Astronomy Picture of the Day 02-17-04

Explanation: Gravity can bend light, allowing whole clusters of galaxies to act as huge telescopes. Almost all of the bright objects in this just-released Hubble Space Telescope image are galaxies in the cluster known as Abell 2218. The cluster is so massive and so compact that its gravity bends and focuses the light from galaxies that lie behind it. As a result, multiple images of these background galaxies are distorted into long faint arcs - a simple lensing effect analogous to viewing distant street lamps through a glass of wine. The cluster of galaxies Abell 2218 is itself about two billion light-years away in the northern constellation Draco. The power of this massive cluster telescope has recently allowed astronomers to detect a galaxy at a redshift of about 7, the most distant galaxy or quasar yet measured. Three images of this young, still-maturing galaxy are faintly visible in the white contours near the image top and the lower right. The recorded light, further analyzed with a Keck Telescope, left this galaxy when the universe was only about five percent of its current age.

Here's what the Mars rovers have been up to lately:

MONDAY, FEBRUARY 16, 2004
1816 GMT (1:16 p.m. EST)
The Spirit rover drove 27 meters overnight, moving ever closer to the Bonneville Crater. The craft performed more science activities with its arm during the just-completed workday, so time wasn't available for the maximum drive of 50 meters as originally scheduled.

Late tonight (U.S. time), Spirit will use its arm to "touch and feel" the current location. Then it will make a short drive over a two-hour period, mission manager Jim Erickson said.

On Tuesday night, the rover is expected to perform up to 50 meters of driving.

On Opportunity, the rover today will dig a shallow trench so its suite of mineral and elemental instruments can examine the soil at a site dubbed "Hematite Slope." The rover will lock one wheel in place and use its other five wheels to move a short distance. This procedure should create a two-and-a-half foot long, six-inch wide trench, Erickson said.

The arm will examine the trench tomorrow, followed by remote sensing work with the panorama camera and Mini-Thermal Emission Spectrometer on Wednesday.

Two news conferences are scheduled on NASA Television this week -- Tuesday and Thursday at 1 p.m. EST.

SUNDAY, FEBRUARY 15, 2004
1818 GMT (1:18 p.m. EST)
Spirit is finishing its science work at the flaky-looking rock named Mimi. It has used its instruments to examine the rock, will take some microscopic images late tonight (U.S. time) and then resume its drive to the Bonneville Crater, mission manager Jim Erickson says.

The rover is about 270 meters from the crater. A 25-meter drive in planned for during the morning of its next workday and possibly another 25 meters later in the day, Erickson said.

The goal is to reach the crater in about 18 days, with some additional science stops expected between now and then.

Meanwhile, the Opportunity rover has reached the spot inside its small crater where it will perform a trenching job to dig a hole for soil studies. The soil is being examined today in preparation for the digging. The trenching is planned for tomorrow, followed by science investigations on Tuesday.

Also today, Opportunity's thermal emission spectrometer is looking up and a similar device on the orbiting Mars Global Surveyor spacecraft is looking down for joint atmospheric studies

The Europeans have their ambitious Rosetta probe/lander ready to launch next week:

Rosetta: A space sophisticate
EUROPEAN SPACE AGENCY NEWS RELEASE
Posted: February 16, 2004

The European Space Agency's Rosetta mission will be getting under way in February 2004. The Rosetta spacecraft will be pairing up with Comet 67P/Churyumov-Gerasimenko and accompanying it on its journey, investigating the comet's composition and the dynamic processes at work as it flies sunwards. The spacecraft will even deposit a lander on the comet.

Artist's concept of Rosetta orbiting a comet with its lander on the surface. Credit: Astrium/Erik Viktor

"This will be our first direct contact with the surface of a comet," said Dr Manfred Warhaut, Operations Manager for the Rosetta mission at ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany.

The trip is certainly not short: Rosetta will need ten years just to reach the comet. This places extreme demands on its hardware; when the probe meets up with the comet, all instruments must be fully operational, especially since it will have been in "hibernation" for two-and-a-half years of its journey. During this 'big sleep', all systems, scientific instruments included, are turned off. Only the on-board computer remains active.

Twelve cubic metres of technical wizardry
Rosetta's hardware fits into a sort of aluminium box measuring just 12 cubic metres. The scientific payload is mounted in the upper part, while the subsystems - on-board computer, transmitter and propulsion system - are housed below. The lander is fixed to the opposite side of the probe from the steerable antenna. As the spacecraft orbits the comet, the scientific instruments will at all times be pointed towards its surface; the antenna and solar panels will point towards the Earth and Sun respectively.

For trajectory and attitude control and for the major braking maneuvers, Rosetta is equipped with 24 thrusters each delivering 10 N. That corresponds to the force needed here on Earth to hold a bag containing 10 apples. Rosetta sets off with 1650 kg of propellant on board, accounting for more than half its mass at lift-off.

Just 20 percent of total mass is available for scientific purposes. So when developing the research instruments the same rule applied as for supermodels: make every gram count. The calculation seems to have worked out right: the main probe will be carrying 11 scientific instruments and the Rosetta lander a further ten. They will analyse the composition and structure of the comet's nucleus and study its interaction with the solar wind and the interplanetary plasma.

Rosetta - unplugged
"To provide the probe with the power it needs in space, we have given it the biggest solar panels ever carried by a European satellite," Manfred Warhaut explained. "These cells are its only source of electricity." They span 32 metres tip to tip while, at 64 m2 the surface area is comparable to that of a two-bedroom flat. The panels may be rotated through 180 degrees to catch the maximum amount of sunlight.

These dimensions are also essential because when Rosetta meets Churyumov-Gerasimenko it will be 675 million kilometres away from the Sun. At that distance solar radiation is very weak and the solar collectors will supply only 440 W of power - compared with 8000 W towards the end of the mission when the two companions come closest to the Sun (at some 150 million kilometres from our star distance).

"The probe is also equipped with a set of four 10-amp-hour batteries to maintain power supply while Rosetta flies in the shadow of the comet."

Rosetta lander - standing on its own three legs
The Rosetta lander is another of the mission's technical highlights. Using its scientific instruments, its job will be to investigate the comet's surface on location. Thanks to a mechanical arm, the lander will operate in a two-metre radius. The soft landing is a particular problem given the extremely weak gravitational force exerted by the very small comet nucleus; the lander, weighing in at 100 kg on Earth, will on the comet be as light as a sheet of paper. If there were the slightest recoil, it would bounce back uncontrollably like a rubber ball. To make sure this doesn't happen, the lander's three legs are equipped with special shock-absorbers which take up most of the kinetic energy. The legs are also fitted with ice pitons; these bore into the ground immediately on touchdown.

At the same moment, the lander fires a harpoon to anchor it to the ground - an opportunity also to investigate the mechanical properties of the surface.

"If everything goes according to plan, the mission results could well fundamentally expand our knowledge of comets, just as the Rosetta Stone, after which the probe is named, helped unravel the mystery of Egyptian hieroglyphics," said Manfred Warhaut.

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