Each month in “Slice of History” we feature a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at https://beacon.jpl.nasa.gov/.

Infrared Ear Thermometer — Photograph Number JPL-17459Ac

In 1991, Diatek Corporation of San Diego put a new infrared thermometer - Model 7000 - on the market. Early electronic thermometers had been used by some hospitals and doctors’ offices for several years before that time, but this Diatek model was a pioneering effort to modify space-based infrared sensors for a medical infrared thermometer. The underlying technology was developed by NASA’s Jet Propulsion Laboratory in Pasadena, Calif., for missions including the Infrared Astronomical Satellite, or IRAS. IRAS measured the temperature of stars and planets by reading the infrared radiation emitted from them, while the thermometer almost instantly determined body temperature by measuring the energy emitted from the eardrum - quite an advancement in medical technology. Diatek was part of the JPL Technology Affiliates Program, or TAP, in the late 1980s and received help from JPL personnel in adapting infrared sensor technology to this new product.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.

In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear Dawnscerning Readers,

Nearly three times as far from Earth as the sun is, the Dawn spacecraft is making very good progress on its ambitious trek from Vesta to Ceres. After a spectacular adventure at the second most massive resident of the main asteroid belt between Mars and Jupiter, Dawn used its extraordinary ion propulsion system to leave it behind and undertake the long journey to a dwarf planet.

Ceres orbits the sun outside Vesta’s orbit, yet Dawn is now closer to the sun than both of these alien worlds. How can it be that as the probe climbs from one to the other, it seems to be falling inward? Perhaps the answer lies in the text below; let’s venture on and find out!

On Halloween we discussed why Dawn is heading in toward the sun, but this question is different. Vesta also is getting closer to the sun, but what’s of interest now is that Dawn, despite its more remote destination, has been approaching the sun more quickly. That earlier log stands out as the best one ever written on this exciting mission in the entire history of October 2012, but if you prefer not to visit it now, we can summarize here the explanation for the spacecraft moving toward the sun. Like all members of the sun’s entourage, Vesta and Ceres follow elliptical orbits, their distances from the master of the solar system growing and shrinking as they loop around it. Even Earth’s orbit, although nearly round, certainly is not perfectly circular. Our planet is a little closer to the sun in the northern hemisphere winter (southern hemisphere summer) than it is in the summer (southern hemisphere winter). Dawn’s orbit is elliptical as well, so it naturally moves nearer to the sun sometimes, and now is such a time. But that does not address why it is currently closer to the sun than Vesta, even though it is seeking out the more distant Ceres.

Because it will orbit Ceres, and not simply fly past it (which would be significantly easier but less valuable), Dawn must make its own orbit around the sun be identical to its target’s. But that is not the entire story. After spending 14 months orbiting Vesta, Dawn’s challenge is more than to change the shape of its orbit to match Ceres’s. The spacecraft also must be at the same place in Ceres’s heliocentric orbit that Ceres itself is.

It would not be very rewarding to follow the same looping path around the sun but always be somewhere else on that path. You can visualize this if you have one of the many defective — er, exotic clocks from the Dawn gift shop on your planet that have two minute hands. If the clock starts with one hand pointed at 12 and another pointed at 1, they will take the same repetitive route, but neither hand will ever catch up with the other. For Dawn’s goal of exploring Ceres, this would not prove satisfying. Therefore, part of the objective of the ion thrusting is to ensure the spacecraft arrives not only on the same heliocentric course as Ceres but is there when Ceres is also.

This is a problem familiar to all readers who have maneuvered in orbit, where the principles of orbital mechanics are the rules of the road. To solve it, we rely on one of the laws that we have addressed many times in these logs: objects in a lower orbit travel faster. We described this in more detail in February, and we can recall the essential idea here. The gravitational attraction of any body, whether it is the sun, Earth, a black hole, or anything else, is greater at shorter ranges. So to balance that strong inward pull, an orbiter is compelled to race around quickly. At higher orbits, where gravity is weaker, a more leisurely orbital pace suffices.

We can take advantage of this characteristic of orbits. If we drop to a slightly lower orbit, we travel along more swiftly. That is precisely what Dawn needs to do in order to ensure that when it finishes expanding and tilting its orbit in 2015 so that it is the same as Ceres’s, it winds up at the same location as its target. This would be like speeding up the minute hand that had begun at the 12, allowing it to catch up with the hand that would otherwise always be leading it.

› Continue reading Marc Rayman’s Dawn Journal

Each month in “Slice of History” we feature a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at https://beacon.jpl.nasa.gov/.

100 kilogauss magnet — Photograph Number 328-430Ac

An intense magnetic field facility was completed in 1964 by the Physics Section of the Space Sciences Division at NASA’s Jet Propulsion Laboratory. It was intended for use in studying superconductors, spectroscopy and new materials, and in other experiments where a wider range of measurements was possible because of the high magnetic field. This photo shows the magnet at center. The system also included a control room, cooling tower, pumps and a heat exchanger. The generator was located in a separate room because of the noise. Water was pumped through the magnet at about 440 gallons per minute, to regulate the temperature of the large copper coil in the center of the magnet. The closed loop system contained distilled water with sodium nitrite for corrosion control.

According to a technical report about the facility, the magnetic field of the magnet and bus bars penetrated nearby rooms to a depth of about 30 feet. Any iron that could be attracted to the magnet had to be removed from the area.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.

Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Indawnstrious Readers,

In the depths of the main asteroid belt between Mars and Jupiter, far from Earth, far even from any human-made object, Dawn remains in silent pursuit of dwarf planet Ceres. It has been more than six months since it slipped gracefully away from the giant protoplanet Vesta. The spacecraft has spent 95 percent of the time since then gently thrusting with its ion propulsion system, using that blue-green beam of high velocity xenon ions to propel itself from one alien world to another.

The ship set sail from Earth more than two thousand days ago, and its voyage on the celestial seas has been wonderfully rewarding. Its extensive exploration of Vesta introduced humankind to a complex and fascinating place that had only been tantalizingly glimpsed from afar with telescopes beginning with its discovery 206 years ago today. Thanks to the extraordinary capability of ion propulsion, Dawn was able to spend 14 months orbiting Vesta, observing dramatic landscapes and exotic features and collecting a wealth of measurements that scientists will continue to analyze for many years.

When it was operating close to Vesta, the spacecraft was in frequent contact with Earth. It took Dawn quite a bit of time to beam the 31,000 photos and other precious data to mission control. In addition, engineers needed to send a great many instructions to the distant adventurer to ensure it remained healthy and productive in carrying out its demanding work in the unforgiving depths of space.

Dawn is now more than 20 times farther from Vesta than the moon is from Earth. Alone again and on its long trek to Ceres, it is not necessary for the ship to be in radio contact as often. As we saw in November, the spacecraft now stops ion thrusting only once every four weeks to point its main antenna to Earth. This schedule conserves the invaluable hydrazine propellant the explorer will need at Ceres. But communicating less frequently does not mean the mission operations team is any less busy. Indeed, as we have explained before, “quiet cruise” consists of a considerable amount of activity.

Each time Dawn communicates with Earth, controllers transmit a second-by-second schedule for the subsequent four weeks. They also load a detailed flight profile with the ion throttle levels and directions for that period. It takes about three weeks to calculate and formulate these plans and to analyze, check, double check, and triple check them to ensure they are flawless before they can be radioed to Dawn.

In addition to all the usual information Dawn needs to keep flying smoothly, operators occasionally include some special instructions. As one example, over the last few months, they have gradually lowered the temperatures of some components slightly in order to reduce heater power. When Dawn stretched out its solar array wings shortly after separating from the Delta rocket on September 27, 2007, its nearly 65-foot wingspan was the longest of any NASA interplanetary probe. The large area of solar cells is needed to collect enough light from the distant sun to power the ion propulsion system and all other spacecraft systems. Devoting a little less power to heaters allows more power to be applied to ionizing and accelerating xenon, yielding greater thrust. With two and a half years of powered flight required to travel from Vesta to Ceres, even a little extra power can make a worthwhile difference to a mission that craves power.

Most temperature adjustments are only two degrees Celsius (3.8 degrees Fahrenheit) at a time, but even that requires careful analysis and investigation, because lowering the temperature of one component may affect another. Xenon and hydrazine propellants need to be maintained in certain ranges, and the lines they flow through follow complicated paths around the spacecraft, so the temperatures all along the way matter. Most of the hardware onboard, from valves and switches to electronics to structural mounts for sensitively aligned units, needs to be thermally regulated to keep Dawn shipshape.

It can take hours for a component to cool down and stabilize at a new setting, and sometimes the change won’t even occur until the spacecraft has turned away to resume thrusting, when the faint warmth of the sun and the deep cold of black space affect different parts of the complex robot. Then it will be another four weeks until engineers will receive a comprehensive report on all the temperatures, so they need to be cautious with each change.

› Continue reading Marc Rayman’s Dawn Journal

Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Impordawnt Readers,

The indefatigable Dawn spacecraft is continuing to forge through the main asteroid belt, gently thrusting with its ion propulsion system. As it gradually changes its orbit around the sun, the distance to dwarf planet Ceres slowly shrinks. The pertinacious probe will arrive there in 2015 to explore the largest body between the sun and Neptune that has not yet been glimpsed by a visitor from Earth. Meanwhile, Vesta, the fascinating alien world Dawn revealed in 2011 and 2012, grows ever more distant. The mini-planet it orbited and studied in such detail now appears only as a pinpoint of light 15 times farther from Dawn than the moon is from Earth.

Climbing through the solar system atop a column of blue-green xenon ions, Dawn has a great deal of powered flight ahead in order to match orbits with faraway Ceres. Nevertheless, it has shown quite admirably that it is up to the task. The craft has spent more time thrusting and has changed its orbit under its own power more than any other ship from Earth. While most of the next two years will be devoted to still more thrusting, the ambitious adventurer has already accomplished much more than it has left to do. And now it is passing an interesting milestone on its interplanetary trek.

With all of the thrusting Dawn has completed, it has now changed its speed by 7.74 kilometers per second (17,300 mph), and the value grows as the ion thrusting continues. For space enthusiasts from Earth, that is a special speed, known as “orbital velocity.” Many satellites, including the International Space Station, travel at about that velocity in their orbits. So does this mean that Dawn has only now achieved the velocity necessary to orbit Earth? The short answer is no. The longer answer constitutes the remainder of this log.

We have discussed some of these principles before, but they are counterintuitive and questions continue to arise. Rather than send our readers on a trajectory through the history of these logs even more complicated than Dawn’s flight through the asteroid belt, we will revisit a few of the ideas here. (After substantial introspection, your correspondent granted and was granted permission to reuse not only past text but also future text.)

While marking Dawn’s progress in terms of its speed is a convenient description of the effectiveness of its maneuvering, it is not truly a measure of how fast it is moving. Rather, it is a measure of how fast it would be moving under very special (and unrealistic) circumstances. To understand this, we need to look at the nature of orbits in general and Dawn’s interplanetary trajectory in particular.

The overwhelming majority of craft humans have sent into space have remained in the vicinity of Earth, accompanying that planet on its annual revolutions around the sun. All satellites of Earth (including the moon) remain bound to it by its gravity. (Similarly, Dawn spent much of 2011 and 2012 as a satellite of distant Vesta, locked in the massive body’s gravitational grip.) As fast as satellites seem to travel compared to terrestrial residents, from the larger solar system perspective, their incessant circling of Earth means their paths through space are not very different from Earth’s itself. Consider the path of a car racing around a long track. If a fly buzzes around inside the car, to the driver it may seem to be moving fast, but if someone watching the car from a distance plotted the fly’s path, on average it would be pretty much like the car’s.

Everything on the planet and orbiting it travels around the sun at an average of 30 kilometers per second (67,000 mph), completing one full solar orbit every year. To undertake its interplanetary journey and travel elsewhere in the solar system, Dawn needed to break free of Earth’s grasp, and that was accomplished by the rocket that carried it to space more than five years ago. Dawn and its erstwhile home went their separate ways, and the sun became the natural reference for the spacecraft’s position and speed on its voyage in deep space.

Despite the enormous push the Delta II rocket delivered (with affection!) to Dawn, the spacecraft still did not have nearly enough energy to escape from the powerful sun. So, being a responsible resident of the solar system, Dawn has remained faithfully in orbit around the sun, just as Earth and the rest of the planets, asteroids, comets, and other members of the star’s entourage have.

Whether it is for a spacecraft or moon orbiting a planet, a planet or Dawn orbiting the sun, the sun orbiting the Milky Way galaxy, or the Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to a sizeable fraction of our readership), any orbit is the perfect balance between the inward tug of gravity and the inexorable tendency of objects to travel in a straight path. If you attach a weight to a string and swing it around in a circle, the force you use to pull on the string mimics the gravitational force the sun exerts on the bodies that orbit it. The effort you expend in keeping the weight circling serves constantly to redirect its path; if you let go of the string, the weight’s natural motion would carry it away in a straight line (ignoring the effect of Earth’s gravity).

The force of gravity diminishes with distance, so the sun’s pull on a nearby body is greater than on a more distant one. Therefore, to remain in orbit, to balance the relentless tug of gravity, the closer object must travel faster, fighting the stronger pull. The same effect applies at Earth. Satellites that orbit very close (including, for example, the International Space Station, around 400 kilometers, or 250 miles, from the surface) must streak around the planet at about 7.7 kilometers per second (17,000 mph) to keep from being pulled down. The moon, orbiting almost 1000 times farther above, needs only to travel at about 1.0 kilometers per second (less than 2300 mph) to balance Earth’s weaker hold at that distance.

Notice that this means that for an astronaut to travel from the surface of Earth to the International Space Station, it would be necessary to accelerate to quite a high speed to rendezvous with the orbital facility. But then once in orbit, to journey to the much more remote moon, the astronaut’s speed eventually would have to decline dramatically. Perhaps speed tells an incomplete story in describing the travels of a spacecraft, just as it does with another example of countering gravity.

A person throwing a ball is not that different from a rocket launching a satellite (although the former is usually somewhat less expensive and often involves fewer toxic chemicals). Both represent struggles against Earth’s gravitational pull. To throw a ball higher, you have to give it a harder push, imparting more energy to make it climb away from Earth, but as soon as it leaves your hand, it begins slowing. For a harder (faster) throw, it will take longer for Earth’s gravity to stop the ball and bring it back, so it will travel higher. But from the moment it leaves your hand until it reaches the top of its arc, its speed constantly dwindles as it gradually yields to Earth’s tug. The astronaut’s trip from the space station to the moon would be accomplished by starting with a high speed “throw” from the low starting orbit, and then slowing down until reaching the moon.

The rocket that launched Dawn threw it hard enough to escape from Earth, sending it well beyond the International Space Station and even the moon. Dawn’s maximum speed relative to Earth on launch day was so high that Earth could not pull it back. As we saw in the explanation of the launch profile, Dawn was propelled to 11.46 kilometers per second (25,640 mph), well in excess of the space station’s orbital speed given three paragraphs above. But it has remained under the sun’s control.

Now we can think of the general problem of flying elsewhere in space as similar to climbing a hill. For terrestrial hikers, the rewards of ascent come only after doing the work of pushing against Earth’s gravity to reach a higher elevation. Similarly, Dawn is climbing a solar system hill with the sun at the bottom. It started part way up the hill at Earth; and its first rewards were found at a higher elevation, where Vesta, traveling around the sun at only about two thirds of Earth’s speed, revealed its fascinating secrets to the visiting ship. The ion thrusting now is propelling it still higher up the hill toward Ceres, which moves even more slowly to balance the still-weaker pull of the sun.

› Continue reading Marc Rayman’s Dawn Journal for more on how Dawn achieved orbital velocity

In this new series on “Big Fat Planet,” we will answer selected questions about Earth’s climate submitted by readers. Recently, a reader asked: “Is there still time to reduce climate change, or is it too late?” The following answer is from Dr. Chip Miller, a researcher specializing in remote sensing of carbon dioxide and other greenhouse gases at NASA’s Jet Propulsion Laboratory. He is principal investigator of the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) and was deputy principal investigator for NASA’s Orbiting Carbon Observatory satellite mission, which was designed to measure atmospheric carbon dioxide from space.

This is a question that has been asked many times and many studies have investigated similar questions: What level of climate change is “acceptable”? What constitutes “dangerous interference” in the climate system?

The short answer is that it’s not too late to act, but our past actions may have already locked in certain outcomes and action is needed to avoid more substantial impacts in the future.

In the 1990s and early 2000s it was generally felt that a doubling of carbon dioxide (CO2) in the atmosphere compared to pre-industrial levels - that is, CO2 concentrations increasing to about 500 parts per million (ppm) - was “acceptable.” However, the series of studies from the Intergovernmental Panel on Climate Change (IPCC) has found that as climate models improve, average worldwide surface temperature is projected to increase well beyond the “acceptable” level of 2.0 degrees Celsius (3.6 degrees Fahrenheit) by 2100. (See the IPCC website for the reports and most recent information.)

Jim Hansen (head of NASA’s Goddard Institute for Space Studies) has been one of the more outspoken advocates of curtailing CO2 emissions immediately to return atmospheric CO2 levels to about 350 ppm (the level of carbon dioxide that was in the air in the late 1980s). The challenge here is that even if human emissions of CO2 were cut to zero today, there is an inertia in the climate system that would continue for hundreds to thousands of years as the system attempts to re-equilibrate. (See Hansen’s Royal Society paper, “Climate change and trace gases,” for more details.)

Michael Oppenheimer [Professor of Geosciences and International Affairs at Princeton University] and colleagues have taken a different approach to assessing climate change risk - they define the likelihood of certain environmental outcomes for different levels of atmospheric CO2 accumulation. (See their 2002 Science paper, “Dangerous climate impacts and the Kyoto Protocol,” for a look at three potential outcomes at different CO2 levels.)

Further reading:

“Perception of climate change,” J. Hansen, M. Sato & R. Ruedy, Proceedings of the National Academy of Sciences (6 August 2012); doi: 10.1073/pnas.1205276109.

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

Each month in “Slice of History” we feature a historical photo from the JPL Archives. See more historical photos and explore the JPL Archives at https://beacon.jpl.nasa.gov/.

Granite Oil Slip Table — Photograph Number P-2784Ac

In 1963, spacecraft vibration tests were conducted in the Environmental Laboratory at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. A slab of granite, coated in oil, provided a smooth and stable base for the magnesium slip plate, test fixture and Ranger 6 spacecraft mounted on it. There were vibration exciters (shakers) on each end, capable of more than 25,000 pounds of force. The horizontal fixture at left was used for low frequency vibration testing, and the equipment was capable of testing along all three spacecraft axes.

During the 1960s, Ranger, Surveyor and Mariner spacecraft were developed, built and tested at JPL. Because of the heavy use, a similar but smaller test fixture was used for vibration tests on spacecraft components and assemblies. Building 144 still contains test facilities, but this equipment was removed and the room now contains an acoustic chamber.

This post was written for “Historical Photo of the Month,” a blog by Julie Cooper of JPL’s Library and Archives Group.

As NASA’s Dawn spacecraft takes off for its next destination, this mosaic synthesizes some of the best views the spacecraft had of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA
› full image and caption

Dear Dawnt Look Backs,

Its long and daring interplanetary journey continuing smoothly, Dawn is making good progress in gradually reshaping its orbit around the sun. Its uniquely efficient ion propulsion system is gently bringing it closer to its next destination, dwarf planet Ceres, and ever farther from its previous one, Vesta. Although the robotic explorer’s sights are set firmly ahead, let’s take one last look back at the fascinating alien world it unveiled during its 14 months in orbit there.

Vesta, the second most massive resident of the main asteroid belt between Mars and Jupiter, was discovered in 1807. For more than two centuries thereafter, the mysterious object appeared as little more than a fuzzy patch of light among the stars. The only one of the millions of main belt asteroids to be bright enough to be visible to the naked eye, Vesta beckoned, but its invitation was not answered until Dawn arrived in July 2011, nearly four years after it left distant Earth. The cosmic ambassador is the only spacecraft ever to have orbited an object in the main asteroid belt, and its ambitious mission would have been impossible without ion propulsion.

Dawn found a complex and exotic place, and it returned a fabulously rich collection of pictures and other measurements that will continue to be analyzed for many, many years. For now, we will simply touch on a very few of the many insights that already have been illuminated by the light of Dawn.

Scientists recognize Vesta as being more like a mini-planet than like the chips of rock most people think of as asteroids. The behemoth is 565 kilometers (351 miles) wide at the equator and has a surface area more than twice that of California (although it is populated by far fewer eccentrics, billionaires, and other colorful characters found in that state). Dawn’s measurements of the gravity field provide good evidence that Vesta separated into layers, much like Earth did as the planet was forming. Vesta’s dense core, composed principally of iron and nickel, may be 200 to 250 kilometers (125 to 150 miles) across. Surrounding that is the mantle, which in turn is covered by the veneer of the crust, about 20 kilometers (12 miles) thick. The once-molten core is now solid (in contrast to Earth’s, which remains hot enough to be liquid), but the differentiation into layers gives Vesta a key distinction from most asteroids. Because it was likely still in the process of accumulating material to become a full-sized planet when Jupiter’s immense gravity terminated its growth, scientists often refer to Vesta as a protoplanet.

Among the most prominent features of the alien landscape is a huge gouge out of the southern hemisphere so large that its presence was inferred from observations with the Hubble Space Telescope. Dawn found this gigantic crater to be even deeper and wider than expected, penetrating about 19 kilometers (12 miles) and spanning more than 500 kilometers (310 miles), or nearly 90 percent of the protoplanet’s equatorial diameter.

The yawning hole is now known as Rheasilvia, after the Vestal Virgin who not only was the mythical mother of Romulus and Remus, but also surely would have been astounded by the spectacular sights on Vesta as well as the spacecraft’s capability to point any user-defined body vector in a time-varying inertial direction defined by Chebyshev polynomials. As Dawn has brought Vesta into focus, cartographers have needed labels for the myriad features it has discovered. The International Astronomical Union names Vestan craters for Vestal Virgins and other famous Roman women; mountains, canyons, and other structures are named for towns and festivals associated with the Vestal Virgins.

Vesta dates to the dawn of the solar system, more than 4.5 billion years ago, and its age shows. Myriad craters tell the story of a timeworn surface that has been subjected to the rough and tumble conditions of life in the asteroid belt ever since. A virtual rain of space rocks has fallen upon it. While Rheasilvia records the most powerful punch, from an object as much as 50 kilometers (30 miles) across, there are at least seven craters, some quite ancient indeed, more than 150 kilometers (nearly 100 miles) in diameter. As the eons pass, craters degrade and become more difficult to discern, their crisp shapes eroded by subsequent impacts large and small.

The long history of cratering is particularly evident in the startling difference between the northern and southern hemispheres. The north is very densely cratered, but the south is not. Why? The titanic blow that carved out Rheasilvia is estimated to have occurred over one billion years ago. It excavated a tremendous volume of material. Much of it fell back to the surface, wiping it clean, so the cratering record had to start all over again. Recall that the crater itself is 500 kilometers (310 miles) in diameter, and scientists estimate that 50 kilometers (30 miles) outside the rim, the debris may have piled about 5 kilometers (3 miles) high. Even at greater distances, preexisting features would have been partially or completely erased by the thick accumulation. The effect did not reach to the northern hemisphere, however, so it retained the craters than had formed before this enormous impact.

Some of the rocks were ejected with so much energy that they broke free of Vesta’s gravitational grip, going into orbit around the sun. They then went their own way as they were yanked around by the gravitational forces of Jupiter and other bodies, and many of them eventually made it to the part of the solar system where your correspondent and some of his readers spend most of their time: Earth. When our planet’s gravity takes hold of one of these Vesta escapees, it pulls the rock into its atmosphere. Some lucky witness might even observe it as a meteor. Its blazing flight to the ground is not the end of its glory, however, for these rocks are prized by planetary geologists and other enthusiasts who want a souvenir from that impact.

Scientists now know that about 6 percent of the meteorites seen to fall to Earth originated on Vesta. Six percent! One of every 16 meteorites! This is an astonishingly large fraction. Apart from Mars and the moon, Vesta is the only known source of specific meteorites. Although rocks from Vesta had to travel much farther, they far outnumber meteorites from these other two more familiar celestial bodies.

Combining laboratory studies of the numerous samples of Vesta with Dawn’s measurements at the source provides an extraordinary opportunity to gain insights into the nature of that remote world. Meteorites from Vesta are so common that they are often displayed in museums (occasionally even without the curators’ awareness of their special history) and can be obtained from many vendors. Anyone who has seen or held one surely must be moved by contemplating its origin, so distant in space and time, from well beyond Mars and long before animal or plant life arose on Earth.

› Continue reading Marc Rayman’s Dawn Journal for more Vesta history

Artist’s concept of NASA’s Dawn spacecraft departing the giant asteroid Vesta. Image credit: NASA/JPL-Caltech

Dawn concluded 2012 almost 13,000 times farther from Vesta than it began the year. At that time, it was in its lowest orbit, circling the alien world at an average altitude of only 210 kilometers (130 miles), scrutinizing the mysterious protoplanet to tease out its secrets about the dawn of the solar system.

To conduct its richly detailed exploration, Dawn spent nearly 14 months in orbit around Vesta, bound by the behemoth’s gravitational grip. In September they bid farewell, as the adventurer gently escaped from the long embrace and slipped back into orbit around the sun. The spaceship is on its own again in the main asteroid belt, its sights set on a 2015 rendezvous with dwarf planet Ceres. Its extensive ion thrusting is gradually enlarging its orbit and taking it ever farther from its erstwhile companion as their solar system paths diverge.

Meanwhile, on faraway Earth (and all the other locations throughout the cosmos where Dawnophiles reside), the trove of pictures and other precious measurements continue to be examined, analyzed, and admired by scientists and everyone else who yearns to glimpse distant celestial sights. And Earth itself, just as Vesta, Ceres, Dawn, and so many other members of the solar system family, continues to follow its own orbit around the sun.

Thanks to a coincidence of their independent trajectories, Earth and Dawn recently reached their smallest separation in well over a year, just as the tips of the hour hand and minute hand on a clock are relatively near every 65 minutes, 27 seconds. On Dec. 9, they were only 236 million kilometers (147 million miles) apart. Only? In human terms, this is not particularly close. Take a moment to let the immensity of their separation register. The International Space Station, for example, firmly in orbit around Earth, was 411 kilometers (255 miles) high that day, so our remote robotic explorer was 575 thousand times farther. If Earth were a soccer ball, the occupants of the orbiting outpost would have been a mere seven millimeters (less than a third of an inch) away. Our deep-space traveler would have been more than four kilometers (2.5 miles) from the ball. So although the planet and its extraterrestrial emissary were closer than usual, they were not in close proximity. Dawn remains extraordinarily far from all of its human friends and colleagues and the world they inhabit.

As the craft reshapes its solar orbit to match Ceres’s, it will wind up farther from the sun than it was while at Vesta. (As a reminder, see the table here that illustrates Dawn’s progress to each destination on its long interplanetary voyage.) We saw recently, however, that the route is complex, and the spacecraft is temporarily approaching the sun. Before the ship has had time to swing back out to a greater heliocentric range, Earth will have looped around again, and the two will briefly be even a little bit closer early in 2014. After that, however, they will never be so near each other again, as Dawn will climb higher and higher up the solar system hill, its quest for new and exciting knowledge of distant worlds taking it farther from the sun and hence from Earth.

› Continue reading Marc Rayman’s Dawn Journal to learn how to approximate Dawn’s position in the sky on Jan. 21 and 22

An interesting recent paper from Dr. Son Nghiem at NASA’s Jet Propulsion Laboratory and colleagues finds that the bottom of the Arctic Ocean controls the pattern of sea ice thousands of feet above on the water’s surface. The seafloor topography exerts its control not only locally, in the Bering, Chukchi, Beaufort, Barents and Greenland Seas, but also spanning hundreds to thousands of miles across the Arctic Ocean.

How? The seafloor influences the distribution of cold and warm waters in the Arctic Ocean where sea ice can preferentially grow or melt. Geological features on the ocean bottom also guide how the sea ice moves, along with influence from surface winds.

Interestingly, the study also links the bottom of the Arctic Ocean with cloud patterns up in the sky. The ocean bottom affects sea ice cover, which affects the amount of vapor coming from the surface of the ocean out into the air, which in turn influences cloud cover.

The researchers, who also come from NASA’s Goddard Space Flight Center, the Applied Physics Laboratory and the National/Naval Ice Center in the U.S., use sea ice maps taken from space with NASA’s QuickSCAT satellite, as well as measurements from drifting buoys in the Arctic Ocean. They compare the sea ice and seafloor topography patterns to identify the connection between the two.

Bottom line:

Since the seafloor does not change significantly over many years, sea ice patterns can form repeatedly and persist around certain underwater geological features. So computer models need to incorporate these features in order to improve their forecasts of how ice cover will change over the short- and long-term. This ‘memory’ of the underwater topography could help refine our predictions of what will happen to ice in the Arctic as the climate changes.

Source:

“Seafloor Control on Sea Ice,” S. V. Nghiem, P. Clemente-Colon, I.G. Rigor, D.K. Hall & G. Neumann, Deep Sea Research Part II: Topical Studies in Oceanography, Volumes 77-80, pp 52-61 (2012).

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