【每日阅读训练第四期——速度越障8系列】【8-4】科技

CHRISTINE2010

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First James Webb telescope mirrors delivered to Nasa


[attachimg=624,351]107031[/attachimg] The first two components of the huge mirror set to fly on the US James Webb Space Telescope (JWST) have been delivered to Nasa.

James Webb - regarded as the successor to Hubble - is due to launch in 2018.
After they have been checked, the hexagonal mirror components will be stored until engineers are ready to assemble them onto the telescope.
Some 18 of them will make up JWST's 6.5m primary mirror, which is more than twice as wide as Hubble's main mirror.
On 17 September, the mirrors left the facility of contractor Ball Aerospace in Boulder, Colorado, for Nasa's Goddard Space Flight Center in Maryland, where the telescope is being assembled.
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"These first two completed flight mirror assemblies arriving at Goddard are an important first step leading towards the integration of the mirrors onto the flight structure," said Lee Feinberg, the optical telescope element manager for JWST.
The remaining 16 mirror "assemblies" will make their way from Boulder to Nasa Goddard over the next 12 months as they await integration into the infrared telescope in 2015.
Each mirror component measures more than 1.3m across and weighs some 40kg.
JWST is the first civilian space observatory to use an actively controlled, segmented mirror.
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• James Webb's instruments will be tuned to light beyond the detection of our eyes - at near- and mid-infrared wavelengths
  
• It is in the infrared that very distant objects will show up, and also those objects that in the visible range are obscured by dust
  
• Hubble is a visible light telescope with some near-infrared capability, but its sensitivity will be dwarfed by JWST's technologies
  
• Europe's far-infrared Herschel space telescope has a bigger mirror than Hubble, but JWST's mirror will be larger still
  

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• James Webb's main mirror has around seven times more collecting area than Hubble's 2.4m primary mirror
  
• The sunshield is about 22m by 12m. There will be a 300-degree difference in temperature between the two sides
  
• James Webb's instruments must be very cold to ensure their own infrared glow does not swamp the observations
  
• The mission will launch in 2018 on an Ariane rocket. The observing position will be 1.5 million km from Earth
  

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It will also carry a shield the size of a tennis court to guard its sensitive vision from the heat and intense glare of the Sun.
James Webb is expected to be the premier space observatory of the next decade and will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of star systems capable of supporting life on planets like Earth.
In order to, for example, look back through time to observe young galaxies, JWST needs a large mirror to see such faint objects.
A telescope's sensitivity, or how much detail it can see, is directly related to the size of the mirror area that collects light from the objects being observed - with larger mirrors collecting more light than smaller ones.
Recommended 16 years ago as the logical evolution beyond Hubble, the JWST has managed to garner a fair amount of controversy.
Technical difficulties and project mismanagement mean the observatory is now running years behind schedule and is billions of dollars over-budget.
Elements of the US Congress wanted to cancel the telescope in summer 2011. That did not happen, but Capitol Hill now has James Webb on a very short leash, with Nasa required to provide monthly updates on milestones met or missed.
The current estimate for the US side of the mission is $8.8bn, which covers the full life cycle of the project from its inception to the end of initial operations.
Extra to that bill is some $650m for the European contributions like the Mid-Infrared Instrument (Miri) - which will help confirm the identity of the first luminous objects in the cosmos - and the Ariane rocket that will launch the observatory in October 2018.
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Eternal Clock Could Keep Time After Universe Dies



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The idea for an eternal clock that would continue to keep time even after the universe ceased to exist has intrigued physicists. However, no one has figured out how one might be built, until now.

Researchers have now proposed an experimental design for a "space-time crystal" that would be able to keep time forever. This four-dimensional crystal would be similar to conventional 3D crystals, which are structures, like snowflakes and diamonds, whose atoms are arranged in repeating patterns. Whereas a diamond has a periodic structure in three dimensions, the space-time crystal would be periodic in time as well as space.

The idea of a 4D space-time crystal was first proposed earlier this year by MIT physicist Frank Wilczek, though the concept was purely theoretical. Now a team of researchers led by Xiang Zhang of California's Lawrence Berkeley National Laboratory has conceived of how to make one a reality.

"The idea of creating a crystal with dimensions higher than that of conventional 3D crystals is an important conceptual breakthrough in physics, and it is very exciting for us to be the first to devise a way to realize a space-time crystal," Berkeley Lab physicist Tongcang Li, a member of the research group, said in a statement.

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Zhang and his colleagues suggest that a space-time crystal could be constructed using an electric field to trap charged atoms (called ions), and taking advantage of the natural repulsion between two like-charged particles (positive and positive, or negative and negative), which is called Coulomb repulsion.



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"The electric field of the ion trap holds charged particles in place and Coulomb repulsion causes them to spontaneously form a spatial ring crystal," Zhang said. "Under the application of a weak static magnetic field, this ring-shaped ion crystal will begin a rotation that will never stop. The persistent rotation of trapped ions produces temporal order, leading to the formation of a space-time crystal at the lowest quantum energy state."

In other words, the scientists would aim to create a ring of charged particles, with the resulting electromagnetic forces causing the structure to rotate perpetually. At its lowest quantum-energy state, also known as its ground state, the system has no disorder, or entropy, and there is no way for its entropy to increase over time. Thus, the crystal's temporal structure and timekeeping ability would continue even after the universe reached a state of "heat death," also known as thermodynamic equilibrium, when it had devolved into entropy.

The researchers describe their idea in a paper published recently in the journal Physical Review Letters.

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Scientists Prevent Heart Failure in Mice

ScienceDaily (Sep. 25, 2012) — Cardiac stress -- for example, a heart attack or high blood pressure -- frequently leads to pathological heart growth and subsequently to heart failure. Two tiny RNA molecules play a key role in this detrimental development in mice, as researchers at the Hannover Medical School and the Göttingen Max Planck Institute for Biophysical Chemistry have now discovered. When they inhibited one of those two specific molecules, they were able to protect the rodent against pathological heart growth and failure. With these findings, the scientists hope to be able to develop therapeutic approaches that can protect humans against heart failure.

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Respiratory distress, fatigue, and attenuated performance are symptoms that can accompany heart failure. A reason for this can be an enlarged heart, a so-called cardiac hypertrophy. It may develop when the heart is subjected to permanent stress, for example, due to persistent high blood pressure or a valvular heart defect. In order to boost the pumping performance, the heart muscle cells enlarge -- a condition that frequently results in heart failure if not treated.
Two small RNA molecules tip the balance
A research team at the Göttingen Max Planck Institute for Biophysical Chemistry and the Hannover Medical School discovered that two small RNA molecules play a key role in the growth of heart muscle cells: the microRNAs miR-212 and miR-132. The scientists had observed that these microRNAs are more prevalent in the cardiac muscle cells of mice suffering from cardiac hypertrophy. To determine the role that the two microRNAs play, the scientists bred genetically modified mice that had an abnormally large number of these molecules in their heart muscle cells. "These rodents developed cardiac hypertrophy and lived for only three to six months, whereas their healthy conspecifics had a normal healthy life-span of several years," explained Dr. Kamal Chowdhury, researcher in the Department of Molecular Cell Biology at the Max Planck Institute for Biophysical Chemistry. "For comparison, we also selectively switched off these microRNAs in other mice. These animals had a slightly smaller heart than their healthy conspecifics, but did not differ from them in behavior or life-span," continued the biologist. The crucial point is when the scientists subjected the hearts of these mice to stress by narrowing the aorta, the mice did not develop cardiac hypertrophy -- in contrast to normal mice.
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A microRNA inhibitor protects mice against hypertrophy
The scientists were also able to protect normal mice against the disease. When they gave them a substance that selectively inhibits microRNA-132, no pathological cardiac growth occurred -- even when the hearts of these mice were subjected to stress. "Thus, for the first time ever, we have found a molecular approach for treating pathological cardiac growth and heart failure in mice," said the cardiologist Professor Dr. Dr. Thomas Thum, MD, Director of the Institute for Molecular and Translational Therapy Strategies (IMTTS) at the Hannover Medical School. With these findings, the researchers hope that they will be able to develop therapeutic approaches that can also protect humans against heart failure. "Such microRNA inhibitors, alone or in combination with conventional treatments, could represent a promising new therapeutic approach," said Thum.
"In mice with an overdosage of the two microRNAs in their heart muscle cells, the cellular 'recycling program' is curbed," explained Dr. Ahmet Ucar, who together with Shashi K. Gupta was responsible for the experiments. In this recycling process, the cell breaks down components that are damaged or no longer required and reuses their constituents -- a vital process that, for example, ensures the organism's survival under stress conditions. In mice without the microRNAs -212 and 132, this recycling program is more active than in their normal conspecifics. Conceivably, the reduced cellular recycling could be a cause of the observed cardiac hypertrophy.
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【越障】

Earth-Sun Distance Measurement Redefined



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One of the stalwart units of astronomy just got a makeover. The International Astronomical Union, the authority on astronomical constants, has voted unanimously to redefine the astronomical unit, the conventional unit of length based on the distance between the Earth and the sun.

"The new definition is much simpler than the old one," says Sergei Klioner of the Technical University of Dresden in Germany, one of a group of scientists who worked decades toward the change, which took effect last month during an IAU meeting

Under the new definition, the astronomical unit (or AU) — the measurement used for the Earth-sun distance — is no longer always in flux, depending on the length of a day and other changing factors. It is now a fixed number: 149,597,870,700 meters, which is the equivalent of almost 92.956 million miles.

Klioner explained the simpler definition is helpful, for instance, for scientists who formulate ephemerides — tables that give the precise position of astronomical objects in the sky. They utilize the astronomical unit to calculate the motion of bodies in the solar system.

"The broader community of astronomers are able now to better, with less efforts, understand what their colleagues — astronomers who are experts in planetary ephemerides — do and how they produce the high-accuracy theories of motion in the solar system," he told SPACE.com by email.

The revision also makes the unit easier for engineers, software designers and students to understand, Klioner and his colleague Nicole Capitaine, of Paris Observatory, noted.

At the same time, the redefinition can serves an epitaph to the bygone era when Earth-bound scientists depended on viewing angles to calculate celestial distances.

An established unit

Lacking precise instrumentation, early astronomers relied heavily on angles to calculate the size of the universe. By studying Mars from two separate points on Earth, 17th-century Italian astronomer Giovanni Cassini was able to use trigonometry to calculate the distance from Earth to the sun with only about a 6 percent error.

"Expressing distances in the astronomical unit allowed astronomers to overcome the difficulty of measuring distances in some physical unit," Capitaine told SPACE.com by email. "Such a practice was useful for many years, because astronomers were not able to make distance measurements in the solar system as precisely as they could measure angles."

Modern instruments can come within a few meters of exactly determining distances of nearly150 billion meters (150 million kilometers), or some 93 million miles.

The astronomical unit eventually came to be defined by a mathematical expression that involved the mass of the sun, the length of a day, and a fixed number known as the Gaussian gravitational constant. Because the Earth orbits its star in an ellipse rather than a circle, the length of a day shifts over the course of a year. At the same time, the sun is constantly transforming mass into energy.

In the 20th century, famed scientist Albert Einstein added general relativity to the mix. According to the famous theory, space-time is relative depending on one's frame of reference.

The new fixed number is the best estimate of the original expression, Klioner said.

"If we would decide to continue with the old definition, we would have to add several additional conventions to make the latter meaningful in the framework of general relativity," he explained. "A better way was to change the definition completely — and this is what we succeeded in doing."

Capitaine said, "The change of definition of the astronomical unit mainly concerns those in the field of high-accuracy solar system dynamics."

Satellites and other crafts traveling in space are not affected, because they rely on set distances.

"The distance between the Earth and the sun, as any physical distance, should be measured and cannot be fixed by any sort of resolution," Klioner said.

The times, they are a-changing

Capitaine and Klioner are among several scientists who worked over the last two decades on a revision of the astronomical unit. Capitaine said she first got involved when she gave a presentation in 1994 with Bernard Guinot, also of Paris Observatory. Over the course of 10 years, several published papers by various scientists discussed the ramifications of changing the stalwart unit. The three scientists presented the issue to the astronomical community on a number of different occasions.

Other astronomers helped to demonstrate the feasibility of the change before it landed on the table of one of the working groups for the International Astronomical Union. The resolution was reworked several times before it won unanimous passage.

"The change as we have it now is really a product of collective work," Klioner said.

He went on to add, "I think that the energy, commitment, and the worldwide scientific reputation of Nicole Capitaine were crucial for getting this change through."

Shifting from a constantly changing value to a fixed number may seem like an easy choice, but the group faced some resistance. Some believed that the overhaul would be too difficult to implement with crucial software, while others were concerned that discrepancies might be introduced into past work. Still others were uncomfortable changing such a historic definition. Eventually, all concerns were apparently met.
"Within the last two years, I have not heard a single objection for the change itself," Klioner said.

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fightfor700

谢谢Christine分享。今天状态不好，周围同事电话、走来走去，被干扰后各种走神和回读。抗干扰能力还是太差了。。
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spencerX

谢谢christine的分享。

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puffywhite

真心感谢 ~~
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obstacle:按错了 ...
a recent meeting decided to redefine the distance between the earth and the sun, namely, the astronomical unit, and set it as a fix number.scientists argued that this change would bring a great deal of benefits.
for example, it will be simpler and help other scientists to locate other planets.
moreover,it will easier for students to understand such a definition.
than the passage provides us the history of measuring the astronomical unit.
at last, C and K, the leading scientists paticipated in the change, told us the difficulties and process during the change.
they said this action is a cooperative work and it has come across a lot of resistence before put into practice.

ENJ0Y

谢谢LZ。。。科技文继续无感 。。。。

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torresAing

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CCatherine

楼主 我想请教一下 文章是看完了 但是也没记住多少啊. 有没有一个着眼点能够让很累的时候能够继续有目标地看下去呢？

miffyhui

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Scientists redefined the AU
Under the new definition, The AU is no longer in influx.
1. This definition is help. Scientists used it to calculate themotion of body.
2. Revision makes the units easier for the engineers…
3. The redefinition can serves an …
The earlier calculation of size of the universe
Spacecraft and satellites will not be affected, because theyrely on the set distance
Shifting from the a changing value to a fixed number is not easier.Several astronomers resist it.