Astronomy Picture of the Day 03-12-04

Explanation: Above, the ringed planet Saturn shines in x-rays. Otherwise beyond the range of human vision, the eerie x-ray view was created by overlaying a computer generated outline of the gas giant's disk and ring system on a false-color picture of smoothed, reconstructed x-ray data from the orbiting Chandra Observatory. The data represents the first detection of Saturn at x-ray energies and held some surprises for researchers. For starters, the x-rays seem concentrated near the planet's equator rather than the poles, in marked contrast to observations of Jupiter, the only other gas giant seen at such high energies. And while Saturn's high energy emission is found to be consistent with the reflection of x-rays from the Sun, the intensity of the reflected x-rays was also found to be unusually strong. Outside the planet's disk, only a faint suggestion of x-rays from Saturn's magnificent ring system is visible at the left.

Like clockwork: on the astronomynow site, and I post it as an addition. The next day or two sees it as an APOD.

Two asteroid fly-bys for Rosetta
EUROPEAN SPACE AGENCY
Posted: March 11, 2004


The European Space Agency probe Rosetta will pass two asteroids named Lutetia and Steins on its journey to comet 67P/Churyumov-Gerasimenko. Image credit: ESA/AOES Medialab

The Rosetta Science Working Team has made the final selection of the asteroids that Rosetta will observe at close quarters during its journey to Comet 67P/Churyumov-Gerasimenko. Steins and Lutetia lie in the asteroid belt between the orbits of Mars and Jupiter.

Rosetta's scientific goals always included the possibility of studying one or more asteroids from close range. However, only after Rosetta's launch and its insertion into interplanetary orbit could the ESA mission managers assess how much fuel was actually available for fly-bys. Information from the European Space Operations Centre (ESOC) in Germany enabled Rosetta's Science Working Team to select a pair of asteroids of high scientific interest, well within the fuel budget.

The selection of these two excellent targets was made possible by the high accuracy with which the Ariane 5 delivered the spacecraft into its orbit. This of course leaves sufficient fuel for the core part of the mission, orbiting Comet 67P/Churyumov-Gerasimenko for 17 months when Rosetta reaches its target in 2014.

Asteroids are primitive building blocks of the Solar System, left over from the time of its formation about 4600 million years ago. Only a few asteroids have so far been observed from nearby. They are very different in shape and size, ranging from a few kilometres to over 100 kilometres across, and in their composition.

The targets selected for Rosetta, Steins and Lutetia, have rather different properties. Steins is relatively small, with a diameter of a few kilometres, and will be visited by Rosetta on 5 September 2008 at a distance of just over 1700 kilometres. This encounter will take place at a relatively low speed of about 9 kilometres per second during Rosetta's first excursion into the asteroid belt.

Lutetia is a much bigger object, about 100 kilometres in diameter. Rosetta will pass within about 3000 kilometres on 10 July 2010 at a speed of 15 kilometres per second. This will be during Rosetta's second passage through the asteroid belt.

Rosetta will obtain spectacular images as it flies by these primordial rocks. Its onboard instruments will provide information on the mass and density of the asteroids, thus telling us more about their composition, and will also measure their subsurface temperature and look for gas and dust around them.

Rosetta began its journey on 2 March, and is well on its way. Commissioning of its instruments has already started and is proceeding according to plan.

"Comets and asteroids are the building blocks of our Earth and the other planets in the Solar System. Rosetta will conduct the most thorough analysis so far of three of these objects," said Prof. David Southwood, Director of ESA's Science Programme. "Rosetta will face lots of challenges during its 12-year journey, but the scientific insights that we will gain into the origin of the Solar System and, possibly, of life are more than rewarding."

Spirit looks down into crater after reaching rim
MISSION CONTROL REPORT
Posted: March 11, 2004

NASA's Spirit has begun looking down into a crater it has been approaching for several weeks, providing a view of what's below the surrounding surface.

Spirit reaches Bonneville. Credit: NASA/JPL
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Spirit has also been looking up, seeing stars and the first observation of Earth from the surface of another planet. Its twin, Opportunity, has shown scientists a "mother lode" of hematite now considered a target for close-up investigation.

"It's been an extremely exciting and productive week for both of the rovers," said Spirit Mission Manager Jennifer Trosper at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Dr. Chris Leger, a rover driver at JPL, said, "The terrain has been getting trickier and trickier as we've gotten close to the crater. The slopes have been getting steeper and we have more rocks." Spirit has now traveled a total of 335 meters (1,099 feet).

Spirit's new position on the rim of the crater nicknamed "Bonneville" offers a vista in all directions, including the crater interior. The distance to the opposite rim is about the length of two football fields, nearly 10 times the diameter of Opportunity's landing-site crater halfway around the planet from Spirit.

Initial images from Spirit's navigation camera do not reveal any obvious layers in "Bonneville's" inner wall, but they do show tantalizing clues of rock features high on the far side, science-team member Dr. Matt Golombek of JPL said at a news briefing today. "This place where we've just arrived has opened up, and it's going to take us a few days to get our arms around it."

Scientists anticipate soon learning more about the crater from Spirit's higher-resolution panoramic camera and the miniature thermal emission spectrometer, both of which can identify minerals from a distance. They will use that information for deciding whether to send Spirit down into the crater.

From the crater rim and during martian nighttime earlier today, Spirit took pictures of stars, including a portion of the constellation Orion. Shortly before dawn four martian days earlier, it photographed Earth as a speck of light in the morning twilight. The tests of rover capabilities for astronomical observations will be used in planning possible studies of Mars' atmospheric characteristics at night. Those studies might include estimating the amounts of dust and ice particles in the atmosphere from their effects on starlight, said Dr. Mark Lemmon, a science team member from Texas A&M University, College Station.

Earth as seen from Mars' surface. Credit: NASA/JPL/Cornell/Texas A&M
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Opportunity has been looking up, too. It has photographed Mars' larger moon, Phobos, passing in front of the Sun twice in the past week, and Mars' smaller moon, Deimos, doing so once.

Opportunity's miniature thermal emission spectrometer has taken upward-looking readings of the atmospheric temperature at the same time as a similar instrument, the thermal emission spectrometer on NASA's Mars Global Surveyor orbiter, took downward-pointed readings while passing overhead. "They were actually looking directly along the same path," said science team member Dr. Michael Wolff of the Martinez, Ga., branch of the Space Science Institute, Boulder, Colo. The combined readings give the first full temperature profile from the top of Mars' atmosphere to the surface."

When pointed at the ground, Opportunity's miniature thermal emission spectrometer has checked the abundance of hematite in all directions from the rover's location inside its landing-site crater. This mineral, in its coarse-grained form, usually forms in a wet environment. Detection of hematite from orbit was the prime factor in selection of the Meridiani Planum region for Opportunity's landing site.

"The plains outside our crater are covered with hematite," said Dr. Phil Christensen of Arizona State University, Tempe, lead scientist for the instrument. "The rock outcrop we've been studying has some hematite. Parts of the floor of the crater, interestingly enough, have virtually none." The pattern fits a theory that the crater was dug by an impact that punched through a hematite-rich surface layer, he said. One goal for Opportunity's future work is to learn more about that surface layer to get more clues about the wet past environment indicated by sulfate minerals identified last week in the crater's outcrop.

Christensen said that before Opportunity drives out of the crater in about 10 days, scientists plan to investigate one area on the inner slope of the crater that he called "the mother lode of hematite."

JPL, a division of the California Institute of Technology in Pasadena, manages the Mars Exploration Rover project for NASA's Office of Space Science, Washington, D.C.

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