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By Daniel Fischer Every page present in Europe & the U.S.!
| Ahead | Awards 
The latest issue!
| An experimental German companion. Current mission news: MGS (latest pictures!) + Cassini + Galileo + NEAR |
on a special TV channel for the SRTM (see below)! It's on DFS-2 Kopernikus, 28.5 deg. East, 11.595 GHz V, in analog PAL, with audio at 7.02 MHz and 6.65 MHz. There will be daily half-hour programs starting at 14:30 UTC: details!
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NEAR in orbit around Eros!This time everything worked: At 16:00 UTC on Feb. 14th, navigation data from the Near Earth Asteroid Rendezvous spacecraft indicated that NEAR had achieved orbit around asteroid 433 Eros! It had been 15:33 UTC, with Eros about 327 kilometers "below", when NEAR's small hydrazine thrusters fired for 57 seconds, slowing the spacecraft's approach to walking speed and easing it into the asteroid's weak gravitational pull. The rendezvous took place about 256 million kilometers from Earth. "NEAR is now the first spacecraft to successfully lock into orbit around an asteroid," said Mission Director Dr. Robert Farquhar: "We're making history here today."Over the next 24 hours, instrument data and pictures of the asteroid taken after the orbit insertion burn were expected to provide more details about NEAR's precise position around Eros. Preliminary data showed the capture orbit having a periapsis of plus or minus 49 km to the expected 327 km and an apoapsis of plus or minus 67 km to the expected 450 km, with an orbital period of plus or minus a half-day of the expected 27.6 days. What might appear to be a large uncertainty in these numbers is only caused by the unknowns about Eros' mass. Researchers say that the understanding of the mass will be improved markedly over the next few days, allowing the precise orbit achieved around the space rock to be refined. NEAR has returned the first batch of images of Eros since the orbit insertion, with 30 meters resolution. The first picture returned from orbit is of a 5 km wide crater that the spacecraft had been monitoring for weeks during its approach. Also, data collected from the pre-arrival observations of the asteroid's northern hemisphere using NEAR's IR spectrometer has been transmitted to Earth. "Key data for the spectrometer were obtained," said Joe Veverka, the leader of NEAR's imaging team: "They are on the ground now and they look absolutely fantastic. We're in the process of putting these spectra together." During the final days of approach, many interesting views had been obtained by NEAR's camera as well, e.g. of an "amazing heart-shaped feature" that mission scientists called "a most curious observation" - actually it is a 5 km long depression in the surface that appears as a heart because of an accidental confluence of shadows. Nonetheless it became another "erotic" aspect of the NEAR mission, the orbital insertion of which had been deliberately set to Valentin's Day, as Farquhar has now admitted: There would have been a window of two weeks to chose from ... |
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SRTM delivers fine data, over half the Earth mapped, but mission may be shortenedWhile the Shuttle Radar Topography Mission has already delivered tons of data and some impressive quicklook views of Earth in 3D, Endeavour's crew and flight controllers are troubleshooting a problem with a small nitrogen thruster mounted at the tip of the radar's outboard antenna. Although gaseous nitrogen propellant is flowing, little or no thrust is being produced, perhaps due to a leak. The thruster was designed to keep the mast from "righting" itself in response to Earth's gravity and remove the need for additional orbiter thruster firings to keep the antenna in its data-taking position. |
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Without the thruster on the antenna, crew members have to fire the
orbiter's thrusters more than expected and exhaust the fuel supply some
8 hours ahead of time - flight controllers are working to develop
propellant-conserving strategies, however. It will take a few more days
before they will know how much, if any, of the mission must be cut, and
if a shortening is required, it would not be more than one day. "This is
going to evolve over time," said Milt Heflin, mission operations
representative. "That's why we're not ready to state that science mission
duration is (some) number because this is going to occur over the next
two or three days. [...] Right now, we are full duration as far as our
mapping is concerned until we know otherwise." The continuing operation of the radar system has thus far not been affected by the thruster trouble, and until early on Feb. 14, 38 percent of the Earth's landmasses had already been mapped. And one day later Endeavour's astronauts had completed mapping well over half the targeted Earth land surface - the mission already has tripled the world's pool of digital terrain data with this much detail. "We are starting to see the first `quick look' results from the X-band and C-band antennas and the details are fantastic," said Dr. Michael Kobrick, SRTM project scientist: "Even in this lower resolution, quick-look results, we can see many topographic features that were completely invisible in the best maps we have today." Up until the thruster problem appeared, the SRTM had "gone so well it's almost a little spooky," as Kobrick had put it: As the big radar mast was untried technology, scientists and astronauts alike had always felt that some surprises would be in store. How the radar system worksI haven't seen one article on the SRTM off- or online that explains the basics of the radar system right, and the only official online source with a correct explanation I've found so far is in German. So here's how "Aperture Synthesis Radar Interferometry" works: 1500 times a second the radar transmitters in the payload bay (one built by JPL, the other by DSS in Germany) send sharp pulses to the ground that are reflected and return to receivers in the payload bay and at the end of the famous 61 meters mast. Even with only one antenna one can already get detailled 2-D images of the ground, as the same radar system demonstrated during two shuttle flights in 1994: As a side-looking radar it keeps every point on the ground in view for an extended time while the orbiter with the radar moves on, and so a large antenna is 'synthesized'.By the same principle many current radar satellites (civilian like the ERS satellites and military like Lacrosse alike) produce their data products, and it is also being used from airplanes. The image reconstruction from the echoes resembles the process by which a hologram is read out with coherent light. Radar interferometry is also based on wave optics: The radar echoes returning to the two antennae 60 meters apart are almost the same - but not quite, as they have phase differences because of slightly different paths the rays took. Once both data sets are brought together (later, in a big computer), interference fringes appear which code the elevation pattern that reflected the radar rays (visible as colorful lines in some of the quicklook data from the SRTM): That is the information from which the digital elevation models are generated as the last step. You see: The process has nothing to do with classical stereo vision where it is the parallax effect that gives the depth (or, when looking down, elevation) information, because two eyes see the scenery from slightly different perspectives. That this is not the method that the SRTM uses is actually evident from the numbers: Endeavour flies at 230 kilometers, the radar antennae are 60 meters apart, and the elevation information will be precise to about 10 meters. 60 meters is almost exactly 1000 times as large as our human eyes are spaced (6 cm), and so the challenge of SRTM would scale to our eyes (or cameras) determining the distance to objects with 1 cm precision 230 meters away, by the parallax method alone - impossible. And this only proves what a powerful tool interferometry can be, compared to classical vision! DF |
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Astro-E loss blamed on breach in booster nozzleIt seems to have been a low-tech problem that doomed the Japanese X-ray satellite Astro-E during its Feb. 10th launch (last Update story 2): A camera mounted on the M-5 rocket "showed an apparent breach of the solid rocket motor's nozzle by exhaust gases at 25 sec. and 41 sec. after liftoff," according to Aviation Week. At 41 seconds after liftoff, the inner pressure of the first stage motor abruptly dropped. The burning and control of the second and the third stage motors were normal and strived to restore the velocity deficiency, but in vain - the satellite couldn't reach orbit and burnt up. The total value of the mission was about 18 billion yen or $170 million. (AW&ST of Feb. 14 p. 18) |
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Starshine's orbit decays, burnup expected for Feb. 18thThe little satellite with mirrors - deployed from a shuttle last June (see Update # 133 last story) is now encountering the Earth's atmosphere and will reenter and burn up in a few days - the call is out to observe the event and perhaps even photograph it: Homepage, special Decay Page and the Observers Page.New storm collision looms on JupiterSoon there will be only one White Oval Spot left, while until 2 years ago there were three. But then WOS BC and DE merged into one big storm called BE - and now BE has approached FA, with their centers only 12 degrees apart. When the distance has shrunk to 10 degrees the (hard to observe) features will start to interact. (H.-J. Mettig in the Planetenbeobachter list on Feb. 14th)How Jupiter's weather is being driven by internal forces (see also Update # 173 small items) is discussed by the NYT and Gannett. Galileo approaches Io again for the closest fly-by ever - with the spacecraft dipping to only 200 kilometers above the fiery surface: This Week on Galileo. |
The whole ring around SN 1987A starts to brightennow that more and more debris from the explosion 13 years ago is hitting it - the light show begins: NYT advance story, STScI and Columbia Press Releases, APOD, CNN, BBC, Space.com, RP, Space Daily coverage.How bacteria could flourish in Lake Vostokand under ice elsewhere in the solar system - the accretion ice above Lake Vostok provides all three of the ingredients essential to life, i.e. water, energy and carbon: Nature Science Update.Cassini's asteroid sighting was also a warmupfor finding more moons of Saturn - the Masursky observations (see last Update story 4 and ESA Science News) marked the initial use of Cassini's automated object-targeting capabilities: Space.com.
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All other historical issues can be found in the Archive.
The U.S. site of this Cosmic Mirror has been visited
since it was issued (the German site has no counter).
Compiled and written by Daniel Fischer
(send me a mail to [email protected]!), Skyweek
