Mars Reconnaissance Orbiter

Lasers Boost Space Communications.

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Before NASA even existed, science-fiction writer Arthur C. Clarke in 1945 imaginedspacecraft that could send messages back to Earth using beams of light. After decades of setbacks and dead ends, the technology to do this is finally coming of age.

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Two spacecraft set for launch in the coming weeks will carry lasers that allow data to be transferred faster than ever before. One, scheduled for take-off on 5 September, is NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE), a mission that will beam video and scientific data from the Moon. The other, a European Space Agency (ESA) project called Alphasat, is due to launch on 25 July, and will be the first optical satellite to collect large amounts of scientific data from other satellites.

“This is a big step forward,” says Hamid Hemmati, a specialist in optical communications at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Europe is going beyond demonstrations for the first time and making operational use of the technology.”

These lasers could provide bigger pipes for a coming flood of space information. New Earth-observation satellites promise to deliver petabytes of data every year. Missions such as the Mars Reconnaissance Orbiter (MRO) already have constraints on the volume of data they can send back because of fluctuations in download rates tied to a spacecraft’s varying distance from Earth. “Right now, we’re really far from Earth, so we can’t fit as many images in our downlink,” says Ingrid Daubar, who works on the MRO’s HiRISE camera at the University of Arizona in Tucson. Laser data highways could ultimately allow space agencies to kit their spacecraft with more sophisticated equipment, says John Keller, deputy project scientist for NASA’s Lunar Reconnaissance Orbiter (LRO). That is not yet possible, he says. “We’re limited by the rate at which we can download the data.”

Today’s spacecraft send and receive messages using radio waves. The frequencies used are hundreds of times higher than those put out by music stations on Earth and can cram in more information, allowing orbital broadcasts to transmit hundreds of megabits of information per second. Lasers, which operate at higher frequencies still, can reach gigabits per second (see ‘Tuned in’). And unlike the radio portion of the electromagnetic spectrum, which is crowded and carefully apportioned, optical wavelengths are underused and unregulated.

Efforts to develop laser communication systems struggled for much of the twentieth century: weak lasers and problematic detectors derailed project after project. But recent advances in optics have begun to change the situation. “The technology has matured,” says Frank Heine, chief scientist at Tesat-Spacecom, a company based in Backnang, Germany.

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In the 1980s, Europe took advantage of improved lasers and optical detectors to begin work on its first laser communication system, the Semiconductor Laser Intersatellite Link Experiment (SILEX). Equipped with the system, the ESA satellite Artemis received 50 megabits of information per second from a French satellite in 2001and then exchanged messages with a Japanese satellite in 2005. The project taught engineers how to stabilize and point a laser in space. But it was abandoned after its intended application — a constellation of satellites to provide Internet services — was dropped in favor of the network of fiber-optic cables now criss-crossing the globe.

Since then, Heine’s team at Tesat-Spacecom has created a laser terminal for satellite-to-satellite communication, at a cost to the German Aerospace Center of €95 million (US$124 million). The laser, amplified by modern fiber-optic technology, achieves a power of watts — compared with the tens of milliwatts reached by SILEX. In 2008, terminals mounted on two satellites transferred information at gigabits per second over a few thousand kilometers.

ESA’s Alphasat will extend the range of this laser terminal to tens of thousands of kilometers once it is positioned high in geostationary orbit. Future satellites that sport laser terminals in lower orbits will be able to beam as much as 1.8 gigabits per second of information up to Alphasat, which will then relay the data to the ground using radio waves. Alphasat’s geostationary orbit means that it can provide a constant flow of data to its ground station — unlike low-Earth-orbit satellites, which can communicate with the ground for only an hour or two each day as they race by overhead. “Other satellites will be able to buy time on our laser terminal,” says Philippe Sivac, Alphasat’s acting project manager.

One client will be another ESA mission due to launch this year: Sentinel-1, the first of several spacecraft to be sent up for Europe’s new global environmental-monitoring program Copernicus. It will beam weather data to Alphasat until the end of 2014. At that point, Europe plans to start deploying a network of dedicated laser-relay satellites that will ultimately handle 6 terabytes of images, surface-temperature measurements and other data collected every day by a fleet of Sentinel spacecraft.

But Europe’s space lasers have a significant drawback. Although they can shuttle information between spacecraft, they have trouble talking to the ground — a task that must still be performed by radio waves. This is because these lasers encode information by slightly varying the frequency of light in a way analogous to modulating an FM radio station. A beam modulated in this way is protected from solar interference but is vulnerable to atmospheric turbulence.

The laser on NASA’s upcoming LADEE mission will communicate directly with Earth using a different approach that is less susceptible to atmospheric interference. It encodes information AM-style by tweaking the amplitudes rather than the frequency of a light wave’s peaks.

NASA hopes that the LADEE demonstration will extend laser communications beyond Earth’s immediate vicinity, to the Moon and other planets. Deep-space missions currently rely on radio transmissions. But radio waves spread out when they travel long distances, weakening the signal and reducing the data-transfer rate.

Laser beams, by contrast, keep their focus, allowing them to shuttle the already greater quantities of information they encode over longer distances without using the extra power needed by radio transmitters. “Laser communication becomes more advantageous the farther out you go,” says Donald Cornwell, mission manager for the Lunar Laser Communication Demonstration project on LADEE at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

In 1992, the Galileo probe, on its way to Jupiter, spotted laser pulses sent more than 6 million kilometers from Earth. A laser on Earth pinged the Mars Global Surveyor in 2005. Another struck the MESSENGER mission en route to Mercury, which responded with its own laser pulses. In January this year, the Lunar Reconnaissance Orbiter received the first primitive message sent by laser to the Moon — an image of the Mona Lisa that travelled pixel by pixel in a sort of Morse code.

LADEE carries NASA’s first dedicated laser communications system. With a bandwidth of 622 megabits per second, more than six times what is possible with radio from the distance of the Moon, the system can broadcast high-definition television-quality video. But even though its AM optical system is good at penetrating Earth’s turbulent atmosphere, it will still need a backup radio link for cloudy days when the laser is blocked. To minimize this problem, LADEE’s primary ground station is in a largely cloudless desert in New Mexico, with alternative sites in two other sunny spots: California and the Canary Islands.

Source: http://www.scientificamerican.com

Marks on Martian Dunes May Reveal Tracks of Dry Ice Sleds.

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NASA research indicates hunks of frozen carbon dioxide — dry ice — may glide down some Martian sand dunes on cushions of gas similar to miniature hovercraft, plowing furrows as they go.
Researchers deduced this process could explain one enigmatic class of gullies seen on Martian sand dunes by examining images from NASA’s Mars Reconnaissance Orbiter (MRO) and performing experiments on sand dunes in Utah and California.

“I have always dreamed of going to Mars,” said Serina Diniega, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and lead author of a report published online by the journal Icarus. “Now I dream of snowboarding down a Martian sand dune on a block of dry ice.”

The hillside grooves on Mars, called linear gullies, show relatively constant width — up to a few yards or meters across — with raised banks or levees along the sides. Unlike gullies caused by waterflows on Earth and possibly on Mars, they do not have aprons of debris at the downhill end of the gully. Instead, many have pits at the downhill end.

“In debris flows, you have water carrying sediment downhill, and the material eroded from the top is carried to the bottom and deposited as a fan-shaped apron,” said Diniega. “In the linear gullies, you’re not transporting material. You’re carving out a groove, pushing material to the sides.”

Images from MRO’s High Resolution Imaging Science Experiment (HiRISE) camera show sand dunes with linear gullies covered by carbon dioxide frost during the Martian winter. The location of the linear gullies is on dunes that spend the Martian winter covered by carbon dioxide frost. The grooves are formed during early spring, researchers determined by comparing before-and-after images from different seasons. Some images have even caught bright objects in the gullies.

Scientists theorize the bright objects are pieces of dry ice that have broken away from points higher on the slope. According to the new hypothesis, the pits could result from the blocks of dry ice completely sublimating away into carbon-dioxide gas after they have stopped traveling.

“Linear gullies don’t look like gullies on Earth or other gullies on Mars, and this process wouldn’t happen on Earth,” said Diniega. “You don’t get blocks of dry ice on Earth unless you go buy them.”

That is exactly what report co-author Candice Hansen, of the Planetary Science Institute in Tucson, Ariz., did. Hansen has studied other effects of seasonal carbon-dioxide ice on Mars, such as spider-shaped features that result from explosive release of carbon-dioxide gas trapped beneath a sheet of dry ice as the underside of the sheet thaws in spring. She suspected a role for dry ice in forming linear gullies, so she bought some slabs of dry ice at a supermarket and slid them down sand dunes.

That day and in several later experiments, gaseous carbon dioxide from the thawing ice maintained a lubricating layer under the slab and also pushed sand aside into small levees as the slabs glided down even low-angle slopes.

The outdoor tests did not simulate Martian temperature and pressure, but calculations indicate the dry ice would act similarly in early Martian spring where the linear gullies form. Although water ice, too, can sublimate directly to gas under some Martian conditions, it would stay frozen at the temperatures at which these gullies form, the researchers calculate.

“MRO is showing that Mars is a very active planet,” Hansen said. “Some of the processes we see on Mars are like processes on Earth, but this one is in the category of uniquely Martian.”

Hansen also noted the process could be unique to the linear gullies described on Martian sand dunes.

“There are a variety of different types of features on Mars that sometimes get lumped together as ‘gullies,’ but they are formed by different processes,” she said. “Just because this dry-ice hypothesis looks like a good explanation for one type doesn’t mean it applies to others.”
The University of Arizona Lunar and Planetary Laboratory operates the HiRISE camera, which was built by Ball Aerospace & Technologies Corp. of Boulder, Colo. JPL manages MRO for NASA’s Science Mission Directorate in Washington. Lockheed Martin Space Systems of Denver built the orbiter.

Source: NASA

 

Snow on Mars: NASA Spacecraft Spots ‘Dry Ice’ Snowflakes.

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A spacecraft orbiting Mars has detected carbon dioxide snow falling on the Red Planet, making Mars the only body in the solar system known to host this weird weather phenomenon.

The snow on Mars fell from clouds around the planet’s south pole during the Martian winter spanning 2006 and 2007, with scientists discovering it only after sifting through observations by NASA’s Mars Reconnaissance Orbiter (MRO). The Martian south pole hosts a frozen carbon dioxide — or “dry ice” — cap year-round, and the new discovery may help explain how it formed and persists, researchers said.

“These are the first definitive detections of carbon-dioxide snow clouds,” lead author Paul Hayne, of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., said in a statement. “We firmly establish the clouds are composed of carbon dioxide — flakes of Martian air — and they are thick enough to result in snowfall accumulation at the surface.”

The find means Mars hosts two different kinds of snowfall. In 2008, NASA’s Phoenix lander observed water-ice snow — the stuff we’re familiar with here on Earth — falling near the Red Planet’s north pole,.

Hayne and his team studied data gathered by MRO’s Mars Climate Sounder instrument during the Red Planet’s southern winter in 2006-2007. This instrument measures brightness in nine different wavelengths of visible and infrared light, allowing scientists to learn key characteristics of the particles and gases in the Martian atmosphere, such as their sizes and concentrations.

The research team examined measurements the Mars Climate Sounder made while looking at clouds — including one behemoth 300 miles (500 kilometers) wide — from directly overhead, and from off to the side. These combined observations clearly revealed dry-ice snow falling through the Red Planet’s skies, researchers said.

“One line of evidence for snow is that the carbon-dioxide ice particles in the clouds are large enough to fall to the ground during the lifespan of the clouds,” said co-author David Kass, also of JPL. “Another comes from observations when the instrument is pointed toward the horizon, instead of down at the surface.”

“The infrared spectra signature of the clouds viewed from this angle is clearly carbon-dioxide ice particles, and they extend to the surface,” Kass added. “By observing this way, the Mars Climate Sounder is able to distinguish the particles in the atmosphere from the dry ice on the surface.”

Astronomers still aren’t entirely sure how the dry ice sustaining Mars’ south polar cap — the only place where frozen carbon dioxide exists year-round on the planet’s surface — is deposited. It could come from snowfall, or the stuff may freeze out of the air at ground level, researchers said.

“The finding of snowfall could mean that the type of deposition — snow or frost — is somehow linked to the year-to-year preservation of the residual cap,” Hayne said.

Dry ice requires temperatures of about minus 193 degrees Fahrenheit (minus 125 Celsius) to fall, reinforcing just how cold the Martian surface is.

The study will be published in an upcoming issue of the Journal of Geophysical Research. Hayne performed the research while a postdoc at Caltech in Pasadena.

Source: Yahoo news

Earth’s plates move slower than thought.

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The mystery of erratic changes in the history of Earth’s past and current plate motions has been cracked by academics from The Australian National University.

Dr Giampiero Iaffaldano, from the Research School of Earth Sciences in the ANU College of Physical and Mathematical Sciences, led a team that found true changes in plate motions occur on timescales no shorter than a few million years.

“The scenario arising from recent data was puzzling because plates appeared to move erratically and significantly over geologically-short periods of less than one million years,” said Dr Iaffaldano.

“This posed a conundrum, as the forces that would be required to explain their sudden motions far exceed the most optimistic estimates we could make.”

Dr Iaffaldano’s research focused on the detailed records of plate motions across the mid-oceanic ridges in the South Pacific, Indian and Atlantic Oceans. After accounting for data noise – the portion of data that was unrelated to Earth’s plate motions – he found that true changes in plate motions occur on timescales no shorter than five million years.

“A major discovery of the study is that, upon noise reduction, true changes in plate motions occur on timescales no shorter than a few million years. This yields simpler movement patterns and more plausible dynamics,” he said.

“We showed that noise is in fact a significant bias to our understanding of the forces shaping Earth’s surface, particularly as more and more measurements of plate motions are made available.

“This does not mean that these measurements are wrong, but we need to reduce noise as much as possible before making any geophysical conclusions. In our study we provide for the first time a method to do so in a simple and efficient way, by statistically determining the likelihood of a certain plate-motion change at a given time in the geologic past.”

Source: Science Alert