Category Archives: asteroid

Potential for Danger from Space

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.

NASA' NEOWISE survey finds more potentially hazardous asteroids in our planet's vicinity than previously thought. Courtesy NASA.

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 rocks’ 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!

Black Hole Munches Asteroids

Is There No End to Its Incessant Demands?

This image from NASA's Chandra X-ray Observatory shows the center of our Galaxy, with a supermassive black hole known as Sagittarius A* (Sgr A* for short) in the center. Using intermittent observations over several years, Chandra has detected X-ray flares about once a day from Sgr A*. The flares have also been seen in infrared data from ESO's Very Large Telescope in Chile. Click to embiggen.

How did I miss this story?  Early in February, the folks working with the Chandra X-ray Observatory announced that they’ve been watching the black hole at the heart of the Milky Way Galaxy munching down on asteroids.  According to them, this happens pretty frequently. They’ve seen the evidence:  once-a-day x-ray flares from Sagittarius A* (that’s the name given to our central black hole), or Sgr A* for short. The flares last a few hours, and have also been seen in infrared light by detectors at the Very Large Telescope in Chile.

The idea is that there is a cloud around Sgr A* containing hundreds of trillions of asteroids and comets. Where did they come from? Astronomers suggest that this debris collection is made of material stripped from the parent stars by the force of gravity.

An asteroid that gets too close to  another object, such as a star or planet, can get thrown into an orbit that places it on a heading for  Sgr A*.  If it gets too close — say within 100 million miles of the black hole (about the distance between Earth and the Sun) it would be torn into pieces as it encountered the  tidal forces from the black hole. These fragments would be vaporized as they pass through the hot, thin gas that continually flows toward Sgr A*.  This is very similar to what happens to a meteor when it encounters Earth’s atmosphere for example. It heats up and glows, and eventually it vaporizes, and we see a flare marking the end of the meteor’s trip. When an object does this near Sgr A*, we can see it in x-rays and infrared.  Whatever’s left of the asteroid gets sucked into the black hole.

This is kind of a cool thing to see at an object so far away, and Chandra’s still on the case. Over the next year or so, the satellite will study more of these things that go “fuff” in the night near Sgr A*.