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再见,卡西尼号:13年土星旅程迎来最后时刻

更新时间:2017-9-16 10:39:57 来源:纽约时报中文网 作者:佚名

Cassini’s Final Moments and Plunge Into Saturn: What to Expect
再见,卡西尼号:13年土星旅程迎来最后时刻

PASADENA, Calif. — Cassini is accelerating to its end.

加州帕萨迪纳——卡西尼号(Cassini)正在加速走向它的终点。

Early Friday, NASA’s Cassini spacecraft, which has been studying Saturn, its rings and its moons for 13 years, will dip a bit deeper into the planet’s atmosphere.

周五早些时候,13年来一直在研究土星、土星环及土星卫星的美国航空航天局(NASA)卡西尼号太空船将会更加深入该行星的大气层。

High above the cloud tops, the atmosphere is still thin, nearly a vacuum. “The analog of that on Earth might be where the International Space Station is,” Earl Maize, Cassini’s project manager, said at a news conference on Wednesday.

在云顶上方,大气依然非常稀薄,几乎是真空。“地球上与之相似的位置可能是国际空间站所在的地点,”卡西尼号项目主管厄尔·梅兹(Earl Maize)于周三在新闻发布会上说。

But at the speed Cassini will be flying, about 76,000 miles per hour, the force of even a few molecules from Saturn’s atmosphere will be enough to tear the spacecraft to pieces.

但是卡西尼号将以每小时约7.6万英里的速度飞行,以这个速度,土星大气层中的几个分子就足以将它撕成碎片。

“Cassini will be vaporized in maybe two minutes,” Dr. Maize said. “But I think more like one. It’s just inevitable.”

“卡西尼号大约会在两分钟后汽化,”梅兹说。“但我觉得一分钟就够了。这是不可避免的。”

That is exactly as he and his team planned it.

这正是他和他的团队计划中的事。

Why is the Cassini mission coming to an end?

为什么卡西尼号的任务即将结束?

With Cassini’s fuel running low, NASA is cleaning up after itself, leaving the Saturn system as pristine as it found it. Any spacecraft, even one launched in 1997, has unwanted microbial hitchhikers aboard. In particular, planetary scientists want to ensure that there is zero chance of the spacecraft crashing and contaminating Titan or Enceladus, two moons that could be hospitable for life, with hidden passengers from Earth.

随着卡西尼号的燃料即将耗尽,NASA目前正在做善后工作,以避免对土星系统构成影响。任何航天器上都会附着有害的微生物,即便是在1997年发射的也是如此。研究行星的科学家们特别希望确保飞船坠毁时,不会让土卫六和土卫二沾染上飞船从地球带来的那些不可见的不速之客,这两颗卫星存在适合人类居住的可能性。

NASA did the same thing with its Galileo orbiter in 2003, sending it plunging into the clouds of Jupiter to protect Europa, another moon where scientists think life could exist.

2003年,NASA对伽利略号(Galileo)轨道飞行器做了同样的处理,令其坠入木星的云层中,以保护木卫二——另一个科学家认为可能有生命存在的卫星。

How will the Cassini mission conclude?

卡西尼号的任务将如何结束?

The beginning of the end was Monday, when Cassini flew close to Titan, the biggest of Saturn’s moons, for the 127th time. The flybys have provided a close-up examination of an intriguing haze-shrouded world; Cassini’s navigators on Earth have also enlisted the flybys as gravitational kicks to send it to the next target.

这一终结时刻于周一开始,当时卡西尼号第127次飞近土星最大的卫星土卫六。这些近天体探测飞行令人们得以详尽观察一个迷雾笼罩、引人入胜的世界;卡西尼号在地球上的导航员还会利用近天体探测飞行进行引力弹射,帮助飞船飞向下一个目标。

This last flyby was “just close enough, just the right orientation to seal Cassini’s fate,” Dr. Maize said.

这最后的一次近天体探测飞行“足够接近,方向也刚刚好够把卡西尼推向命运的终点,”梅兹说。

On Wednesday and Thursday, Cassini started taking a final set of photographs, of the rings, Enceladus, Titan and Saturn itself. One image will be the spot where Cassini will disintegrate.

周三和周四,卡西尼号开始拍摄最后一组照片,包括土卫二、土卫六和土星。它还会在卡西尼号最终分解的地方拍摄一张照片。

At 5:45 p.m. Eastern time on Thursday, that final stream of images will start arriving on Earth. When that is complete, more than 10 hours later, “We will then reconfigure Cassini for its very final transmissions,” Dr. Maize said.

东部时间周四下午5:45,最终一波图像流开始到达地球。传输完成后,也就是10多个小时后,“我们将重新配置卡西尼的最终传输方式,”梅兹说。

For most of the mission, Cassini collected observations and stored them in its memory to transmit to Earth later. On Thursday and Friday, there will be no time. Instead, Cassini will keep its main antenna pointed toward Earth and send data back almost as soon its instruments collects it. The transmission is too slow for photographs, so the camera will be turned off during those final hours.

在大部分任务中,卡西尼号收集观测资料,将其放在存储器里,稍后传送到地球。周四和周五没有时间这样做了。届时,卡西尼将保持其主天线指向地球,并在仪器收集数据后立即发送回去。对于照片来说,这样的传输方式太慢,所以在最后几个小时内,太空船上的相机将被关闭。

Cassini’s final moments

卡西尼号的最后时刻

The intake collector for an instrument to identify the constituents of the atmosphere will be pointed forward, giving scientists their deepest look so far at the gases that make up Saturn. “Essentially it’s just getting full blast of atmosphere,” Dr. Maize said.

太空船上将会探出一个用于识别大气成分的进气口收集器,令科学家可以进一步深入了解组成土星的气体。“基本上它将面对的是大气的全力冲击,”梅兹说。

But the data collection will be short-lived. As the drag of gas molecules starts twisting the spacecraft, eight small thrusters will fire to keep it upright. But each thruster is tiny, exerting about an eighth of a pound of force, and Cassini is about the size and mass of a 30-passenger school bus.

但数据收集将会是短暂的。由于气体分子的阻力开始令太空船飞行发生扭曲,它将发动八个小型推进器,以保持笔直的飞行方向。但是每个推进器都很小,只能施加约八分之一磅的力量,而卡西尼号的体积和重量则相当于一辆30座校车。

For the last five orbits, the spacecraft has dipped into the upper wisps of Saturn’s atmosphere, its thrusters keeping the spacecraft oriented in the right direction.

在最后五次绕轨道运行过程中,太空船已初步进入土星大气层的上层,其推进器使它保持正确方向。

But on this final plunge, Cassini will not emerge on the other side.

但在最后一次下行时,卡西尼号不会再绕到土星的另一边。

“What will happen is the thrusters will eventually be overpowered by the atmosphere,” Dr. Maize said.

“接下来,推进器的力量最终将被大气的力量压倒,”梅兹说。

Soon afterward, Cassini will be ripped apart.

不久后,卡西尼号就会被撕裂。

Much of the spacecraft, made of aluminum will melt very quickly. The most resilient bits will likely be the casings around its plutonium power source — the 72 marshmallow-sized pellets are encased in iridium and graphite containers designed to withstand re-entry into Earth’s atmosphere or an explosion at launch. “Those will be the last pieces,” Dr. Maize said.

太空船大部分由铝制成,很快就会融化。它所采用的钚电机是72个棉花糖大小的颗粒,被包裹在由铱和石墨制成的容器中,这个外壳能够承受重新进入地球大气层或发射时的爆炸,可能是飞船上最耐久的部分。“它们将会是最后的残骸,”梅兹说。

Cassini’s radio transmissions will disappear at 7:55 a.m., according to calculations by NASA engineers. The time of death at Saturn will actually have been one hour, 23 minutes earlier, but that is the time it takes the signals, moving at the speed of light, to travel the 1 billion miles that currently separate Saturn and Earth, picked up by radio telescopes in Australia and then sent to NASA’s Jet Propulsion Laboratory here.

根据NASA工程师的计算,卡西尼号的无线电传输将在上午7:55消失。这距离它在土星上死亡的时间实际上已经过去1小时23分钟,但信号正是在这段时间里以光速移动,穿越届时土星和地球之间相隔的10亿英里,被位于澳大利亚的射电望远镜收集,然后传送到NASA的喷气推进实验室。

Then, for the foreseeable future, there will be no new data coming from Saturn.

之后,在可预见的未来,不会再有来自土星的新数据了。

NASA TV will broadcast live commentary online of Cassini’s end, beginning at 7 a.m. Eastern time on Friday.

NASA电视台将于东部时间周五上午7时开始在线直播卡西尼号的最后时刻。

A science mission to the very end

延续到最后一刻的科学任务

Scientists will be studying for years the information that Cassini gathered. But the engineers working on the mission will disperse to new projects.

卡西尼号所收集的信息将供科学家们研究多年。但是,目前从事这一任务的工程师将会分散到新的项目中去。

“It’s a mix of sadness of Cassini ending, saying goodbye to this Cassini family we talk about,” said Linda Spilker, the project scientist. “We’ve been together, for lots of us, for multiple decades.”

“卡西尼号的结束让人有些伤感,要和我们所说的这个卡西尼大家庭说再见了,”项目科学家琳达·斯皮尔克(Linda Spilker)表示。“我们当中很多人在一起合作已经几十年了。”

Although the orbital maneuvers that will bring the mission to an end have been a top priority in recent weeks, Cassini’s scientific mission has carried on.

虽然最近几周以来,终结该任务的轨道飞行操作一直是重中之重,但卡西尼号的科学任务仍在继续。

Since April, Cassini has been making daring plunges between Saturn and its innermost ring. For almost all of the 13 years there, Cassini has steered far from the rings to avoid the possibility of a fatal collision with a piece of rock or ice. With the impending end, now was the time to take bigger risks.

自四月份以来,卡西尼号一直在土星与土星环最内侧之间进行大胆冒险。探测土星的13年以来,卡西尼号大部分时间远离土星环,避免同其石块或冰块发生致命碰撞的可能。随着终点即将到来,现在是冒险的时候了。

Cassini passed through the gap, unharmed, 22 times, providing a closest view yet of the rings.

卡西尼22次穿过空隙,毫发无损,提供了迄今距离土星环最近的照片。

Along the way, it took a few final pictures of old favorites, Enceladus and Titan.

这一路上,它还为我们格外关心的土卫二和土卫六拍了最后的几组照片。

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