By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

NASA’s Dawn spacecraft obtained this image with its framing camera on September 20, 2011. This image was taken through the camera’s clear filter. The distance to the surface of Vesta is 673 km and the image resolution is about 66 meters per pixel. Image credit: NASA/ JPL-Caltech/ UCLA/ MPS/ DLR/ IDA
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Dear Dawnderfuls,

Dawn has completed another wonderfully successful phase of its exploration of Vesta, studying it in unprecedented detail during the past month. From the time of its discovery more than two centuries ago until just a few months ago, this protoplanet appeared as hardly more than a fuzzy blob, an indistinct fleck in the sky. Now Dawn has mapped it with exquisite clarity, revealing a fascinatingly complex alien world.

The high altitude mapping orbit (HAMO) includes the most intensive and thorough imaging of the entire year Dawn will reside at Vesta. Spectacular as the results from survey orbit were, the observations from HAMO are significantly better. From four times closer to the surface, Dawn’s sensors provided much better views of the extraordinary surface of craters large and small, tremendous mountains, valleys, towering cliffs, ridges, smooth and flat regions, gently rolling plains, systems of extensive troughs, many clusters of smaller grooves, immense landslides, enormous boulders, materials that are unusually bright and others that are unusually dark (sometimes adjacent to each other), and myriad other dramatic and intriguing features. There is no reason to try to capture in words what visual creatures like humans can best appreciate in pictures. To see the sites, which literally are out of this world, either go to Vesta or go here.

Circling the colossus 680 kilometers (420 miles) beneath it in HAMO, the probe has spent most of its time over the illuminated side taking pictures and other scientific measurements and most of the time over the dark side beaming its precious findings back to eager Earthlings.

Dawn revolves in a polar orbit around Vesta, passing above the north pole, then traveling over the day side to the south pole, and then soaring north over the night side. Each circuit takes 12.3 hours. Meanwhile, Vesta completes a rotation on its axis every 5.3 hours. Mission planners choreographed this beautiful cosmic pas de deux by choosing the orbital parameters so that in 10 orbits, nearly every part of the lit surface would come within the camera’s field of view. (Because it is northern hemisphere winter on that world, a region around the north pole is hidden in the deep dark of night. Its appearance in Dawn’s pictures will have to wait for HAMO2.) A set of 10 orbits is known to Dawn team members (and now to you) as a mapping cycle.

Although the HAMO phase was extremely complex, it was executed almost flawlessly, following remarkably well the intricate plan worked out in great detail last year. It consisted of six mapping cycles, and they were conducted in order of their overall importance. In the first cycle, Dawn aimed its camera straight down and took pictures with all of the instrument’s color filters. In addition to showing the startling diversity of exotic features, the color images provide scientists some information about the composition of the surface materials, which display an impressive variation on this mysterious protoplanet. Cycle 1 yielded more than 2500 photos of Vesta, nearly as many as were acquired in the entire survey orbit phase. These observations were deemed so important that not only were they first, but cycle 6 was designed to acquire nearly the same data. This strategy was formulated so that if problems precluded the successful mapping in cycle 1, there would be a second chance without requiring the small and busy operations team to make new plans. As it turned out, there were only minor glitches that interfered with some of the pictures in cycle 1, but the losses were not important. Nevertheless, cycle 6 did fill in most of the missing views.

Cycles 2 through 5 were devoted to acquiring images needed to develop a topographical map. Instead of flying over the sunlit side with its camera pointed straight down, the spacecraft looked at an angle. Each direction was chosen to provide scientists the best combination of perspective and illumination to build up a three dimensional picture of the surface. Knowing the elevations of different features and the angles of slopes is essential to understanding the geological processes that shaped them.

In cycle 2, the camera constantly was directed at the terrain ahead and a little to the left of the point directly below the spacecraft. Cycle 3, in contrast, looked back and slightly to the left. Cycle 4 pointed straight ahead but by a smaller angle than in cycle 2. Cycle 5 did not look forward or backwards; it only observed the surface to the right. With the extensive stereo coverage in each of these 10-orbit mapping cycles, most of the terrain now has been photographed from enough different directions that the detailed shape of the alien landscape can be determined.

The HAMO observations constitute the most comprehensive visible mapping of Vesta for the mission. The survey orbit images were obtained from a higher altitude and so do not show as much detail. When Dawn flies down to its low altitude mapping orbit (LAMO), its primary objectives will be to measure the atomic constituents with the gamma ray and neutron detector (GRaND) and to map the gravitational field. While some images will be acquired, they will be a secondary objective. The principal resources, both for the spacecraft and for the operations team, will be devoted to the higher priority science. In addition, the probe will be too close in LAMO for its camera to collect enough pictures for a global map. The subsequent observations in HAMO2 will be designed mostly to glimpse some of the northern latitudes that are currently too dark to see.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

NASA’s Dawn spacecraft is less than two months away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

Artist’s concept of the Dawn spacecraft using its ion propulsion system during the approach to Vesta. Image credit: NASA/JPL-Caltech

Dear Dependawnble Readers,

Dawn remains healthy and on course as it continues to approach Vesta. Thrusting with its ion propulsion system, as it has for most of its interplanetary journey so far, the spacecraft is gradually matching its solar orbit to that of the protoplanet just ahead.

As these two residents of the asteroid belt, one very new and one quite ancient, travel around the sun, they draw ever closer. Vesta follows its own familiar path, repeating it over and over, just as Earth and many other solar system bodies do. Dawn has been taking a spiral route, climbing away from the sun atop a blue-green pillar of xenon ions. With an accumulated total in excess of two and a half years of ion thrusting, providing an effective change in velocity of more than 6.5 kilometers per second (14,500 mph), the probe is close to the end of the first leg of its interplanetary trek. On July 16, Vesta’s gravity will capture the ship as it smoothly transitions from spiraling around the sun to spiraling around Vesta, aiming for survey orbit in August. For several reasons, the date for the beginning of the intensive observations there has not yet been set exactly.

Astronomers have estimated Vesta’s mass, principally by measuring how it occasionally perturbs the orbits of some of its neighbors in the asteroid belt and even the orbit of Mars, but this method yields only an approximate value. Because the mass is not well known, there is some uncertainty in the precise time that Dawn will become gravitationally bound to the colossal asteroid. As we have seen before, entry into orbit is quite unlike the highly suspenseful and stressful event of missions that rely on conventional chemical propulsion. Dawn simply will be thrusting, just as it has for 70 percent of its time in space. Orbit entry will be much like a typical day of quiet cruise. That Vesta will take hold at some point will matter only to the many Dawnophiles throughout the cosmos following the mission. The ship will continue to sail along a gently curving arc to survey orbit.

› Continue reading Marc Rayman’s May 27, 2011 Dawn Journal

By Amber Jenkins

I stumbled upon this video earlier today. It’s Isaac Asimov, famous science fiction writer and biochemist, talking about global warming — back in January 1989. If you change the coloring of the video, the facial hair style, and switch out Asimov for someone else, the video could pretty much have been made today.

Asimov was giving the keynote address at the first annual meeting of The Humanist Institute. “They wanted me to pick out the most important scientific event of 1988. And I really thought that the most important scientific event of 1988 will only be recognized sometime in the future when you get a little perspective.”

What he was talking about was the greenhouse effect, which, he goes on to explain, is “the story everyone started talking about [in 1988], just because there was a hot summer and a drought.” (Sound familiar, letting individual weather events drive talk of whether the Earth’s long-term climate is heating up or cooling down??)

The greenhouse effect explains how certain heat-trapping (a.k.a. “greenhouse”) gases in our atmosphere keep our planet warm, by trapping infrared rays that Earth would otherwise reflect back out into space. The natural greenhouse effect makes Earth habitable — without our atmosphere acting like an electric blanket, the surface of the earth would be about 30 degrees Celsius cooler than it is now.

The problem comes in when humans tinker with this natural state of affairs. Our burning of fossil fuels (coal, oil and gas) constantly pumps out carbon dioxide — a heat-trapping gas — into the atmosphere. Our cutting down of forests reduces the number of trees there are to soak up some of this extra carbon dioxide. All in all, our atmosphere and planet heats up, (by about 0.6 degrees Celsius since the Industrial Revolution) with the electric blanket getting gradually thicker around us.

“I have been talking about the greenhouse effect for 20 years at least,” says Asimov in the video. “And there are other people who have talked about it before I did. I didn’t invent it.” As we’ve stressed here recently, global warming, and the idea that humans can change the climate, is not new.

As one blogger notes, Asimov’s words are as relevant today as they were in 1989. “It’s almost like nothing has happened in all this time.” Except that Isaac Asimov has come and gone, and the climate change he spoke of is continuing.

Asimov’s full speech can be seen here.

This post was written for “My Big Fat Planet,” a blog hosted by Amber Jenkins on NASA’s Global Climate Change site.

By Jane Houston Jones

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.

Julie Webster

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!

César Bertucci

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.

Cassini flies to within 880 kilometers (547 miles) of Titan’s surface during its 71st flyby of Titan, known as “T70,” the lowest in the entire mission. Image credit: NASA/JPL/Space Science Institute
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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!

Glenn Orton

We’ve had such great feedback and comments to our earlier post about the recent impact at Jupiter that we wanted to give you more details, plus answer some questions. My name is Glenn Orton, a senior research scientist at JPL. My colleague and fellow JPL blogger Leigh Fletcher is on a well-deserved vacation for a bit, and he filled in for me while I was at a conference talking about another aspect of our research and the Jupiter impact last week.

I’ve been on Anthony Wesley’s email list (as I am for many in the amateur astronomy community) for some time, so it wasn’t happenstance that I was aware of his Jupiter observation. Anthony is the Australian-based amateur astronomer who alerted the world to this big impact. When we received news of his discovery, we immediately wanted to verify it with some of the sophisticated telescopes NASA uses. Having actively observed in both the visible and infrared during the Shoemaker-Levy-9 impacts in 1994, I was aware that a quick verification was possible by looking at a wavelength with lots of gaseous absorption, which suppresses light reflected from Jupiter’s deep clouds.

This image shows a large impact shown on the bottom left on Jupiter’s south polar region captured on July 20, 2009, by NASA’s Infrared Telescope Facility in Mauna Kea, Hawaii. Image credit: NASA/JPL/Infrared Telescope Facility

Luck was on our side. Several months before the impact, our JPL team had been awarded observing time on NASA’s Infrared Telescope Facility (IRTF) atop Mauna Kea in Hawaii. We had the midnight to 6 a.m. shift (from our Pasadena office, which meant we started work at 3 a.m.) so much of our observing time would take place before Jupiter rose over Australian skies. Another piece of luck is that Anthony’s “day job” involves software engineering so he was able to watch the same telescope instrument status and data screens as we were, while we did remote-style observing from the IRTF over the Internet. He would also be doing his own (now *very important*) post-impact observing. Weather was just as “iffy” over Mauna Kea as in Australia, so it was lucky for all of us that we could catch this event.

With Leigh, several JPL summer interns and me huddled at our side-by-side computers at JPL (one with instrument controls and one showing the data), and Anthony online from Australia, we got started. We knew the location of Anthony’s dark spot would be coming over Jupiter’s rising limb (edge) just as our allotted time was beginning. A near-infrared spectrometer was in the center of the telescope from the previous observer. Although it wasn’t our instrument of choice (we wanted images!), it has a very nice guide camera sensitive to the near infrared, so we used it rather than waiting for the 20-40 minute hiatus needed by the telescope operator to move it out of the way and put our preferred instrument in its place. This turned out to be a good decision because the very first image showed us something brighter than anyplace else on the planet — exactly where Anthony’s dark feature was located. For me, this totally clinched the case that this was an impact. Even better was the fact that Anthony was looking on in real time. We e-mailed him what was obvious - he was *definitely* the father of a new impact!

Right after this we collected data that may help us sort out any exotic components of the impactor or of Jupiter’s atmosphere and just how high the particulates have spread. Then we switched instruments to something at much longer wavelengths that told us the temperatures were higher, and that ammonia gas had probably been pushed up from Jupiter’s troposphere (the lower part of the atmosphere) and ejected into its stratosphere (higher up in the atmosphere). We finished up with our preferred (more versatile) near-infrared camera and ended up, pretty tired, at 9 a.m. (this was a midnight to 6 a.m. run in Hawaii, and in California we were three hours ahead). Then we took some of the screen shots we’d been making and used them to submit a press release. Another person had already alerted a clearinghouse for important astronomical bulletins, so that was another thing that was important but that we didn’t need to do.

Now some responses to posts:

Good post from Mike Salway who is another one of the cadre of the world’s talented Jupiter observers. I should note that, in fact, there aren’t all that many of us who track the time evolution of phenomena in the planets in the professional community, either (see the web pages for the International Outer Planet Watch: http://dawn.ucla.edu/IJW/).

Asim. Neither NASA nor JPL is capable of observing everything in the sky. There is a program to search for asteroids whose orbits will intersect the Earth’s, but not at Jupiter. In fact, it’s unlikely this object could have been seen, given that it may have been at most a half kilometer in size. For Shoemaker-Levy 9, we were both lucky and the disruption of the comets left a lot of very shiny material around it which made it easier to see.

Denise. It hit quite a bit further south than the Shoemaker-Levy 9 fragments, almost at 60 deg S latitude.

Patrick, Jim, BobK. I suspect that the only link between this and the SL9 fragments is the voracious appetite of Jupiter, the great gravitational vacuum cleaner in that part of the solar system! SL9 fragments impacted from the south; this was from the east.

Leigh Fletcher

What an incredible few hours it’s been for astronomers everywhere, as we witness a chance of a lifetime event: evidence of a space rock of some sort slamming into Jupiter. Images taken after the impact show the debris field and aftermath of a gigantic collision that occurred in the southern polar region of the enormous planet.

An extremely dedicated and meticulous team of amateur astronomers observe Jupiter’s changing cloud patterns on a regular basis, and it came as an amazing surprise when Anthony Wesley, near Canberra, Australia, reported his Sunday-morning (July 19, 2009) observations (http://jupiter.samba.org/jupiter-impact.html) of a dark scar that bore all the hallmarks of the Shoemaker Levy 9 impacts at Jupiter in 1994. By an amazing coincidence, I was part of a team that had already been allocated time to observe Jupiter from the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Based on Anthony’s discovery, we were crowded around our computers at 3 a.m. PDT (with Anthony observing with us remotely from Australia) as the first near- and mid-infrared images started to come in… it was such an exciting moment, seeing the high altitude particles that had been lofted by the impact (they appear bright in the infrared). Anthony celebrated with us, but then the real work began. We celebrated and then rolled up our sleeves and began an exciting night of observations.

This image shows a large impact shown on the bottom left on Jupiter’s south polar region captured on July 20, 2009, by NASA’s Infrared Telescope Facility in Mauna Kea, Hawaii. Image credit: NASA/JPL/Infrared Telescope Facility

With the assistance of William Golisch at the IRTF, Glenn Orton and I viewed the impacts in as many wavelengths and spectra as we possibly could, as Jupiter rotated and carried the impact scar out of Earth’s view. We used these many views to show evidence for high temperatures at the impact location, and suggestions of ammonia and aerosols that had been carried high into the atmosphere. The observations were repeated again today, Tuesday morning, to track the shape and properties of the site. The scar is extremely large, almost as big as Earth and will continue to grow as Jupiter’s atmospheric winds and jet streams redistribute the material, and then, like Shoemaker-Levy 9, it will begin to fade in the coming weeks and months. Based on comparisons to SL-9, the impactor was likely to be small despite the large aftermath, maybe a few hundreds of metres across. Not only will this tell us a lot about impacts in the outer solar system, and how they contribute to the nature of the planets and icy moons, but they’ll also serve as a probe for the fundamental weather patterns in Jupiter’s high atmosphere.

Amateur observers continue to flood the Internet with new images of the dark spot at approximately 60 degrees south on Jupiter, and so far it looks as though the impact took place sometime in the 24 hours preceding Anthony’s discovery. The debris field now extends out to the west and northwest, with additional high-resolution images from the Keck telescope (Marchis, Wong, Kalas, Fitzgerald and Graham http://keckobservatory.org/index.php/news/jupiters_adds_a_feature/) showing the detailed morphology of the impact region. The hard work continues today, as an international team of planetary astronomers scrambles for time on some of the world’s largest astronomical facilities.

Finally, it’s a shame but perhaps not surprising that we didn’t see the collision, or the impactor itself, given the great distance to Jupiter. Like throwing a rock in a pond, we’re seeing and analyzing the splash that it’s made, and we can’t yet infer many details about the rock itself - the detailed shape of the impact site could help determine the trajectory and energy of the collision. But it certainly made quite a splash, and we hope to learn a lot about Jupiter from this event!

Anthony’s discovery is truly astounding, as it united astronomers in looking again at the gas giant Jupiter. It’s overwhelming and spectacularly exciting to watch this event unfolding before our eyes!

You can follow Leigh on Twitter at http://twitter.com/LeighFletcher