太陽系下一個矚目焦點：木星

中國科技網5月4日報道（張微 編譯）我們最近總是談論冥王星、土星或火星。但好像沒什麼人再去討論有關木星的話題。為什麼會這樣?我想說的是，要知道它可是太陽系中最大的行星。318倍於地球的質量，這可是非常重要的，怎麼能被人遺忘呢，對吧?難道不是嗎?

木星是太陽系中最重要的地方之一。而且它讓人記憶深刻：比地球強大的多的古氣旋風暴，一個讓你覺得無法想像的強大磁層圈。

拜訪木星最讓人信服的理由之一，就是因為它的衛星，木衛二、木衛四和木衛三都可能在冰殼之下存在大型的液態水海洋。而且，正如你可能了解的，在地球上，哪裡有液態水，哪裡就會有生命存在。

因此，在整個太陽系中，木星的冰衛星是最有可能存在生命的地方。

當我在2016年初錄製視頻時，木星和它的衛星上沒有任何航天器。實際上，那裡已經冷清多年了。上一次宇宙飛船拜訪木星是2007年NASA的新視野號。火星被軌道器和火星車頻繁造訪，我們也剛剛靠近了冥王星給它拍攝了照片!但是，我們在近10年的時間裡都沒有近距離地觀測木星了。這到底是為什麼?

部分原因是木星實在住的太遠了，需要很長時間才能到達目的地。

第一個從地球到木星的太空船是美國航空航天局的先驅者10號。它於1972年3月3日發射，並於1973年12月3日到達。飛行時間總計640天。

但是先驅者10號只是在探索外太陽系的時候順路飛掠了木星，沒有真正停留。它在距離行星13萬公里以內的距離，為木星拍攝了首個特寫照片，然後就繼續深入太空進行其持續11年的飛行，直到與美國航天局失去聯絡。

先驅者11號一年前發射，在一年之後到達。在這606天的旅程中，它做了更加近距離的飛掠，到達了距離木星2萬1千公里的地方，而且還訪問了土星。

繼任者是旅行者號探測器。旅行者1號只花了546天時間，於1979年3月5日到達木星，旅行者2號用時688天。

所以，如果你也打算做這樣一次旅行，那麼你需要花550-650天的時間在路上。

但是，如果你想要放慢速度，進入木星軌道，圍繞它運轉，你就要花更長的時間在旅途上。目前，唯一一個曾經在木星周圍徘徊的飛行器是美國宇航局的伽利略號宇宙飛船，它發射於1989年10月18日。

它沒有採用直接飛向木星的路徑，伽利略號在其軌道上一次飛掠過金星，兩次飛掠過地球，利用天體的引力獲得助推力來加快速度，最後到達木星的時間是1995年12月8日，共花費2242天

為什麼伽利略號花費的時間比別的航天器都長呢?這是因為當你接近木星的時候，你需要行進的足夠緩慢，你才能真正進入這個行星的軌道並圍繞其運轉，而不是從它身邊加速飛過。

現在，經過長期的對木星的外圍偵查，我們將有另一個宇宙飛船到達這個巨大行星並進入它的軌道。2011年8月5日，朱諾號木星探測器開始踏上遠征木星之旅，它一直在內太陽系中繁忙，忙於積累速度來完成木星之旅。

它在2013年重新返回了地球，如果一切順利，那麼朱諾號將於2016年7月4日進入木星系統。總飛行時間1795天。

我們就再一次有了一個航天器，能夠觀察木星和它的衛星。

這僅僅是個開始。我們還有多個針對木星的任務正在計劃中。歐洲航天局將在2022年發射木星冰衛星任務(JupiterIcyMoonsMission)，這次任務將用時8年，航天器於2030年達到木星。

美國宇航局的木衛二多飛掠任務(EuropaMultiple-Flyby Mission)(編者註：之前被稱作Europa Clipper)可能會在相同的時間表內發射航天器，並圍繞木衛二運轉。它可能無法探測到冰面下的任何生命，但是它會準確地弄清這個星球上海洋開始的地方。

那麼，到木星需要多長時間?600天左右，如果你只想做一次飛掠而不是在它周圍徘徊的話，或約2000天，如果你想真正進入木星軌道。

How long does it take to get to Jupiter?

We re always talking aboutPluto, orSaturnor Mars. But nobody ever seems to talk about Jupiter any more. Why is that? I mean, it s the largest planet in thesolarsystem. 318 times the mass of the Earth has got to count for something, right? Right?

Jupiter is one of the most important places in the solar system. The planet itself is impressive; with ancient cyclonic storms larger than the Earth, or a magnetosphere so powerful it defies comprehension.

One of the most compelling reasons to visit Jupiter is because of its moons. Europa, Callisto and Ganymede might all contain vast oceans of liquid water underneath icy shells. And as you probably know, wherever we find liquid water on Earth, we find life.

And so, the icy moons of Jupiter are probably the best place to look for life in the entire solar system.

And yet, as I record this video in early 2016, there are no spacecraft at Jupiter or its moons. In fact, there haven t been any there for years. The last spacecraft to visit Jupiter was NASA sNewHorizons in 2007. Mars is buzzing with orbiters and rovers, we just got close up pictures ofPluto! and yet we haven t seen Jupiter close up in almost 10 years. What s going on?

Part of the problem is that Jupiter is really far away, and it takes a long time to get there.

How long? Let s take a look at all the spacecraft that have ever made this journey.

The first spacecraft to ever cross the gulf from the Earth to Jupiter was NASA sPioneer10. It launched on March 3, 1972 and reached on December 3, 1973. That s a total of 640 days of flight time.

But Pioneer 10 was just flying by, on its way to explore the outer solar system. It came within 130,000 km of the planet, took the first close up pictures ever taken of Jupiter, and then continued on into deep space for another 11 years before NASA lost contact.

Pioneer11 took off a year later, and arrived a year later. It made the journey in 606 days, making a much closer flyby, getting within 21,000 kilometers of Jupiter, and visitingSaturntoo.

Next came theVoyagerspacecraft. Voyager 1 took only 546 days, arriving on March 5, 1979, andVoyager2 took 688 days.

So, if you re going to do a flyby, you ll need about 550-650 days to make the journey.

But if you actually want to slow down and go into orbit around Jupiter, you ll need to take a much slower journey. The only spacecraft to ever stick around Jupiter was NASA s Galileo spacecraft, which launched on October 18, 1989.

Instead of taking the directpathto Jupiter, it made two gravitational assisting flybys of Earth and one of Venus to pick up speed, finally arriving at Jupiter on December 8, 1995. That s a total of 2,242 days.

So why did Galileo take so much longer to get to Jupiter? It s because you need to be going slow enough that when you reach Jupiter, you can actually enter orbit around the planet, and not just speed on past.

And now, after this long period of Jupiterlessness, we re about to have another spacecraft arrive at the massive planet and go into orbit. NASA s Juno spacecraft was launched back on August 5, 2011 and it s been buzzing around the innersolarsystem, building up the velocity to make the journey to Jupiter.

It did a flyby of Earth back in 2013, and if everything goes well, Juno will make its orbital insertion into the Jovian system on July 4, 2016. Total flight time: 1,795 days.

Once again, we ll have a spacecraft observing Jupiter and its moons.

This is just the beginning. There are several more missions to Jupiter in the works. The European Space Agency will be launching the Jupiter IcyMoonsMission in 2022, which will take nearly 8 years to reach Jupiter by 2030.

NASA s Europa Multiple-Flyby Mission [Editor s note: formerly known as the Europa Clipper] will probably launch in the same timeframe, and spend its time orbiting Europa, trying to get a better understand the environment onEuropa. It probably won t be able to detect any life down there, beneath the ice, but it ll figure out exactly where the ocean starts.

So, how long does it take to get toJupiter? Around 600 days if you want to just do a flyby and aren t planning to stick around, or about 2,000 days if you want to actually get into orbit.

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