T**********e
发帖数: 29576 | 1 In a Breathtaking First, NASA Craft Exits the Solar System
Since the launch of the Voyager spacecraft in 1977, Voyager 1 has traveled
over 11.7 billion miles from the launchpad pictured here. That is equivalent
to traveling to the moon and back almost 25,000 times.
PASADENA, Calif. — The spacecraft’s technology was laughable by today’s
standards: it carried an 8-track tape recorder and computers with 240,000
times less memory than a low-end iPhone. When it left Earth 36 years ago, it
was designed as a four-year mission to Saturn, and everything after that
was gravy.
But Voyager I has become — unexpectedly — the Little Spacecraft That Could
. On Thursday, scientists declared that it had become the first man-made
object to exit the solar system, a breathtaking achievement that NASA could
only fantasize about back when it was launched in 1977, the same year that
“Star Wars” was released.
http://www.nytimes.com/2013/09/13/science/in-a-breathtaking-fir | w********r
发帖数: 14958 | 2 同年，我国光荣伟大的无产阶级文化大革命胜利结束。 | x**h
发帖数: 1375 | | c*********e
发帖数: 16335 | 4 可惜不能传回图像了。
equivalent
it
Could
【在 T**********e 的大作中提到】
: In a Breathtaking First, NASA Craft Exits the Solar System
: Since the launch of the Voyager spacecraft in 1977, Voyager 1 has traveled
: over 11.7 billion miles from the launchpad pictured here. That is equivalent
: to traveling to the moon and back almost 25,000 times.
: PASADENA, Calif. — The spacecraft’s technology was laughable by today’s
: standards: it carried an 8-track tape recorder and computers with 240,000
: times less memory than a low-end iPhone. When it left Earth 36 years ago, it
: was designed as a four-year mission to Saturn, and everything after that
: was gravy.
: But Voyager I has become — unexpectedly — the Little Spacecraft That Could
| b*******y
发帖数: 4304 | 5 How Do We Know When Voyager Reaches Interstellar Space?
Sept 12, 2013
Artist's concept of Voyager's distances
You Are Here, Voyager: This artist's concept puts huge solar system
distances in perspective. The scale bar is measured in astronomical units (
AU), with each set distance beyond 1 AU representing 10 times the previous
distance. Each AU is equal to the distance from the sun to the Earth. It
took from 1977 to 2013 for Voyager 1 to reach the edge of interstellar space.
Image Credit: NASA/JPL-Caltech
Full image and caption
Illustration of Voyager 1
Layers of Intrigue: This illustration shows the outer layers of our solar
bubble, or heliosphere, and the interstellar space that Voyager 1 is
currently investigating.
Image Credit: NASA/JPL-Caltech
Full image and caption
Whether and when NASA's Voyager 1 spacecraft, humankind's most distant
object, broke through to interstellar space, the space between stars, has
been a thorny issue. For the last year, claims have surfaced every few
months that Voyager 1 has "left our solar system." Why has the Voyager team
held off from saying the craft reached interstellar space until now?
"We have been cautious because we're dealing with one of the most important
milestones in the history of exploration,” said Voyager Project Scientist
Ed Stone of the California Institute of Technology in Pasadena. “Only now
do we have the data -- and the analysis -- we needed."
Basically, the team needed more data on plasma, which is ionized gas, the
densest and slowest moving of charged particles in space. (The glow of neon
in a storefront sign is an example of plasma.) Plasma is the most important
marker that distinguishes whether Voyager 1 is inside the solar bubble,
known as the heliosphere, which is inflated by plasma that streams outward
from our sun, or in interstellar space and surrounded by material ejected by
the explosion of nearby giant stars millions of years ago. Adding to the
challenge: they didn't know how they'd be able to detect it.
"We looked for the signs predicted by the models that use the best available
data, but until now we had no measurements of the plasma from Voyager 1,"
said Stone.
Scientific debates can take years, even decades to settle, especially when
more data are needed. It took decades, for instance, for scientists to
understand the idea of plate tectonics, the theory that explains the shape
of Earth's continents and the structure of its sea floors. First introduced
in the 1910s, continental drift and related ideas were controversial for
years. A mature theory of plate tectonics didn't emerge until the 1950s and
1960s. Only after scientists gathered data showing that sea floors slowly
spread out from mid-ocean ridges did they finally start accepting the theory
. Most active geophysicists accepted plate tectonics by the late 1960s,
though some never did.
Voyager 1 is exploring an even more unfamiliar place than our Earth's sea
floors -- a place more than 11 billion miles (17 billion kilometers) away
from our sun. It has been sending back so much unexpected data that the
science team has been grappling with the question of how to explain all the
information. None of the handful of models the Voyager team uses as
blueprints have accounted for the observations about the transition between
our heliosphere and the interstellar medium in detail. The team has known it
might take months, or longer, to understand the data fully and draw their
conclusions.
"No one has been to interstellar space before, and it's like traveling with
guidebooks that are incomplete," said Stone. "Still, uncertainty is part of
exploration. We wouldn't go exploring if we knew exactly what we'd find."
The two Voyager spacecraft were launched in 1977 and, between them, had
visited Jupiter, Saturn, Uranus and Neptune by 1989. Voyager 1's plasma
instrument, which measures the density, temperature and speed of plasma,
stopped working in 1980, right after its last planetary flyby. When Voyager
1 detected the pressure of interstellar space on our heliosphere in 2004,
the science team didn't have the instrument that would provide the most
direct measurements of plasma. Instead, they focused on the direction of the
magnetic field as a proxy for source of the plasma. Since solar plasma
carries the magnetic field lines emanating from the sun and interstellar
plasma carries interstellar magnetic field lines, the directions of the
solar and interstellar magnetic fields were expected to differ.
Most models told the Voyager science team to expect an abrupt change in the
magnetic field direction as Voyager switched from the solar magnetic field
lines inside our solar bubble to those in interstellar space. The models
also said to expect the levels of charged particles originating from inside
the heliosphere to drop and the levels of galactic cosmic rays, which
originate outside the heliosphere, to jump.
In May 2012, the number of galactic cosmic rays made its first significant
jump, while some of the inside particles made their first significant dip.
The pace of change quickened dramatically on July 28, 2012. After five days,
the intensities returned to what they had been. This was the first taste
of a new region, and at the time Voyager scientists thought the spacecraft
might have briefly touched the edge of interstellar space.
By Aug. 25, when, as we now know, Voyager 1 entered this new region for good
, all the lower-energy particles from inside zipped away. Some inside
particles dropped by more than a factor of 1,000 compared to 2004. The
levels of galactic cosmic rays jumped to the highest of the entire mission.
These would be the expected changes if Voyager 1 had crossed the heliopause
, which is the boundary between the heliosphere and interstellar space.
However, subsequent analysis of the magnetic field data revealed that even
though the magnetic field strength jumped by 60 percent at the boundary, the
direction changed less than 2 degrees. This suggested that Voyager 1 had
not left the solar magnetic field and had only entered a new region, still
inside our solar bubble, that had been depleted of inside particles.
Then, in April 2013, scientists got another piece of the puzzle by chance.
For the first eight years of exploring the heliosheath, which is the outer
layer of the heliosphere, Voyager's plasma wave instrument had heard nothing
. But the plasma wave science team, led by Don Gurnett and Bill Kurth at the
University of Iowa, Iowa City, had observed bursts of radio waves in 1983
to 1984 and again in 1992 to 1993. They deduced these bursts were produced
by the interstellar plasma when a large outburst of solar material would
plow into it and cause it to oscillate. It took about 400 days for such
solar outbursts to reach interstellar space, leading to an estimated
distance of 117 to 177 AU (117 to 177 times the distance from the sun to the
Earth) to the heliopause. They knew, though, that they would be able to
observe plasma oscillations directly once Voyager 1 was surrounded by
interstellar plasma.
Then on April 9, 2013, it happened: Voyager 1's plasma wave instrument
picked up local plasma oscillations. Scientists think they probably stemmed
from a burst of solar activity from a year before, a burst that has become
known as the St. Patrick's Day Solar Storms. The oscillations increased in
pitch through May 22 and indicated that Voyager was moving into an
increasingly dense region of plasma. This plasma had the signatures of
interstellar plasma, with a density more than 40 times that observed by
Voyager 2 in the heliosheath.
Gurnett and Kurth began going through the recent data and found a fainter,
lower-frequency set of oscillations from Oct. 23 to Nov. 27, 2012. When they
extrapolated back, they deduced that Voyager had first encountered this
dense interstellar plasma in August 2012, consistent with the sharp
boundaries in the charged particle and magnetic field data on August 25.
Stone called three meetings of the Voyager team. They had to decide how to
define the boundary between our solar bubble and interstellar space and how
to interpret all the data Voyager 1 had been sending back. There was general
agreement Voyager 1 was seeing interstellar plasma, based on the results
from Gurnett and Kurth, but the sun still had influence. One persisting sign
of solar influence, for example, was the detection of outside particles
hitting Voyager from some directions more than others. In interstellar space
, these particles would be expected to hit Voyager uniformly from all
directions.
"Now that we had actual measurements of the plasma environment – by way of
an unexpected outburst from the sun – we had to reconsider why there was
still solar influence on the magnetic field and plasma in interstellar space
," Stone said.
"The path to interstellar space has been a lot more complicated than we
imagined."
Stone discussed with the Voyager science group whether they thought Voyager
1 had crossed the heliopause. What should they call the region were Voyager
1 is?
"In the end, there was general agreement that Voyager 1 was indeed outside
in interstellar space," Stone said. "But that location comes with some
disclaimers – we're in a mixed, transitional region of interstellar space.
We don't know when we'll reach interstellar space free from the influence of
our solar bubble."
So, would the team say Voyager 1 has left the solar system? Not exactly –
and that's part of the confusion. Since the 1960s, most scientists have
defined our solar system as going out to the Oort Cloud, where the comets
that swing by our sun on long timescales originate. That area is where the
gravity of other stars begins to dominate that of the sun. It will take
about 300 years for Voyager 1 to reach the inner edge of the Oort Cloud and
possibly about 30,000 years to fly beyond it. Informally, of course, "solar
system" typically means the planetary neighborhood around our sun. Because
of this ambiguity, the Voyager team has lately favored talking about
interstellar space, which is specifically the space between each star's
realm of plasma influence.
"What we can say is Voyager 1 is bathed in matter from other stars," Stone
said. "What we can't say is what exact discoveries await Voyager's continued
journey. No one was able to predict all of the details that Voyager 1 has
seen. So we expect more surprises."
Voyager 1, which is working with a finite power supply, has enough
electrical power to keep operating the fields and particles science
instruments through at least 2020, which will mark 43 years of continual
operation. At that point, mission managers will have to start turning off
these instruments one by one to conserve power, with the last one turning
off around 2025.
Voyager 1 will continue sending engineering data for a few more years after
the last science instrument is turned off, but after that it will be sailing
on as a silent ambassador. In about 40,000 years, it will be closer to the
star AC +79 3888 than our own sun. (AC +79 3888 is traveling toward us
faster than we are traveling towards it, so while Alpha Centauri is the next
closest star now, it won't be in 40,000 years.) And for the rest of time,
Voyager 1 will continue orbiting around the heart of the Milky Way galaxy,
with our sun but a tiny point of light among many.
The Voyager spacecraft were built and continue to be operated by NASA's Jet
Propulsion Laboratory, in Pasadena, Calif. Caltech manages JPL for NASA. The
Voyager missions are a part of NASA's Heliophysics System Observatory,
sponsored by the Heliophysics Division of the Science Mission Directorate at
NASA Headquarters in Washington.
【在 c*********e 的大作中提到】
: 可惜不能传回图像了。
:
: equivalent
: it
: Could
| T**********e
发帖数: 29576 | 6 旅行者一号的能量是没问题，飞行速度每秒17公里， 应该不是用火箭推动，如果利用
星体重力，如何掌握方向比较有意思。
Voyager 1 has three radioisotope thermoelectric generators (RTGs) mounted on
a boom. Each MHW-RTG contains 24 pressed plutonium-238 oxide spheres. The
RTGs generated about 470 Watts of electric power at the time of launch, with
the remainder being dissipated as waste heat.[18] The power output of the
RTGs does decline over time (halving every 87.7 yrs), but the RTGs of
Voyager 1 will continue to support some of its operations until around 2025. | b*******y
发帖数: 4304 | 7 插！这都什么乱七八糟， 都靠得是重力slingshot 才能走这么远
on
with
2025.
【在 T**********e 的大作中提到】
: 旅行者一号的能量是没问题，飞行速度每秒17公里， 应该不是用火箭推动，如果利用
: 星体重力，如何掌握方向比较有意思。
: Voyager 1 has three radioisotope thermoelectric generators (RTGs) mounted on
: a boom. Each MHW-RTG contains 24 pressed plutonium-238 oxide spheres. The
: RTGs generated about 470 Watts of electric power at the time of launch, with
: the remainder being dissipated as waste heat.[18] The power output of the
: RTGs does decline over time (halving every 87.7 yrs), but the RTGs of
: Voyager 1 will continue to support some of its operations until around 2025.
| T**********e
发帖数: 29576 | 8
那靠什么掌握方向和速度？
【在 b*******y 的大作中提到】
: 插！这都什么乱七八糟， 都靠得是重力slingshot 才能走这么远
:
: on
: with
: 2025.
| b***n
发帖数: 1210 | 9 应该远远不止每秒17公里, 引力加速后速度至少快2-3倍吧
on
with
2025.
【在 T**********e 的大作中提到】
: 旅行者一号的能量是没问题，飞行速度每秒17公里， 应该不是用火箭推动，如果利用
: 星体重力，如何掌握方向比较有意思。
: Voyager 1 has three radioisotope thermoelectric generators (RTGs) mounted on
: a boom. Each MHW-RTG contains 24 pressed plutonium-238 oxide spheres. The
: RTGs generated about 470 Watts of electric power at the time of launch, with
: the remainder being dissipated as waste heat.[18] The power output of the
: RTGs does decline over time (halving every 87.7 yrs), but the RTGs of
: Voyager 1 will continue to support some of its operations until around 2025.
| T**********e
发帖数: 29576 | 10
这是wiki上说现在的速度。
【在 b***n 的大作中提到】
: 应该远远不止每秒17公里, 引力加速后速度至少快2-3倍吧
:
: on
: with
: 2025.
|
|
