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Interplanetary Superhighway :: Articles :: Astronomy @Stardust Sand

[2007-04-15]

此文刊登於香港太空館2007年4-6月通訊中的天文新知專欄及太空館網頁


行星際高速公路 Interplanetary Superhighway

了脫離地球重力的綑綁,火箭成為了解開重力枷鎖的鎖匙。但一般現在所使用的化學火箭效率並不高,以為人熟悉的阿波羅11號登月任務所使用的土星5型火箭為例,火箭重量的90%為燃料,真正登月艙的重量只有10%;而登月艙本身的61%也是燃料,這樣才可以讓太空人登陸月球,和讓他們離開月面返回地球。然而,隨著太空探測船重量和目的地距離的增加,所需的燃料卻以指數方式地上升,如前往土星的卡西尼探測船,發射的泰坦4B型火箭,當中99.5%重量均為燃料,可想而知,化學燃料推進是多麼低效率的方法。

在太陽系的行星中穿梭,直線航行並非理想的路線,現在的探測船均使用赫曼轉移軌道(以太陽為焦點連接行星的橢圓軌道),再配合行星推助的技術航行(探測船經過行星時偷取行星的能量加速或減速)。可是這方法的成本仍很高,如木星軌道器伽利略號中,它總重量的42%為化學燃料。為了解決這問題,科學家現透過數學方法發展出一套可以用極少燃料甚至是零燃料便可以在行星中穿梭的方法,稱為「行星際傳輸網絡」。

就像橫渡越海洋的船隻借助洋流一樣,天空中也有類似連接起行星的管道,只要探測船進入了管道,推進探測船就不須能量,便會沿管道滑行在太陽系中穿梭。形成這無形管道的力量,就是宇宙間的萬有引力。以太陽和地球為例,太陽和地球之間的重力在5處不同的地方剛好互相抵消,稱為拉格朗日點L1-L5,物體可以停留在拉格朗日點中而不受重力影響,即使點中沒有物質,物體也可以圍繞拉格朗日點公轉。選取適當的初始速度,探測船便可以由地球到達繞L1或L2的軌道,這些軌道的集合形成了行星際傳輸網絡的管道。數學家發現,在兩顆互相公轉的天體,總會存在這些管狀的曲面,當物體位處管上,便會被驅往或驅離L1或L2。在管上行走,不用能量,重力自然會把物體送到目的地。近期美國太空總署收集太陽風樣本的創世號探測器便是利用這軌道,僅需負載4%的燃料,便可以環繞L1兩年時間收集樣本再滑行把樣本送返地球。這方法雖然令任務時間增加,卻大大節省燃料。

當不同行星之間的管道在重力之舞下連接,物體更可由一條管道跳往另一管道,因此,在精密的數學計算下,行星間的管道可以變成一條行星際的高速公路,讓探測船在行星中穿梭。科學家正考慮,在未來的日子,首先在地月之間的L1點興建永久的中轉站,成為往後前往各行星旅程的起點,那麼,到太陽系旅行,就不再是夢想了。

Rocket launcher is the key for us to escape from the gravitational pull of our home planet. Unfortunately, from the energy point of view, it is far from being efficient. Take Saturn V, the rocket launcher used in the well-known Apollo 11 mission as example. Chemical fuel alone contributed to 90% of the rocket weight, while the lunar module accounted for the remaining 10%. Worse still, the fuel required by the lunar module to carry out a round-trip to the Moon and back accounted for another 61% of the weight of the module. With space probes becoming increasingly massive and destinations becoming more remote, fuel consumption is escalating exponentially. The Titan 4B rocket employed in the Cassini-Huygens mission to planet Saturn had 99.5% of its weight coming from fuel. We can see how inefficient chemical fuel propulsion actually is.

In terms of efficiency in energy consumption, traveling among planets in straight lines not from desirable. Today, space probes customarily make use of Hohmann Transfer Orbits and gravity assists in their interplanetary endeavours. This method, though still quite costly, does help to save fuel to a certain extent. For instance, fuel occupied only 42% of the weight of Galileo, the probe that visited Jupiter several years ago. Scientists are now developing mathematical models that may allow space probes to travel among planets with minimal, or even nil fuel requirements. This model is called the Interplanetary Transport Network.

Much as water currents helping vessels to move across oceans, there exist pathways among planets that allow probes to glide through without expending much energy. These pathways are actually consequences of universal gravitation. Take the Sun and Earth as example. There are five points in space, called Lagrangnian Points L1 to L5, where the gravity of the Sun and Earth exactly cancels out each other. Objects residing at these points are in some sort of gravitational equilibrium. More intriguingly, probes can orbit about these Lagrangnian points even if no object is present there. With suitable initial speed, space probes can be put to orbit around L1 or L2. The collection of this kind of orbits constitutes the so-called interplanetary highways. Mathematicians found that tube-shpae manifold always exist whenever two celestial bodies are revolving around each other.

Any object in these tubes will always be attracted towards or driven away by gravity from L1 and L2, without extra energy input. The Genesis probe recently launched by NASA to collect samples of the solar wind was traveling along such kind of orbits. Fuel accounted for only 4% of the weight of the probe, which revolved around L1 for two years before returning to Earth. Traveling by means of this method is more time-consuming yet saves much fuel.

Tubes connecting different planets may even coalesce together, allowing space probes to jump from one tube to another. Mathematically speaking, these tubes form a giant network of interplanetary superhighways for probes to speed along. Scientists are now planning to build a transfer station at L1 between the Earth and the Moon as the starting point to reach for more remote planets. If successful, interplanetary tourism may no longer be a distant dream.



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