Astronomy Picture of the Day 08-11-04

Explanation: The ongoing Perseid Meteor Shower should be at its strongest tonight and tomorrow night. Although meteors should be visible all night long, the best time to watch will be between 2:00 AM and dawn each night. In dark, moonless, predawn skies you may see dozens of meteors per hour. Sky enthusiasts in Europe and Asia might see an unusual burst of meteors near 2100 hours UT. Grains of cosmic sand and gravel shed from Comet Swift-Tuttle will streak across the sky as they vaporize during entry into Earth's atmosphere. Tracing the meteor trails backwards, experienced skygazers will find they converge on the constellation Perseus, thus this annual meteor shower's name. Pictured above is a Perseid meteor from 2002 over a rock formation in the US Southwest desert. Shadowing and blurring are caused by the long 10-minute exposure. The brightest Perseids can be seen from anywhere on Earth by monitoring the continuously returning images from the Night Sky Live cameras.

MESSENGER's checkout time
JHU-APL MISSION REPORT
Posted: August 10, 2004

With a successful launch behind them and a long cruise ahead, MESSENGER mission operators are checking out the systems on the Mercury-bound spacecraft.

"MESSENGER is in great shape and well on its way," says Mission Operations Manager Mark Holdridge, of the Johns Hopkins University Applied Physics Laboratory, which built and operates MESSENGER.

Artist's impression shows MESSENGER departing Earth. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Since liftoff on August 3 the operations team has checked out MESSENGER's communications system, getting strong signals from its high-, medium- and low-gain antennas through NASA's Deep Space Network. It will check the guidance and control system this week before moving on to some of the spacecraft's science instruments. The team also continues to monitor power and temperature levels on the spacecraft, now more than 1.5 million miles from Earth.

MESSENGER is tentatively scheduled to carry out its first trajectory correction maneuver on Aug. 24, firing its small thrusters to adjust the course that will bring it back to Earth for a gravity-assist flyby next summer.

"We're going to take as long as we need to make sure every system works the way it's supposed to," Holdridge says. "Now that we've launched, we have time on our side."

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a scientific investigation of the planet Mercury, and the first NASA mission designed to orbit the planet closest to the Sun. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, built and will operate the MESSENGER spacecraft and manages the Discovery-class mission for NASA.

Solar sail systems tested
NASA-MSFC NEWS RELEASE
Posted: August 10, 2004

NASA engineers and their industry partners have successfully deployed two solar sails - each nearly 33 feet in length along one side - reaching a critical milestone in the development of a unique propulsion technology that could enable future deep space missions.

Solar sail propulsion uses the Sun's energy to travel through space, much the way wind pushes sailboats across water. The technology bounces a stream of solar energy particles called photons off giant, reflective sails made of lightweight material 40 to 100 times thinner than a piece of writing paper. The continuous pressure provides sufficient thrust to perform maneuvers, such as hovering at a point in space and rotating the space vehicle's plane of orbit, which would require too much propellant for conventional rocket systems. Because the Sun provides the necessary propulsive energy, solar sails also require no onboard propellant, thus reducing payload mass.

In July, L'Garde, Inc., of Tustin, Calif., successfully deployed a solar sail technology system. Earlier this year, Able Engineering of Goleta, Calif., successfully completed testing of its own solar sail design. The work of both contractors is led by the In-Space Propulsion Technology Projects Office at NASA's Marshall Space Flight Center in Huntsville, Ala.

"We are making the stuff of science fiction into reality," said Les Johnson, manager of the In-Space Propulsion Technology Projects Office at the Marshall Center. "It has been a tremendous engineering challenge, and I'm pleased and proud of the teams that have made it happen."

L'Garde's sail deployment was conducted in a 100-foot-diameter vacuum chamber at NASA's Glenn Research Center Plum Brook Station in Sandusky, Ohio. The tests included temperatures as cold as minus 112 degrees Fahrenheit to simulate the conditions of open space.

The sail technology used an inflatable, thermally rigidized boom system, which inflates and becomes stiff in space environment conditions. The boom is the core of the support structure for the thin, reflective solar sail itself - merely a fraction of the thickness of a human hair -- and includes a stowage structure and built-in deployment mechanism. Engineers used a computer-controlled boom pressurization system to initiate deployment of the boom and sail system.

In May, Able Engineering also successfully completed testing of a solar sail design at NASA's Langley Research Center in Hampton, Va. This sail employed a "coilable" graphite boom, which is extended or uncoiled via remote control - much the way a screw is rotated to remove it from an object. The boom supports the lightweight sail, which is made of an aluminized, temperature-resistant material called CP-1. Named NASA's 1999 Invention of the Year, CP-1 was invented by the Langley Research Center and is produced under exclusive license by SRS Technologies of Huntsville. The boom system also includes a central stowage structure and deployment mechanism.

Tests of the coilable boom were conducted in a 50-foot-diameter vacuum chamber. Engineers remotely initiated deployment of the boom and sail in April, then spent the next five weeks studying its shape and system dynamics - or how the solar sail functions in relation to force, weight and tension.

Data from both tests will be used to make improvements to future solar sail design and modeling. In March 2005, NASA plans a laboratory deployment of a sail more than 65 feet in length.

Solar sail technology was selected for development in August 2002 by NASA's Office of Space Science in Washington. Along with the sail system design projects by L'Garde and Able Engineering, NASA's Jet Propulsion Laboratory in Pasadena, Calif., was tapped to develop an integrated set of computer-based solar sail simulation tools. These are just three of a number of efforts undertaken by NASA Centers, industry and academia to develop solar sail technology.

Solar sail technology is being developed by the In-Space Propulsion Technologies Program, managed by NASA's Office of Space Science and implemented by the In-Space Propulsion Technology Projects Office at Marshall. The program's objective is to develop in-space propulsion technologies that can enable or benefit near and mid-term NASA space science missions by significantly reducing cost, mass and travel times.

YES! The Mars rovers Spirit and Opportunity are still functioning! Here is an update:

SPIRIT UPDATE: The quest for the top of the hills continues - sol 201-204, August 03, 2004

Mars has seasons like the Earth does, but the seasons are twice as long due to Mars' larger orbit around the Sun. Right now, Mars is approaching northern summer. That also means that it's approaching southern martian winter at the same time. So Spirit is headed for winter, being 14 degrees south of the equator. Because martian winter is setting in, solar array energy continues to be a concern for Spirit. If Spirit parks with a northerly tilt, the rover will see between 350 and 380 watt-hours of energy, but if Spirit stops on flat ground or with a southerly tilt, solar energy is as low as 280 watt-hours. So engineers make a concerted effort to find the north-facing islands along Spirit's path.

On Sol 201, Spirit was commanded to drive 98 feet (30 meters) across terrain that was pretty steep. Spirit accomplished 8.2 feet (2.5 meters) then stopped due to an excessive tilt angle of 25.6 degrees. Engineers had set the maximum tilt angle limit at 25 degrees. Spirit did complete pre-drive science observations and post-drive imaging.

On sol 202, Spirit repeated the drive plan from sol 201 with the maximum tilt angle set to 32 degrees. This time the rover completed the drive as planned, traveling 83.6 feet (25.5) meters up the hill. Spirit then performed post-drive imaging.

On sol 203, scientists' hope was to find rock outcropping in this location, but none were found. So the decision was made to continue the drive up the hill to find a better rock outcrop. Spirit performed another six-wheel, 62-foot (19-meter) drive. This drive was completed successfully; however, at the end of the drive, Spirit drove into a small hollow. As a result, Spirit was pitched 15 degrees toward the southwest, and ended up with a southerly tilt.

Planning for sol 204 was very exciting due to the late downlink of information from sol 203. Very late in the planning cycle, available power on sol 204 was reduced from 370 watt-hours to 288 watt-hours. Ouch! Pre-drive observations were cut back to 17 minutes, during which the motors were heated for driving. Spirit drove only 0.82 feet (0.25 meters). Because the drive was so short, the power situation is not as bad as it could have been.

Total odometry after sol 204, which ended on July 30, is 2.21 miles (3,565.57 meters). Total elevation above the plains of Gusev Crater is estimated to be 30 feet (9 meters).

Over the next few sols, scientists and engineers hope to make it to "Clovis" rock outcrop and to recharge the batteries.

OPPORTUNITY UPDATE: Opportunity Turns to Talk to Odyssey - sol 188-189, August 09, 2004

Sol 188 was devoted to finishing an examination of a target patch called "Tuktoyuktuk" where the rover's rock abrasion tool had ground the surface coating off of a rock called "Inuvik." Opportunity then drove partly up-slope and partly cross-slope as both a mobility test and the start of a traverse to the next target. It slipped down-slope about as much as expected, a good result. The rover's current terrain consists of rocky plates lightly covered with sand and soil, plus some deeper sandy patches between the plates. The sandy patches result in more slip and sometimes cause the vehicle to yaw a little (as more slip on one side of the vehicle than the other causes it to turn). The overall tilt of the rover is about 18 degrees.

On sol 189 the rover drove about 4 meters (13 feet) eastward across the inner slope of the crater. The drive went well despite substantial down-slope slip. Slippage averaged about 33 percent, with a peak of about 56 percent on one half-meter (1.6-foot) drive segment, but the rover team expected that and compensated in advance for it. The team then asked the rover to conduct a series of turns in place during the communications relay pass with the Mars Odyssey orbiter to optimize the communications link. The idea was to keep Odyssey in the sweet spot of the rover's ultra-high-frequency antenna pattern as the orbiter swept across the sky. The total data return was about 135 megabits. The best possible return predicted by models if the rover had just sat in one orientation was about 115 megabits.

The next target the scientists would like the rover to approach, "Axel Heiberg," is a rocky outcrop about 18 meters (59 feet) away to the east and a bit deeper in the crater.

NASA's Mars Exploration Rover Spirit captured this interesting view of martian topography
just below the "West Spur" portion of the "Columbia Hills" on sol 208 (Aug. 2, 2004).
The view is looking southwest. The rover's wheel tracks show the contrast between soft
martian soil and the harder "Clovis" rock outcrop, which scientists are now studying.

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