Sizing Up Near-Earth Asteroids

by Amy Mainzer
Scientist and Engineer

Asteroids. The word conjures images of pitted rocks zooming through space, the cratered surfaces of planets and moons, and for some, memories of a primitive video game. Just how hazardous are these nearest neighbors of ours? We think that one contributed to the extinction of the dinosaurs, giving rise to the age of mammals. How likely is this to happen again?

The Wide-field Infrared Explorer (WISE) mission, an infrared telescope launching in about a year, will observe hundreds of near-Earth asteroids, offering unique insights into this question. The risk posed by hazardous asteroids is critically dependent on how many there are of different sizes. We know that there are more small asteroids than large ones, but how many more, and what are they made of?

Asteroids reflect sunlight (about half of which is the visible light that humans see), but the sun also warms them up, making them glow brightly in infrared light. The problem with observing asteroids in visible light alone is that it is difficult to distinguish between asteroids that are small and highly reflective, or large and dark. Both types of objects, when seen as distant points of light, can appear equally bright in visible light. However, by using infrared light to observe asteroids, we obtain a much more accurate measurement of their size. This is because the infrared light given off by most asteroids doesn’t depend strongly on reflectivity.

This image of near-Earth asteroid 433 Eros reveals that its ancient surface has been scarred by numerous collisions with other small objects. Image credit: NASA/JPL/JHUAPL

WISE will give us a much more accurate understanding of how many near-Earth asteroids there are of different sizes, allowing astronomers to better assess the hazard posed by asteroids. The danger posed by a near-Earth asteroid depends not only on its size, but also on its composition. An asteroid made of dense metals is more dangerous than one of the same size made mostly of less dense silicates. By combining infrared and visible measurements, we can determine how reflective the asteroids are, which gives us some indication of their composition.

Posted in Earth, Solar System | 14 Comments »

Exciting Times for Cassini

by Amanda Hendrix
Scientist

It’s an exciting time in Cassini-land these days! We are well into the Equinox Mission, an extension to Cassini’s mission that includes seven flybys of the Saturnian moon Enceladus, discovered in July 2005 to be geologically active. Prior to the prime mission, we knew that Enceladus was interesting and unique, and thus planned and executed three targeted flybys for the prime mission. With the tremendous discovery of water plumes at the south pole of this small icy moon (which happened on the second targeted flyby), we planned a more in-depth investigation for the Equinox Mission. And we are well into it! Our first Enceladus flyby of the Equinox Mission was in August, and we had two in October.

My job on Cassini is two-fold: I am on the science planning team, helping to plan out the science activities that occur during each icy moon encounter, and I am on the team for the ultraviolet imaging spectrograph instrument, studying ultraviolet data of the surfaces of these icy moons. So it’s really fantastic to be involved in planning each encounter, and then analyzing data to understand the moons.

Close-up view of Enceladus, taken on the Oct. 31 flyby. Image credit: NASA/JPL

In order to learn as much as we can about crazy Enceladus (it’s so small and icy — yet it’s got these geysers!), we want to let all of the instruments make measurements, and it isn’t possible to simultaneously get measurements from all instruments. (That’s just the way the spacecraft is built.) We know that the cameras will tell us a lot about the current and historical geology of the surface, the ultraviolet and infrared imagers will tell us about the surface composition, and the long-wavelength infrared instrument will reveal surface temperatures. These four “remote-sensing” instruments can take data simultaneously. But if we want to get the best data from the “in situ” instruments (like the ion and neutral mass spectrometer and cosmic dust analyzer), we need to orient the spacecraft such that it’s nearly impossible to get remote sensing data. So we divide up the flybys and allow many instruments the opportunity to get data. The period around the closest approach during the August flyby (called “E4”) was allocated to the remote-sensing instruments — and this resulted in the highest-resolution images of the active “tiger stripes” ever! (See one of these images here: http://photojournal.jpl.nasa.gov/catalog/PIA11113) The closest approach of the next Enceladus flyby – called “E5,” on October 9 — took the spacecraft deeper into the south polar plume than ever before. Here the priority was given to the in situ instruments, which obtained great, high-signal data of the plume, telling us about the composition of both the gaseous and particle components. And the October 31 flyby – called “E6” — was again dedicated to remote-sensing, for a last look at the south pole before it heads mostly into seasonal darkness.

It’s so fortunate that Cassini has multiple opportunities to execute close encounters of an object as dynamic as Enceladus. The Voyager spacecraft had just one shot as they flew through the Saturn system, but Cassini, as an orbiter, gives us the chance to analyze our data, figure out what we’ve learned, and make thoughtful decisions on what experiments we need to make to follow up on those discoveries.

Things aren’t completely within our control, however! For instance, southern summer in the Saturn system is coming to a close, limiting the amount of sunlight illuminating the fascinating south polar region of Enceladus. But there’s plenty of important science to do in the dark with the in situ instruments, as well as the composite infrared spectrometer and radar, which is great. Who knows — we’ll see what the equinox season (and hopefully the following solstice) has in store for us! We may get some surprises!

Posted in Saturn, Solar System | 1 Comment »

Shakeout for Southern California

by Maggi Glasscoe
Geophysicist

For those of us living in southern California, the risk of earthquakes is a constant fact of life.  In fact, small earthquakes occur daily, we simply may not notice them.  It’s the larger, more damaging earthquakes that are cause for concern.  The infamous San Andreas fault twists its way through much of California, posing significant risk to southern and northern California both-- and as many scientists have said, it’s not a question of if, it’s a question of when, a large earthquake will occur.

Even though the risk of earthquakes is always present, I am sure most people are not thinking about this on the way to work, or as they are watching TV at night, or just generally going about their daily lives.  Establishing an earthquake preparedness plan probably doesn’t even come to mind, except possibly when there is a major earthquake elsewhere, or a minor earthquake nearby.

We here at JPL are working on ways to extend our ability to forecast earthquakes. We are combining the state of the art in high performance computing resources and modeling software with satellite observations made from space of small scale motion on Earth. This will enhance our understanding of the fundamental earthquake processes. With projects like NASA/JPL’s QuakeSim, which aims to improve our ability to forecast earthquakes, much as we do the weather, we will also be able to help prepare ourselves for the inevitable.

This is a portion of the 1,200-kilometer (800-mile) San Andreas fault, the longest fault in California. Image credit: NASA/JPL

Unfortunately, should a large earthquake catch us unprepared-- and remember, it’s not a question of if, it’s a question of when-- this could have disastrous consequences.  According to FEMA, the annualized loss due to earthquakes is $5.3 billion per year, with 66% ($3.5 billion) concentrated in the state of California alone.  A moderate-sized earthquake in the metropolitan Los Angeles region could lead to loss of vital infrastructure-- water via the aqueduct, freeways, possibly even the ports or the airports, rendering us isolated and without resources for not days, but possibly months.

We are told to be prepared in case of an earthquake with 72 hours’ worth of water and food and other necessary emergency provisions.  That will certainly see us through the first few days, but if the vital infrastructural resources like our water distribution, sewers, freeways, and other pipelines are taken out, we could be looking at much more than 72 hours without proper services, especially water and power.  Are you prepared for such a circumstance?

On November 13, 2008, the United States Geological Survey will lead a disaster preparedness scenario called “The Great Southern California Shakeout.”  It will be based on a magnitude 7.8 earthquake along the southern San Andreas fault.   Shaking from an earthquake of this size is projected to last up to two minutes, and the modeling that they have done has predicted that sediments in the various basins around the Los Angeles area will trap and magnify seismic waves, amplifying ground motions, much like what occurred in the Northridge earthquake. (To learn more about the “Great Shakeout,” please visit: www.shakeout.org)

This earthquake scenario will also be the basis for the statewide emergency response exercise, Golden Guardian 2008.  These complementary exercises are meant to demonstrate our ability to deal with an earthquake scenario in which there would be 1800 deaths, 50,000 injuries, and $200 billion in damage.  An earthquake of this magnitude could produce destruction on the scale of the recent Gulf Coast hurricanes or worse.

One thing to keep in mind, though, is that we need to be proactive, rather than simply reactive.  That way, when the inevitable moderate to large earthquake does hit, we will be as ready as we can be to deal with it.  Exercises like the ShakeOut certainly help to keep the community more aware of the ever-present risk of earthquakes, but we as individuals also need to take the time to make sure that we are disaster prepared as well.  That way we can be not only prepared, but resilient.

Posted in Earth | 1 Comment »