probably remain undiscovered. Suppose we someday discover a Kuiper belt comet that is larger than Pluto. How would this affect the debate over Pluto’s planetary status? What if we found a dozen other objects larger than Pluto? (Discussion points: If we call Pluto a planet, wouldn’t we be obligated to call any larger object a planet as well? If we found one such object, would we make it the ninth planet and Pluto the "tenth planet"? If we found a dozen such objects, would it make sense to say that our solar system has 20+ planets? And why draw the line at Pluto? Would we then count Sedna or other objects as planets? How should we define the term "planet"? Under the proposed spherical size definition above, our solar system already had more than 9 planets. Is this OK or a problem?)
All for now!
Best wishes,
Jeffrey Bennett