Archive for the ‘Columns’ Category

Slice of History: Granite Oil Slip Table

Tuesday, February 5th, 2013

By Julie Cooper

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
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.


The Giant Asteroid: A Retrospective

Thursday, January 31st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
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 that have been found 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


My Big Fat Planet: In Essence: Science Boiled Down

Thursday, January 10th, 2013

By Amber Jenkins

Map of the Arctic Sea and environs

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.


My Big Fat Planet: Pick of the Pics

Thursday, January 10th, 2013

By Amber Jenkins

View of Earth at Night    Earth at night, as seen by the Suomi National Polar-orbiting Partnership (NPP) satellite, a joint effort by NASA and the National Oceanic and Atmospheric Administration (NOAA). Courtesy of NASA Earth Observatory and NOAA National Geophysical Data Center.

This is a new image of our planet at night, as taken by a new NASA and National Oceanic and Atmospheric Administration (NOAA) satellite orbiting above us. Scientists recently unveiled this global composite image (and the one below), constructed using cloud-free nighttime images. They show the glow of natural and man-made phenomena across the planet in greater detail than ever seen before. City lights can tell us about how humans have spread across the globe.

View of Earth at Night

Many satellites are equipped to look at Earth during the day, when they can observe our planet fully illuminated by the sun. But with a new sensor onboard the NASA-NOAA Suomi National Polar-orbiting Partnership (NPP) satellite launched last year, scientists now can observe Earth’s atmosphere and surface during nighttime hours.

For more Earth at night images, see this article.

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


Slice of History: Viking Stereo Viewer

Tuesday, December 4th, 2012

By Julie Cooper

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/.

Viking Stereo ViewerViking Stereo Viewer — Photograph Number 324-1954

This interactive computer-based stereo viewing system was used to analyze Mars topography images generated by the cameras on NASA’s Viking 1 Mars lander. Two 17-inch video monitors faced a scanning stereoscope mounted between them on a table. Left and right lander camera image data were sent to the left and right monitors. Panning controls on the stereoscope helped align one image with the other to create a stereo image, 640 by 512 pixels in size. A mouse was used for finely controlled rotation of the monitors. An article about the system described a prototype mouse, used before this photo was taken in 1976. “The track ball is a baseball-sized sphere protruding from the top of a retaining box and capable of being rotated freely and indefinitely about its center …”

The resulting images could be displayed on additional monitors and were used to create contour maps and other images that aided lander surface operations. The system was developed by Stanford University and NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

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


Short Puffs Keep Dawn Chugging Along

Tuesday, December 4th, 2012

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft at Ceres
Artist’s concept of NASA’s Dawn spacecraft at its next target, the protoplanet Ceres. Image credit: NASA/JPL-Caltech

Dear Dawndroids,

Dawn is continuing to gently and patiently change its orbit around the sun. In September, it left Vesta, a complex and fascinating world it had accompanied for 14 months, and now the bold explorer is traveling to the largest world in the main asteroid belt, dwarf planet Ceres.

Dawn has spent most of its time since leaving Earth powering its way through the solar system atop a column of blue-green xenon ions emitted by its advanced ion propulsion system. Mission controllers have made some changes to Dawn’s operating profile in order to conserve its supply of a conventional rocket propellant known as hydrazine. Firing it through the small jets of the reaction control system helps the ship rotate or maintain its orientation in the zero-gravity of spaceflight. The flight team had already taken some special steps to preserve this precious propellant, and now they have taken further measures. If you remain awake after the description of what the changes are, you can read about the motivation for such frugality.

Dawn’s typical week of interplanetary travel used to include ion thrusting for almost six and two-thirds days. Then it would stop and slowly pirouette to point its main antenna to Earth for about eight hours. That would allow it to send to the giant antennas of NASA’s Deep Space Network a full report on its health from the preceding week, including currents, voltages, temperatures, pressures, instructions it had executed, decisions it had made, and almost everything else save its wonderment at operating in the forbidding depths of space so fantastically far from its planet of origin. Engineers also used these communications sessions to radio updated commands to the craft before it turned once again to fire its ion thruster in the required direction.

Now operators have changed the pace of activities. Every turn consumes hydrazine, as the spacecraft expels a few puffs of propellant through some of its jets to start rotating and through opposing jets to stop. Instead of turning weekly, Dawn has been maintaining thrust for two weeks at a time, and beginning in January it will only turn to Earth once every four weeks. After more than five years of reliable performance, controllers have sufficient confidence in the ship to let it sail longer on its own. They have refined the number and frequency of measurements it records so that even with longer intervals of independence, the spacecraft can store the information engineers deem the most important to monitor.

Although contact is established through the main antenna less often, Dawn uses one of its three auxiliary antennas twice a week. Each of these smaller antennas produces a much broader signal so that even when one cannot be aimed directly at Earth, the Deep Space Network can detect its weak transmission. Only brief messages can be communicated this way, but they are sufficient to confirm that the distant ship remains healthy.

In addition to turning less often, Dawn now turns more slowly. Its standard used to be the same blinding pace at which the minute hand races around a clock (fasten your seat belt!). Engineers cut that in half two years ago but returned to the original value at the beginning of the Vesta approach phase. Now they have lowered it to one quarter of a minute hand’s rate. Dawn is patient, however. There’s no hurry, and the leisurely turns are much more hydrazine-efficient.

With these two changes, the robotic adventurer will arrive at Ceres in 2015 with about half of the 45.6-kilogram (101-pound) hydrazine supply it had when it rocketed away from Cape Canaveral on a lovely September dawn in 2007. Mission planners will be able to make excellent use of it as they guide the probe through its exploration of the giant of the main asteroid belt.

Any limited resource should be consumed responsibly, whether on a planet or on a spaceship. Hydrazine is not the only resource that Dawn’s controllers manage carefully, but let’s recall why this one has grown in importance recently.

› Continue reading Marc Rayman’s Dawn Journal


Slice of History: 1944 Map of JPL

Thursday, November 1st, 2012

By Julie Cooper

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/.

1944 Map of JPL1944 Map of JPL — Photograph Number HC 3-1294

On October 31, NASA’s Jet Propulsion Laboratory in Pasadena, Calif., celebrated its 76th anniversary. It began with a few individuals working on the Caltech campus and testing rocket motors in the Arroyo Seco. By the time this 1944 map of “The Project” was created, JPL was supported by Army Air Corps contracts and the site included more than 50 offices, labs and test facilities.

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


Dawn Comes Closer to Go Farther

Thursday, November 1st, 2012

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft at Ceres
Artist’s concept of NASA’s Dawn spacecraft at its next target, the protoplanet Ceres. Image credit: NASA/JPL-Caltech

Dear Indawnspensable Readers,

Dawn is making good progress on the second segment of its cosmic travels. Following more than a year of arduous but sensationally productive and exciting work revealing the fascinating character of the giant protoplanet Vesta, it is now patiently pursuing its next target, the mysterious dwarf planet Ceres, which resides farther from the sun. For the second (and final) time in its interplanetary journey, however, Dawn is about to turn around, going closer to the sun rather than farther away.

In August 2008, we saw in detail how it could be that even as the bold explorer travels outward in the solar system from Earth, past Mars, to Vesta, and then on to Ceres, it could occasionally appear to reverse course temporarily. We present here a shorter explanation for those readers who did not memorize the log explaining this perplexing behavior (you know who you are, and we do as well, but your secret remains safe under the terms of our reader privacy agreement).

Dawn orbits the sun, as do Vesta, Ceres, the other residents of the main asteroid belt, and the planets. All orbits, whether of these objects around the star at the center of our solar system, artificial satellites or the moon in orbit around Earth, or even Dawn when it was in orbit around Vesta, are ellipses (like flattened circles). Earth, for example, orbits the sun at an average distance of 150 million kilometers (93.0 million miles), which astronomers call one astronomical unit (AU). During its year-long revolution, however, our planet comes in to 0.98 AU from the sun and goes out to 1.02 AU. Earthlings manage quite nicely with these small variations. (Note that the seasons are not caused by the changes in distance but instead are a result of the tilt of Earth’s axis and thus the differing angles at which the warming rays of the sun arrive during the year. If the sun’s distance were all that mattered, the northern and southern hemispheres would have the same seasons.) So, orbiting bodies move smoothly between a minimum and a maximum range from their gravitational masters rather than remaining at a constant distance.

When Dawn was in orbit around Vesta, it accompanied that world on its regular journey around the sun. The table last month showing the probe’s progress over the five years of its deep space trek reminds us that Vesta’s path brings it as close to the sun as 2.15 AU and takes it out to 2.57 AU.

If Dawn had remained in orbit around Vesta, it would have continued to follow the same elliptical course as its host in the asteroid belt. The pair would have reached their maximum solar distance next month and then would have fallen back to 2.15 AU in September 2014. While visiting Vesta was extremely gratifying, this explorer’s ambitions are greater. It broke free of Vesta’s grip, its sights set on a new and distant alien destination.

Now the spacecraft is in its own independent orbit around the sun, and the persistent but gentle pressure of its advanced ion propulsion system gradually reshapes that orbit. At any moment, the orbit is an ellipse, and an instant later, it is a slightly different ellipse, courtesy of the thrust. As Dawn departed from Vesta only last month, its orbit is not yet dramatically different, but over the course of the coming years, the effect of the thrusting will be to change the orbit tremendously. To reach Ceres in 2015, the ship will enlarge and tip its elliptical course to match the motion of the dwarf planet around the sun. (Some of the parameters characterizing each object’s orbit are shown here.)

Although the ship’s orbit is growing, it will reach the current high point on Nov. 1. It will then be 2.57 AU from the sun and, just as in 2008 (albeit at a smaller distance), it will begin moving closer, even as it continues to thrust.

If Dawn stopped thrusting on Nov. 1, its elliptical orbit would carry it down to 2.19 AU from the sun in September 2014. That’s a higher orbit than Vesta’s but still well below what it needs to be for the rendezvous with Ceres. Astute readers have already anticipated that the plan is not to stop thrusting but to continue reworking the trajectory, just as a ceramicist gradually achieves a desired shape to create the envisioned artistic result. The ongoing thrusting will raise the low point of the orbit, so if the ship follows the flight plan, it will descend only to 2.45 AU in October 2013 before sailing outward again. By May 2014 it will have risen to the same solar altitude as it is now. All the thrusting in the interim will have altered its course so much, however, that it will not turn around then; rather, it will continue ascending to keep its 2015 appointment with Ceres.

› Continue reading Marc Rayman’s Dawn Journal