Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2004 January 22 
Posted on 01/22/2004 2:53:40 AM PST by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2004 January 22
Explanation: After leaving its nest, the Spirit rover turned to captured this spectacular view over the Colombia Memorial Station and the floor of Gusev crater on the 16th sol of its visit to Mars. The sharp picture looks toward the northeast. Over 2 meters wide, the lander platform surrounded by deflated airbags, and the egress ramp used by the rover to complete its journey to the martian surface, are in the foreground. In the background lie Spirit's likely future waypoints and destination - initially toward a ridge on the left bordering an impact crater about 200 meters across and finally toward. the hills visible on the horizon at the right. The crater is about 250 meters away while the hills are about 3 kilometers distant. Searching for evidence of ancient watery environments, Spirit's scientific instruments have begun to return data on the composition of the surface in the lander's vicinity, suggesting that iron bearing volcanic minerals are present.
The Mars Spirit rover stopped beaming scientific data to flight controllers Wednesday following thunderstorms at an Australian ground station that may have interfered with the daily uplink of critical computer commands.
The rover acknowledged receiving its daily instructions, but during a subsequent overflight of the Mars Odyssey orbiter, Spirit did not transmit science data as expected. Another relay opportunity, this one with the Mars Global Surveyor, began around 11:30 p.m. EST but as of 12:30 a.m., a spokesman for the Jet Propulsion Laboratory said he had been unable to reach flight controllers to confirm whether any data were received.
"Ground controllers were able to send commands to the Mars Exploration Rover Spirit early Wednesday and received a simple signal acknowledging that the rover heard them, but they did not receive expected scientific and engineering data during scheduled communication passes during the rest of that martian day," said a status report from the Jet Propulsion Laboratory in Pasadena, Calif.
"Project managers have not yet determined the cause, but similar events occurred several times during the Mars Pathfinder mission. The team is examining a number of different scenarios, some of which would be resolved when the rover wakes up after powering down at the end of the martian day (around midday Pacific time Wednesday)."
It is possible the glitch has, in fact, been resolved. But JPL's public affairs office is not set up to provide overnight updates from mission control.
During a news briefing Wednesday afternoon, mission manager Jennifer Trosper said the daily transmission of commands to Spirit was affected by thunderstorms at the Deep Space Network's ground station in Canberra, Australia. Whether that had anything to do with subsequent events was not addressed in the JPL status report.
"You've heard about the rain in Spain and how that can cause problems? Well, today it was the rain in Canberra," Trosper said. "When the rover woke up this morning, we were actually over the Deep Space Network station in Canberra, Australia, because that was the part of the Earth that was pointed toward Mars at the time. There was lightning and there was rain and there was a thunderstorm in Canberra.
"The morning timeframe, local solar time at the Gusev [Crater landing] site, between about 9 a.m. and 9:45 a.m., is an important time for us to take all of the work we did overnight, all of the sequences we built, the commands to the rover, we transmit them to the rover and then it will start to act on those about 9:45 local solar time.
"As a result of the rain in Canberra today, the signal strength was not able to be received by the rover and so we weren't able to transmit those commands to the rover. It received a weak signal from Earth because of the rain and so, it actually didn't get all the data we wanted it to get. As a result, the rover did exactly what it was supposed to do, it continued to run yesterday's master sequence, which takes care of the rover in terms of keeping it awake during the day and continues to do communications."
Trosper said she expected to hear from Spirit during an afternoon overflight of the Mars Odyssey spacecraft. But no data were received then, or presumably later, during an evening pass by the Mars Global Surveyor. Another opportunity was available around 1:30 a.m. EST Thursday.
"If necessary, the flight team will take additional recovery steps early Thursday morning (the morning of sol 19 on Mars) when the rover wakes up and can communicate directly with Earth," the status report said.
The next Spirit news conference is planned for noon EST Thursday.
Distance measured to star of ancient literature and legend
CALIFORNIA INSTITUTE OF TECHNOLOGY NEWS RELEASE
Posted: January 21, 2004
The cluster of stars known as the Pleiades is one of the most recognizable objects in the night sky, and for millennia has been celebrated in literature and legend. Now, a group of astronomers has obtained a highly accurate distance to one of the stars of the Pleiades known since antiquity as Atlas. The new results will be useful in the longstanding effort to improve the cosmic distance scale, as well as to research the stellar life-cycle.
In the January 22 issue of the journal Nature, astronomers from the California Institute of Technology and the Jet Propulsion Laboratory report the best-ever distance to the double-star Atlas. The star, along with "wife" Pleione and their daughters, the "seven sisters," are the principal stars of the Pleiades that are visual to the unaided eye, although there are actually thousands of stars in the cluster. Atlas, according to the team's decade of careful interferometric measurements, is somewhere between 434 and 446 light-years from Earth.
The range of distance to the Pleiades cluster may seem somewhat imprecise, but in fact is accurate by astronomical standards. The traditional method of measuring distance is by noting the precise position of a star and then measuring its slight change in position when Earth itself has moved to the other side of the sun. This approach can also be used to find distance on Earth. If you carefully record the position of a tree an unknown distance away, move a specific distance to your side, and measure how far the tree has apparently "moved," it's possible to calculate the actual distance to the tree by using trigonometry.
However, this procedure gives only a rough estimate to the distance of even the nearest stars, due to the gigantic distances involved and the subtle changes in stellar position that must be measured.
Further, the team's new measurement settles a controversy that arose when the European satellite Hipparcos provided a distance measurement to the Pleiades so much nearer the distance than assumed that the findings contradicted theoretical models of the life cycles of stars.
This contradiction was due to the physical laws of luminosity and its relationship to distance. A 100-watt light bulb one mile away looks exactly as bright as a 25-watt light bulb half a mile away. So to figure out the wattage of a distant light bulb, we have to know how far away it is. Similarly, to figure out the "wattage" (luminosity) of observed stars, we have to measure how far away they are. Theoretical models of the internal structure and nuclear reactions of stars of known mass also predict their luminosities. So the theory and measurements can be compared.
However, the Hipparcos data provided a distance lower than that assumed from the theoretical models, thereby suggesting either that the Hipparcos distance measurements themselves were off, or else that there was something wrong with the models of the life cycles of stars. The new results show that the Hipparcos data was in error, and that the models of stellar evolution are indeed sound.
The new results come from careful observation of the orbit of Atlas and its companion -- a binary relationship that wasn't conclusively demonstrated until 1974 and certainly was unknown to ancient watchers of the sky. Using data from the Mt. Wilson stellar interferometer (located next to the historic Mt. Wilson Observatory in the San Gabriel range) and the Palomar Testbed Interferometer at Caltech's Palomar Observatory in San Diego County, the team determined a precise orbit of the binary.
Interferometry is an advanced technique that allows, among other things, for the "splitting" of two bodies that are so far away that they normally appear as a single blur, even in the biggest telescopes. Knowing the orbital period and combining it with orbital mechanics allowed the team to infer the distance between the two bodies, and with this information, to calculate the distance of the binary to Earth.
"For many months I had a hard time believing our distance estimate was 10 percent larger than that published by the Hipparcos team," said the lead author, Xiao Pei Pan of JPL. "Finally, after intensive rechecking, I became confident of our result."
Coauthor Shrinivas Kulkarni, MacArthur Professor of Astronomy and Planetary Science at Caltech, said, "Our distance estimate shows that all is well in the heavens. Stellar models used by astronomers are vindicated by our value."
"Interferometry is a young technique in astronomy and our result paves the way for wonderful returns from the Keck Interferometer and the anticipated Space Interferometry Mission that is expected to be launched in 2009," said coauthor Michael Shao of JPL. Shao is also the principal scientist for the Keck Interferometer and the Space Interferometry Mission.
The Palomar Testbed Interferometer was designed and built by a team of researchers from JPL led by Shao and JPL engineer Mark Colavita. Funded by NASA, the interferometer is located at the Palomar Observatory near the historic 200-inch Hale Telescope.
The device served as an engineering testbed for the interferometer that now links the 10-meter Keck Telescopes atop Mauna Kea in Hawaii.
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Now that is a very important scientific discovery!
Step by step, Astronomers can calibrate their measurements by using known "standards" and using those calibrations, develop another set of "standards."
When the basic "standards" are measure with more precision, all other measurements that were inferred from the original calibrations are also altered.
Now that is a very important scientific discovery!
Step by step, Astronomers can calibrate their measurements by using known "standards" and using those calibrations, develop another set of "standards."
When the basic "standards" are measure with more precision, all other measurements that were inferred from the original calibrations are also altered.
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