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The first near-Earth asteroid discovered by WISE (red dot) stands out from the stars (blue dots). The asteroid is much cooler than the stars, so it emits more of its light at the longer, infrared wavelengths WISE uses. This makes it appear redder than the

This is a screenshot from the WISE moving-object quality assurance system, which helps weed out false asteroid candidates. The top two rows show an asteroid candidate detected in 16 different WISE snapshots, at two different infrared wavelengths. The lowe

Over the course of the nine months we've been operating WISE, we've observed over 150,000 asteroids and comets of all different types. We had to pick all of these moving objects out of the hundreds of millions of sources observed all over the sky -- so you can imagine that sifting through all those stars and galaxies to find the asteroids is not easy!

We use a lot of techniques to figure out how to distinguish an asteroid from a star or galaxy. Even though just about everything in the universe moves, asteroids are a whole lot closer to us than your average star (and certainly your average galaxy), so they appear to move from place to place in the WISE images over a timescale of minutes, unlike the much more distant stars. It's almost like watching a pack of cyclists go by in the Tour de France. Also, WISE takes infrared images, which means that cooler objects like asteroids look different than the hotter stars. If you look at the picture below, you can see that the stars appear bright blue, whereas the sole asteroid in the frame appears red. That's because the asteroid is about room temperature and is therefore much colder than the stars, which are thousands of degrees. Cooler objects will give off more of their light at longer, infrared wavelengths that our WISE telescope sees. We can use both of these unique properties of asteroids -- their motion and their bright infrared signatures -- to tease them out of the bazillions of stars and galaxies in the WISE images.

Thanks to the efforts of some smart scientists and software engineers, we have a very slick program that automatically searches the images for anything that moves at the longer, infrared wavelengths. With WISE, we take about a dozen or so images of each part of the sky over a couple of days. The system works by throwing out everything that appears again and again in each exposure. What's left are just the so-called transient sources, the things that don't stay the same between snapshots. Most of these are cosmic rays -- charged particles zooming through space that are either spat out by our sun or burped up from other high-energy processes like supernovae or stars falling into black holes. These cosmic rays hit our detectors, leaving a blip that appears for just a single exposure. Also, really bright objects can leave an after-image on the detectors that can persist for many minutes, just like when you stare at a light bulb and then close your eyes. We have to weed the real asteroid detections out from the cosmic rays and after-images.

The data pipeline is smart enough to catch most of these artifacts and figure out what the real moving objects are. However, if it's a new asteroid that no one has ever seen before, we have to have a human inspect the set of images and make sure that it's not just a collection of artifacts that happened to show up at the right place and right time. About 20 percent of the asteroids that we observe appear to be new, and we examine those using a program that we call our quality assurance (QA) system, which lets us rapidly sift through hundreds of candidate asteroids to make sure they're real. The QA system pops up a set of images of the candidate asteroid, along with a bunch of "before" and "after" images of the same part of the sky. This lets us eliminate any stars that might have been confused for the asteroids. Finally, since the WISE camera takes a picture every 11 seconds, we take a look at the exposures taken immediately before the ones with the candidate asteroid -- if the source is really just an after-image persisting after we've looked at something bright, it will be there in the previous frame. We've had many students -- three college students and two very talented high school students -- work on asteroid QA. They've become real pros at inspecting asteroid candidates!

Meanwhile, the hunt continues -- we're still trekking along through the sky with the two shortest-wavelength infrared bands, now that we've run out of the super-cold hydrogen that was keeping two of the four detectors operating. Even though our sensitivity is lower, we're still observing asteroids and looking for interesting things like nearby brown dwarfs (stars too cold to shine in visible light because they can't sustain nuclear fusion). Our dedicated team of asteroid inspectors keeps plugging away, keeping the quality of the detections very high so that we leave the best possible legacy when our little telescope's journey is finally done.


  • Amy Mainzer

Diagram showing the solar arrays of the Rosetta and Dawn spacecraft

Recently, one of our fans on the NASAJPL Facebook page asked a good question about the efficiency of solar arrays on the Dawn and Rosetta spacecraft.

"A question about Dr. Marc D. Rayman's comment in his Dawn journal, saying that 'its tremendous solar arrays [are] the most powerful ever used on an interplanetary mission.' Is that really true? According to JPL's Dawn website, the solar arrays have a total span of 19.7 meters. By comparison, each of Rosetta's two arrays is 14 meters in length (28m total). Are Dawn's arrays so much more efficient? Thanks."

Here's an answer from Dawn Chief Engineer Marc Rayman:

Yes, this is really true. Dawn's solar arrays, although smaller than Rosetta's, could indeed produce more power because they are more efficient. Fortunately, it is not a competition! Both missions seek fascinating new insights into the complex history and character of the solar system as they take all of us on adventures to exotic destinations. To overcome some of the daunting challenges of traveling moderately far from the sun, engineers on each mission have turned to powerful solar arrays.

Rosetta is a fabulous mission, promising exciting results from comet Churyumov-Gerasimenko. It is with the greatest enthusiasm that I look forward to the astonishing discoveries that await its rendezvous with this solar system relict.

The spacecraft carries the largest solar arrays ever flown on an interplanetary mission. The two 14-meter (47-foot) arrays project in opposite directions from the main spacecraft itself, creating a structure about 32 meters (105 feet) tip-to-tip, and the total area of solar cells is 53 square meters (573 square feet). Composed of silicon, these cells could have produced somewhat in excess of seven kilowatts when at Earth's distance from the sun. Of course, Rosetta did not need that much power, but as it travels into the depths of space, every watt will be precious. When the spacecraft is more than five times Earth's distance from the sun and the light from our star is much weaker, the giant arrays still will generate 400 watts, just enough to keep the probe operating. (Rosetta will arc out to that distance on its way to the comet, but it will be closer to the sun, and hence able to produce more power, when it arrives and conducts its investigations of this mysterious body).

Dawn's solar arrays, while the largest used on a NASA interplanetary mission, are smaller than Rosetta's. This bird's wingspan is about 20 meters (65 feet), and the solar arrays, each more than eight meters (27 feet) in length, have a total of about 32 square meters (341 square feet) to capture sunlight. The panels are populated with advanced cells composed of three different materials that work together to convert a larger percentage of the incident light into electrical power. The combination of indium gallium phosphide, indium gallium arsenide, and germanium makes these cells so much more efficient that despite the smaller collecting area, together they produce higher power under the same conditions. These arrays could have generated more than 10 kilowatts at Earth's distance from the sun. Dawn not only did not need such tremendous power, but like Rosetta, it was not even capable of using it all. But it too ventures far from home to remote locations where sunlight is less abundant.

Dawn's ambitious mission to orbit the two most massive residents of the asteroid belt, Vesta and Ceres, would be quite impossible without its use of ion propulsion. The key to ion propulsion's extraordinary capability is its conversion of electrical power into thrust, so Dawn carries such powerful arrays to ensure that even when exploring dwarf planet Ceres at three times Earth's distance from the sun, it can produce sufficient power to thrust and operate all other systems. I describe more about the importance of power to the mission in my Dawn Journal of July 27, 2008.

I appreciate your interest in Dawn, and I hope you will continue to join us as we travel to two of the last unexplored worlds in the inner solar system. In only 10 months, Dawn will become the first spacecraft ever to orbit a resident of the main asteroid belt as it begins its exploration of protoplanet Vesta, and to put it quite simply, this is going to be really cool!

Join the Facebook conversation at http://www.facebook.com/NASAJPL.


  • Marc Rayman

Optimal viewing in the night skies!

Are you eager to see the annual Perseids meteor shower tonight? You'll have to wait until near midnight to see it, so why not pass the time by viewing Venus, Saturn and Mars right from your doorstep? Step outside for the planetary warm-up act just as soon as the sun sets. (Viewing times will be best over the next week. By August 20, the planets set lower on the horizon and are harder to see.)

All you have to do is look towards the west for bright Venus to appear. Now hold your clenched fist up to the sky, covering Venus. To the right of Venus, about half of a clenched fist away, is a second planet: That's Saturn! And to the upper left of Venus is another planet: Mars!

That's not all you'll be able to see. Look below Venus for the slender crescent moon. If you don't see the moon, look again on the night of Friday, August 13 -- it will be a larger crescent to the left of Venus.

Though the three planets appear together in our line of sight, they are really far apart from each other. Mars is about 300 million kilometers (about 185 million miles) from Earth, while Venus is 112 million kilometers (about 70 million miles) away. Saturn? It's 1,535 million kilometers (about 954 million miles) from Earth. And finally, the moon is only 363 thousand kilometers (about 225 thousand miles) away. It's fun to compare the size of the moon and Mars, especially if you received that annual email incorrectly stating that Mars will be as big as the moon this month.


  • Jane Houston Jones

This plot shows asteroids and comets observed by NASA's Wide-field Infrared Survey Explorer, or WISE.

It's hard to believe that we've just crossed the six-month mark on WISE -- seems like just yesterday when we were all up at Vandenberg Air Force Base, near Santa Barbara, shivering in the cold at night while watching the countdown clock. But the time is flying (literally!) as WISE whips by over our heads. We're analyzing data ferociously now, trying to get the images and the data ready for the public release next May. Even though the mission's lifetime is short, we've gotten into a semblance of a routine. We receive and process images of stars, galaxies and other objects taken by the spacecraft every day, and we're running our asteroid-hunting routine on Mondays and Thursdays. We've got a small army (well, okay, three -- but they do the work of a small army!) of extremely talented students who are helping us verify and validate the asteroid detections, as well as hunt for new comets in the data. Plus, there is an unseen, yet powerful, cadre of observers out there all over the world following up our observations.

And so it's come to pass that we've achieved some milestones. We completed our first survey of the entire sky on July 17 -- and we just discovered our 100th new near-Earth object! That's out of the approximately 25,000 new asteroids we've discovered in total so far; most of these hang out in the main belt between Mars and Jupiter and never get anywhere near Earth's orbit. These new discoveries will allow us to conduct an accurate census of both the near-Earth and main belt asteroid populations. We're really busy chewing on the data right now and calculating what it all means.

Because it's so short, this mission reminds me a little bit of what the first days of college felt like -- a tidal wave of new ideas, new sights and new thoughts. The pace of learning has been incredibly quick, whether I'm trying to get up to speed on asteroid evolution theories or tinkering with the software we use to write papers.

Speaking of papers, we're in the process of preparing to submit several to science journals; in fact, I've already submitted one. The gold standard of science, of course, is the peer-review process. We submit our paper to a journal, and the scientific editor assigns another scientist who is an expert in the field but not involved in the project (and who usually remains anonymous) to read it and offer comments. The referee's job is to "kick the tires," so to speak, and ask tough questions about the work to make sure it's sound. We get a chance to respond, and the referee gets a chance to respond to our responses, and then when everybody's convinced the results are right, the paper is accepted and can be published. So stay tuned -- we should have some of the first papers done soon telling us what these milestones mean for asteroid science.

› Read more from "Rocks and Stars with Amy"


  • Amy Mainzer

Titan's atmosphere makes Saturn's largest moon look like a fuzzy orange ball in this natural color view from the Cassini spacecraft.

On Sunday evening, my eyes were glued to eight windows on my computer screen, watching data pop up every few seconds. NASA's Cassini spacecraft was making its lowest swing through the atmosphere of Saturn's moon Titan and I was on the edge of my seat. Trina Ray, a Titan orbiter science team co-chair, was keeping me company. Five other members of my team were also at JPL. Between us, we were keeping an eye on about 2,000 data channels.

One of the 34-meter antennas at the Deep Space Network's Goldstone complex, DSS-24, was pointed at Saturn and listening for the signal that was expected to be here in just a few minutes. The data would be arriving at my computer as quickly as they could be sent back to Earth, though there was an agonizing hour-and-18-minute delay because of the distance the data had to travel. (We call this flyby T70, but it is actually Cassini's 71st flyby of Titan.)

It was a nervous time for me -- the previous night we had been at JPL to send some other real-time commands to the spacecraft when an alarm came in indicating that the magnetometer, the prime instrument taking data for the T70 flyby, needed a reset. Fortunately, the controller on duty immediately called the magnetometer instrument operations team lead in England. Within 90 minutes, the commands were on their way to do a computer reset and clear the alarm. At 2 a.m. Pacific time on Sunday, we got the email indicating all was well and the magnetometer was ready for the Titan closest approach.

So here we were, past one hurdle, hoping nothing else would come up. We had run hundreds of simulations over the past three-and-a-half years, so I knew we had done everything we could think to do. We did more training for this event than anything else we had done since we dropped off the Huygens probe in January 2005 for a descent through the moon's hazy atmosphere.

Right on time, at 7:26 p.m., the Deep Space Network locked on the spacecraft downlink, a good start. I was focused on the data for spacecraft pointing. As long as we stayed within an eighth of a degree of the expected pointing, everything would be fine. At 7:45 p.m., we got the data from closest approach, a mere 880 kilometers (547 miles) in altitude. Over the vocabox, a cross between a telephone and walkie-talkie, the attitude control team reported that the thrusters were firing about twice as much as we expected. The Titan atmosphere appeared to be a little thicker than we expected, even though we had fed about 40 previous low Titan flybys by Cassini and the descent data from Huygens into our modeling.

But spacecraft control was right on the money, keeping the pointing within our predicted limits. Even with the extra thrusting, we stayed well within our safety margin.

At 7:53 p.m., the spacecraft turned away to go to the next observation. I let out a sigh of relief, happy that everything during closest approach had gone just as we planned. Five attitude control guys crowded into my office with smiles on their faces. Trina and I were marveling at what a wonderful spacecraft we have to work with. Another first for the Cassini mission!

Now, as Trina says, we have to finish the job by returning all the great science data. We have data playbacks today at two different Deep Space Network stations to make sure we have - as we say here - both belts and suspenders. Engineers will also go back to analyze the data with the scientists to see just how dense the Titan atmosphere turned out to be at our flyby altitude.

But last night, at least, my team and I went home happy!


  • Julie Webster

Artist's concept of Cassini's Titan flyby

This weekend, Cassini will embark on an exciting mission: trying to establish if Titan, Saturn's largest moon, possesses a magnetic field of its own. This is important for understanding the moon's interior and geochemical evolution.

For Titan scientists, this is one of the most anticipated flybys of the whole mission. We want to get as close to the surface with our magnetometer as possible for a one-of-a-kind scan of the moon. Magnetometer team scientists (including me) have a reputation for pushing the lower limits. In a world of infinite possibilities, we would have liked many flybys at 800 kilometers. But we went back and forth a lot with the engineers, who have to ensure the safety of the spacecraft and fuel reserves. We agreed on one flyby at 880 kilometers (547 miles) and both sides were happy.

Flying at this low altitude will mark the first time Cassini will be below the moon's ionosphere, a shell of electrons and other charged particles that make up the upper part of the atmosphere. As a result, the spacecraft will find itself in a region almost entirely shielded from Saturn's magnetic field and will be able to detect any magnetic signature originating from within Titan.

Titan orbits within the confines of the magnetic bubble around Saturn and is permanently exposed to the planet's magnetic disturbances. Previous measurements by NASA's Voyager spacecraft and Cassini at altitudes above 950 kilometers (590 miles) have shown that Titan does not possess an appreciable magnetic field capable of counterbalancing Saturn's. However, this does not imply that Titan's field is zero. We'd like to know what the internal field might be, no matter how small.

The internal structure of Titan can be probed remotely from its gravitational field or its magnetic properties. Planets with a magnetic field -- like Titan's parent Saturn or our Earth -- are believed to generate their global-scale magnetic fields from a mechanism called a dynamo. Dynamo magnetic fields are generated from currents in a molten core where charge-conducting materials such as metals are flowing around each other and also undergoing other stresses because of the planet's rotation.

We might not find a magnetic field at all. A positive detection of an internal magnetic field from Titan could imply one of the following:

a) Titan's interior still bears enough energy to sustain a dynamo.
b) Titan's interior is "cold" (and therefore has no dynamo), but its crust is magnetized in a similar way as Mars' crust. If this is the case, we should find out how this magnetization took place.
c) Something under the surface of Titan got charged temporarily by Saturn's magnetic field before this Cassini flyby. While I said earlier that the ionosphere shields the Titan atmosphere from Saturn's magnetic bubble, the ionosphere is only an active shield when the moon is exposed to sunlight. During part of its orbit around the planet, Titan is in the dark and magnetic field lines from Saturn can reach the Titan surface. A temporary magnetic field can be created if there is a conducting layer, like an ocean, on or below the moon's crust.

Once Cassini leaves Titan, the spacecraft will perform a series of rolls to fine-calibrate its magnetometer in order to assess T70 measurements with the highest precision. We're looking forward to poring through the data coming down, especially after all the negotiations we had to make for them!


  • Cesar Bertucci

Rhea captured by Cassini

Helene captured by Cassini

Cassini's closest-ever flyby of Saturn's moon Rhea went quite smoothly and teams are busy checking out their data! These flybys never fail to amaze me. And the raw images -- which give us an unprocessed first look -- are really cool!

Raw image N00152150 gives us a view of part of the bright, fractured terrain we refer to as "wispy terrain" from about 14,000 kilometers (8,900 miles) away. We know that Rhea's albedo overall is quite high. (When I say "albedo," I basically mean "brightness" or "reflectivity." Studying the albedo can tell a lot about surface composition, geologic processes, and interactions with external environment.) But this image demonstrates how bright these cracks are since they are so shiny that the surrounding terrain looks quite dark. There are also some interesting apparent albedo variations seen in this image, which are really intriguing.

This raw image (N00152175) from Cassini's narrow-angle camera image was taken about 40 minutes after closest approach. The image shows a region adjacent to the wispy terrain --craters, craters everywhere! And wow, are those crater rims bright compared to the surrounding terrain.

Cassini captured a full portrait of the serene moon with its wide-angle camera (raw image W00063107) on the outbound leg of the flyby, about 1.25 hours after closest approach. Keep in mind that the phase angle is quite low here (only about 2.5 degrees), meaning that the sun is almost directly behind Cassini and Rhea is nearly fully illuminated -- so there are no shadows. Large-scale albedo variations are apparent across the surface.

The spacecraft also obtained a cool image of little Helene with raw image N00152211. We're basically looking at the night side of the body -- but it doesn't appear very dark, because it's illuminated by sunlight reflecting off Saturn. During the later image sequence of Helene, this small moon was transiting Saturn - so you can see Saturn in the background. Sometimes,pointing at these little guys can be very tricky, especially so close after a targeted flyby. It can be difficult (or impossible!) to get the positions of the spacecraft, the moon and the instruments all lined up -- but boy are these close-up Helene images incredible! The detail on the surface is tremendous, and should go a long way to informing geologists about surface properties and processes.

As the imaging team is taking a closer look at images such as these, other instrument teams -- including those for the radar instrument, composite infrared spectrometer, visual and infrared mapping spectrometer and the ultraviolet imaging spectrograph (the instrument I work on) -- are also busy processing their data. At a science meeting Friday, we talked about a few of the preliminary results. Some of the magnetospheric and plasma science instruments teams reported that they’re seeing some really interesting and surprising results! So stay tuned to hear more about those!

Of course, after one successful flyby, we get right to work on another. Coming up next: Dione on April 7!


  • Amanda Hendrix

Bright, wispy markings stretch across a region of darker terrain on Saturn's moon Rhea.

On Tuesday, March 2, 2010, NASA's Cassini spacecraft will make its closest encounter yet with Saturn’s second largest moon.

Here in Cassini-land, we are really excited about Tuesday's Rhea flyby! This will be the mission's second targeted flyby of the moon in the mission, so it's sometimes referred to as R-2 or Rhea-2.

The spacecraft will fly by Rhea at an altitude of about 100 kilometers (60 miles), the closest encounter yet with Saturn's second largest moon. (Our first targeted flyby of Rhea in 2005 was at an altitude of 500 kilometers, or 300 miles, so this is way closer.)

We've been focusing a lot on the moon Enceladus because it is sort of the darling of the Saturn system -- but Rhea is a good example of why the other moons are interesting too. We know a decent amount about this moon, but we still have more questions, especially about the debris that could make up a ring around the moon and the composition of its surface.

The first targeted flyby in 2005 was focused on a radio science experiment doing gravity measurements to understand Rhea's interior structure. We also got some nice remote-sensing data from the cameras and spectrometers (see for example PIA07764) as well as radar measurements for surface and subsurface composition. We also did a much more distant flyby (5,000 kilometers or 3,000 miles) of Rhea in August 2007; that flyby was dedicated to remote sensing of the moon, including imaging (such as PIA08402). So we have a pretty good understanding of Rhea as being pretty heavily cratered with no super obvious signs of activity. It has this "wispy terrain" (see PIA08120), which is a lot like the type of feature seen on another Saturnian moon, Dione, and is basically a large series of fractures that are relatively bright compared to the surrounding regions.

One of the most interesting results to come out of the 2005 and 2007 flybys came from the fields and particles instruments: the mysterious signature of electron depletion around Rhea, suggestive of a debris ring. (Basically, solid material appears to be absorbing electrons in the vicinity of Rhea.) So Rhea could be a moon with its own ring! The ring has not been seen by any of the remote sensing instruments on Cassini, however. It can be difficult to get the viewing geometry just right in order to see this type of thing -- recall that the Cassini cameras didn't definitely see Enceladus' plume until after being in orbit for more than one year!

Tuesday's flyby should give us some clues about the suspected debris disk around the moon, but the slam-dunk experiment to "see" Rhea's debris disk is what we call a stellar occultation through the ring plane - looking to see if debris particles or clumps block out light from stars. Unfortunately we won't get to do such an occultation on this flyby. This is a tricky experiment to do because you have to get the timing and the geometry just right, but we're hoping to do it at some point later in the mission.

Anyway, on to Tuesday's flyby! To get a sense of what we're going to do, check out the movie made by Cassini navigator Brent Buffington that shows each of the activities performed during the flyby.

We will approach Rhea on the night side, so the moon will be dark. This is an especially good opportunity for the radar instrument to make measurements. (The cameras and imaging spectrometers typically prefer to observe the dayside, not the nightside.) Radar will do synthetic aperture radar imaging scans similar to those at Titan and will also do measurements to understand the surface composition. Previous measurements had suggested an asymmetry in brightness (which could be due to compositional differences) between the leading and trailing hemispheres of the moon, so this flyby will help with investigating that.

At closest approach, the fields and particles instruments will take data that will help us understand the environment of Rhea -- its interaction with Saturn's magnetosphere, its debris disk, and its ejecta cloud density. Ejecta clouds are dust or material that is being ejected or sputtered or otherwise lost from Rhea and its environment and contributing to populations of neutral particles and plasma in the Saturn system. This material may also be contaminating Saturn's rings.

Outbound, the remote sensing instruments will take over. They will make measurements -- in wavelengths as short as the ultraviolet all the way to the far infrared -- of Rhea's surface terrains and composition, as well as its surface temperature. The cameras have seen some "bluish spots" that could be related to the debris ring material - so those regions will be investigated more during this encounter, as will the fractured "wispy" terrain. The visual and infrared mapping spectrometer and the ultraviolet imaging spectrograph will do imaging spectroscopy to search for and map out water ice grain sizes, carbon dioxide, ammonia and fine-grained iron particles, among other materials. The composite infrared spectrometer will map temperatures across portions of Rhea's sunlit disk at high resolution. Ninety minutes after closest approach, Rhea will enter Saturn's shadow, giving the composite infrared mapping spectrometer a good opportunity to measure the cooling of the surface, which will provide information about the texture of the uppermost surface layers.

But wait - there's more! Not only do you get a Rhea flyby, but we're going to throw in a close approach to the small moon Helene! Helene is one of the "co-orbitals" of Dione. That means it orbits Saturn at the same radial distance as Dione, but it happens to be 60 degrees ahead of Dione. Helene is only about 30 or 35 km across (19 or 22 miles) and it's not spherical (see PIA10544). Cassini will approach Helene within about 1,825 kilometers (1,130 miles) -- by FAR the closest we've ever gotten to Helene -- allowing the cameras and imaging spectrometers to obtain information about individual regions across the surface.

So this promises to be an exciting period. Please stay tuned to see the great results!


  • Amanda Hendrix


Galileo's Letter to Prince of Venice

The end of 1609 and the first months of 1610 mark the beginning of modern astronomy. 400 years ago today, January 7th, Galileo Galilei looked up towards the constellation Orion. He aimed his telescope at an object brighter than any of the surrounding stars - the planet Jupiter.

The view through his telescope startled him. He did not see only one object, but rather, one large world, with four smaller objects nearby.

These four objects are the moons we now call Io, Europa, Ganymede and Callisto.

Galileo wrote in his book Sidereus Nuncius, which was published in 1610 the following words:

"I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world up to our own times, their positions, and the observations made during the last two months about their movements and their changes of magnitude; and I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day . . . On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavons through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude . . .When on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night."

"I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter."

On the 400th anniversary of their first sighting, use a pair of binoculars to spot Galileo's four tiny moons directly next to the planet. On the evening of January 7 look to the southwest after sunset. Europa and Ganymede will appear to the upper left and Io and Callisto on the lower right of Jupiter.

Letters with translations http://www2.jpl.nasa.gov/galileo/ganymede/discovery.html


  • Jane Houston Jones

Artist concept of Mars Opportunity Rover

I almost didn't get to drive the rovers.

As one of the five developers of the software used to build the command sequences and rehearse and visualize the rover activities, I really wanted to be one of the people using it in flight. Unfortunately, only three members of the team were selected to be Rover Planners (a job title we believe was chosen in place of Rover Drivers to make the job sound very boring and reduce competition for it). I was not one of them.

I was originally slated to be a downlink analyst looking at the telemetry from the rover to assess the driving and arm operations. This entailed months of training to learn how to run somebody else's software, a much more difficult task than using your own. Fortunately for me, and unfortunately for someone else, a position opened up on the Rover Planner team and I was transferred over. This entailed more months of training to learn the procedures, but the fuse was very short since Spirit was careening towards its landing. The fact that I knew the software tools already was the saving grace that allowed me to be ready to go on landing day.

Looking back on these six years, I'm tired, but amazed, when I think about how much we've accomplished and continue to accomplish. During the prime mission, I remember hearing Steve Squyres say how much we would like Spirit to go explore the hills in the distance but that we would never get there. Well, we have driven to the top of those hills and down the other side.

I remember when Opportunity drove into Purgatory and the Rover Planners immediately said that we needed to back out of the sand dune. After months of testbed activities and review, the decision was made to back out of the sand dune. I can remember looking over at Scott Maxwell, another Rover Planner, and saying to each other "This is so cool!!" (We still say that).

Some of my favorite memories are of giving talks to school kids about what I do, though one of my saddest was being asked by one of the kids, an honor student, if the moon landings were faked. I especially enjoyed calling up Car Talk and asking the guys how to keep our electric vehicle running through the winter on Mars. I laugh when I think about a recent talk I had with Scott when the right front wheel of Spirit seemed to work again after four years of being dragged around. Scott said he didn't know if we were driving with six wheels or only five. Immediately I jumped in, Dirty Harry-style, with, "I know what you're thinking punk. Are we driving with six wheels or only five? To tell the truth, I don't know myself. The question you have to ask yourself is, 'Do I feel lucky?' Well, do ya', punk?"

As we work on getting Spirit out of the current sand trap, I feel manic-depressive about our chances. One day I am sure we will have no problem but the next day I am equally convinced that all is lost. This is about the toughest situation we have ever had to get out of. When we are stuck, it seems as if we are always running out of daylight, which translates to power. It happened at Tyrone, it happened at Tartarus, and it has happened at Troy.

Hmmm, maybe we should stop giving names to locations that start with T.