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.

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!