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Why are so many nations going to Mars this summer?
今年各國扎堆探索火星 卻是為何？
Traveling to Mars is usually a lonely business—
火星之行通常都是一件十分孤獨的事情——
with a single spacecraft taking off from a single launchpad for the seven-month trip to the Red Planet.
一艘航天器從一個發射臺上起飛，開始為期七個月的火星之行。
That appeared to be the case again on July 30,
7月30日，NASA的“毅力號”火星車
when NASA’s Perseverance rover roared off the pad at Cape Canaveral atop an Atlas V rocket.
搭載著阿特拉斯V號火箭從（佛羅里達州）卡納維拉爾角發射臺呼嘯而出時，情況似乎依然如此。
But this time the ship will have plenty of company.
不過，此次發射的飛船會有很多同伴。
On July 19, the United Arab Emirates made its first bid to join the Mars game,
7月19日，阿聯酋有史以來首次加入火星競賽，
launching the 3,000-lb., 10-ft.-tall Amal, or "Hope," spacecraft on a mission to orbit Mars for at least two years while studying its atmosphere.
發射了重3000磅、高10英尺的阿邁勒，又名“希望號”宇宙飛船，執行繞火星飛行，研究火星大氣層至少兩年的任務。
Four days later, China launched its Tianwen-1, or "Questions to Heaven," spacecraft,
四天后，中國發射“天問一號”（Questions to Heaven）飛船，
a threepart ship with an orbiter, a lander and a sixwheel, 529-lb. rover.
由環繞器、著陸器和一個重達529磅的六輪巡視器三部分組成。
And a fourth mission, a joint Russian-European project, ExoMars, carrying a rover of its own, was also planned for this summer,
近期的第四個探索火星項目則是俄羅斯與歐洲的合作項目“ExoMars”，該探測器攜帶有自己的火星車，原也計劃今年夏天發射，
though it has been postponed to 2022 because of engineering problems with its parachute and avionics.
然而，由于降落傘和航空電子設備方面的工程問題，該項目已經推遲到2022年。
So why all the interest in Mars—and why right now?
為何大家都——又為何都在此時——對火星有了如此大的興趣？
The timing issue has everything to do with planetary mechanics.
這一時間安排與行星力學有很大關系。
As they fly their differing solar orbits—Earth on the inner track, Mars on the outer—
地球和火星在不同的環日軌道——地球軌道位于內側，火星軌道位于外側——上繞太陽飛行時，
the distance between the two worlds is forever changing.
兩個行星之間的距離一直是一個變量。
At their greatest remove, when they are on opposite sides of the sun, they are up to 250 million miles apart.
距離最大，也就是它們分別位于太陽兩端時，它們之間的距離足足可達2.5億英里。
But once every two years, they line up on the same side of the sun, with just 35 million miles separating them.
但每隔兩年，它們就會出現在太陽的同側，排成一條直線，此時相距僅3500萬英里。
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