May 23, 2012 at 10:00 am | Leave a Comment
A WISE Survey of Nearby Space Rocks
There was a busy space probe out there called the Wide-field Infrared Survey Explorer (WISE, for short). As its name suggests, it was sensitive to infrared wavelengths of light and cataloged millions of objects before it went into hibernation in 2011. Many things radiate in the infrared, including some potentially hazardous asteroids (PHAs) that have the propensity to stray across Earth’s orbit from time to time. WISE has been sweeping its gaze across near-Earth space in search of these asteroids, which are detectable in infrared. Because the telescope detected the infrared light, or heat, of asteroids, it was able to pick up both light and dark objects, which gave astronomers a pretty good and pretty representative survey of what’s “out there”. The infrared data allowed them to make good measurements of the asteroids’ diameters and, when combined with visible light observations, how much sunlight they reflect.
So, what is it about these PHAs that are so intriguing? First, they have the closest orbits to Earth’s of many asteroids. Some of them come within five million miles (about eight million kilometers) of our planet in their orbits. However, if one of them strayed across our orbit andgot too close to our planet, it would be sufficiently big that it would survive passing through Earth’s atmosphere and smashing into the surface (or the ocean). This would cause damage on a regional, or greater, scale.
WISE sampled 107 PHAs that it actually observed, and used that sampling to come up with a decently accurate estimate of how many more are out there. Based on WISE’s data and the estimates, there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 330 feet (about 100 meters). That’s just an estimate of what’s out there. Not all of them have actually been observed — only about 20 to 30 percent of these objects have actually been found.
WISE’s analysis also suggests that about twice as many PHAs as previously thought are likely to reside in lower-inclination orbits. That’s a fancy way of saying that their orbits are which are more aligned with the plane of Earth’s orbit. In addition, they appear to be somewhat brighter and smaller than the other near-Earth asteroids that spend more time far away from Earth. Why the difference? One possible explanation is that many of the PHAs may have originated from a collision between two asteroids in the main belt lying between Mars and Jupiter. A larger body with a low-inclination orbit may have broken up in the main belt, causing some of the fragments to drift into orbits closer to Earth and eventually become PHAs.
Asteroids with lower-inclination orbits would be more likely to encounter Earth and would be easier to reach. The results therefore suggest more near-Earth objects might be available for future robotic or human missions. And that’s kind of exciting, because traveling out to asteroids and studying them is something we’re learning to do, with experience from such missions as the NEAR project.
The discovery that many PHAs tend to be bright says something about their composition; they are more likely to be either stony, like granite, or metallic. This type of information is important in assessing the space rocksâ€™ potential hazards to Earth. The composition of the bodies would affect how quickly they might burn up in our atmosphere if an encounter were to take place. You might wonder why all the fuss about PHAs. The short answer is pretty obvious: they have a chance to hit Earth and cause significant damage. There are people studying them, trying to figure out ways to deflect them if they do head for us. But, as I mentioned above, these asteroids also give us a chance to out and study them and learn more about the basic makeup of objects that, until late in the 20th century, were something of a mystery to astronomers. Now, we know that asteroids hold a lot of information that would help us understand the origin and evolution of our solar system — making them historical troves of great significance!
May 21, 2012 at 8:14 am | Leave a Comment
What Does That Mean?
One of the most commonly used terms in astronomy is the compound word “light-year”. I posted a tweet about light-years a while back and I got a private message from someone telling me that it scared them. I don’t see how it could be, but then again, I’m so used to it I don’t think twice about using that unit of measure. And that’s all it is — a unit of measure.
Put simply, a light-year is the distance light travels in a year at an average speed of 186,282 miles per second (roughly 300,000 meters per second if you think in metric). The nearest star to us is about 4.3 light-years away. The next nearest spiral galaxy to us — the Andromeda Galaxy — is about 2.5 MILLION light-years away. So, knowing a distance to something tells you how long it takes for light from that object to reach us.
When I was a kid, I used to outside with a flashlight and send little blasts of light up to the sky. All things being equal, in one second, those little beams traveled immensely fast and were gone before I’d even turned off the switch. Of course, as a kid, I didn’t know about our atmosphere absorbing light, and dust bouncing it around, but the concept was still sound. Light travels incredibly fast, and if you send light to the sky, it’s headed out to space never to return.
If you think about this concept of light-speed for a bit, you can come up with all kinds of interesting ideas. Like, the light you see from Andromeda left it before modern humans evolved on our planet. Or, the light you see from the Sun shows you how our star looked just under 10 minutes ago. Or, if you look at Mars in the sky, you’re seeing it as it was as little as 4.3 minutes ago or as much as 21 minutes ago. (This is because Mars’s orbit is elliptical and at certain times it’s farther from us than other times.)
Light-travel time affects communications. For example, signals going out to the Cassini spacecraft travel at the speed of light, and they take about an hour and a half to get to the probe’s antennas. Our earliest radio and TV transmissions are spreading out radially from the planet — at the speed of light. They’ve gone not quite 100 light-years out to space. If there’s anybody within that expanding signal radius, then they’re detecting us as we were back in the early 20th century. Maybe that’s scarier than thinking of light speeding along across the light-years. Our early radio and TV programs really don’t say much about what we were actually like — but they do give insight into what we found funny, scary, and interesting. And, light-years from anywhere, our presence is heralded by that expanding ring of electromagnetic debris. It’s an interesting and sobering thought.
May 19, 2012 at 6:54 am | Leave a Comment
The Annular Eclipse
Will you be checking out the upcoming annular eclipse? If so, you’ve probably heard all the safety warnings about not looking directly at the Sun. They’re good advice. When I was a kid I remember some eclipses occurring and being told not to look at the Sun. Of course, I did. Luckily, I didn’t get any severe eye damage — but it’s worth saying again that even a little sungazing without proper protection is too much. So, if you’ve got eclipse glasses, use ‘em! There’s no substitute for safety.
If you’re planning on viewing the eclipse, check here or here for the latest info on where the path of totality is, and what you can expect to see.
The joint JAXA/NASA Hinode mission will also observe the eclipse and provide images and movies that will be available on the NASA website. Due to Hinode’s orbit around the Earth, Hinode will actually observe 4 separate partial eclipses. Scientists often use an eclipse to help calibrate the instruments on the telescope by focusing in on the edge of the moon as it crosses the sun and measuring how sharp it appears in the images. As an added bonus, the satellite’s x-ray telescope will be able to provide images of the peaks and valleys of the lunar surface silhouetted against the glow of the solar corona.
You may be wondering what an annular eclipse is. It’s simply an eclipse where the Moon’s apparent size isn’t big enough to cover the Sun as the Moon moves between Earth and the Sun. It only covers a portion of the Sun. For people in the path of totality, they’ll see a ring of light around the Moon. Those not in the path will see only a part of the Sun’s light blocked. No matter what portion you see, though, the sunlight will be too bright to look directly at the event.
That’s why you need eye protection — and by that, I mean more than sunglasses. You need eclipse shades. Your local planetarium or science center may have them, so if you’re planning to watch the eclipse, check it out.
You can also watch the eclipse via pinhole projection — that is, shining light from the Sun through a pinhole in a piece paper — and letting it shine onto a wall or another piece of paper. The image on the second surface will show you the eclipsed Sun. It’s just about foolproof! For more details on that, check out this page from the Exploratorium. Whatever you do, have a safe solar eclipse viewing experience. It’ll be good practice for the Transit of Venus in June!
May 10, 2012 at 12:00 pm | Leave a Comment
It’s Not Planet X, That’s for Sure
Astronomers and planetarium folk have been getting the usual phone calls about “something bright in the West” after sunset. It’s true. There is something bright out there… but it’s disappearing fast. It’s the planet Venus, and it’s sinking lower into the western sky each day, and brightens up the post-sunset sky like a jewel hanging there against the dusk.
I love going out to look at Venus. It’s really quite beautiful, and it’s easy to understand why some early observers would call it a goddess. It just gleams in the sky.
Venus looks bright because it’s a cloud-covered world, and those clouds reflect sunlight. Also, it’s a bit closer to us in its orbit right now, and thus appears bigger and brighter.
Venus has a long history in science fiction of being a swamp world or a desert world. I remember reading some early science fiction where people from Earth were eking out a living among dinosaur-type creatures. Another book in my library, written in the late 50s, had Earthlings settling on a dry and dusty cloud-covered Venus, and ultimately launching attacks on Earth.
Those stories were WAY off the mark however. In the 1960s, we sent our first probes to Venus, and right away discovered the truth: a world with a poisonous atmosphere that is so heavy it destroyed the probes that landed on the planet. Later on, orbiters such as the Magellan mission mapped the volcanoes of Venus, showing us once and for all that our “sister planet” is not a very hospitable place.
But, or course, you don’t see that when you gaze at the orb of Venus hanging low in the western sky these May nights. That doesn’t make it less lovely to ponder as the sky darkens. Before too long, Venus will be a morning object, right after it transits the Sun on June 5/6. So, go check it out. Here’s a map to get you started!
May 8, 2012 at 12:05 pm | Leave a Comment
They’re Up There: You Can’t Miss Them
The planets Mars and Saturn are gleaming up there in the sky these May nights. Saturn is in the constellation Virgo, not far from the bright star Spica. I’ve seen a lot of images people are posting of Saturn, and the rings are standing out. So, if you have a decent pair of binoculars, or even better, a small telescope, you can spot those rings yourself.
When I was a kid, one of the first images of a planet I saw was of Saturn. Those rings absolutely mystified me. They were the most alien thing I could think of “out there” and I often wondered how such things could form. Astronomers knew the rings were made up of particles, but it wasn’t until the Voyager mission to Saturn that they knew just how complex the system really is. And, of course, the Cassini Mission is taking their understanding to new heights. You can find some gorgeous views of Saturn from Cassini at the mission Web site, and I think they’ll whet your appetite to see the planet for yourself.
Mars is close to the bright star Regulus, in the constellation of Leo the Lion. It’s a reddish-looking blob of light and if you have a fairly powerful backyard telescope, you can make out some of the surface markings (dark and light) on the planet. Of course, the best way to look at Mars (after you’ve spotted it in your night sky) is to surf over to NASA’s Mars site, or ESA’s Mars Express page. There, you’ll find many fascinating images of the Red Planet taken by spacecraft visiting the planet “up close and personal”.
Go out these May nights and check out the planets. They’re not hard to find and they’ll pique your curiosity. I guarantee it!
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Copyright 2013, Carolyn Collins Petersen
Image of Horsehead Nebula: T.A.Rector (NOAO/AURA/NSF) and Hubble Heritage Team (STScI/AURA/NASA)
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