火星探測窗口期

斯文·格雷戈里

Mars may not be the kind of place to raise your kids， but chances are that one day [Elton John]s famous lyrics will be wrong about there being no one there to raise them1. For now， we have probes， orbiters， and landers. Mars missions are going strong this year， with three nations about to launch their rockets towards the Red Planet： the United States sending their Perseverance rover， Chinas Tianwen-1 mission， and the United Arab Emirates sending their Hope orbiter.

As all of this is planned to happen still within the month of July， it almost gives the impression of a new era of wild space races where everyone tries to be first. Sure， some egos will certainly be boosted here， but the reason for this increased run within such a short time frame has a simple explanation： Mars will be right around the corner later this year—relatively speaking—providing an ideal opportunity to travel there right now.

In fact， this year is as good as it gets for quite a while. The next time the circumstances will be （almost） as favorable as this year is going to be in 2033， so its understandable that space agencies are eager to not miss out on this chance. Not that Mars missions couldnt be accomplished in the next 13 years—after all， several endeavors are already in the wings for 2022， including the delayed Rosalind Franklin rover launch2. Its just that the circumstances wont be as ideal.

A window to Mars

Even the simplest model of our solar system will show how Earth and Mars revolve differently around the sun， with distance and speed being the most obvious ones. Earth rotates in a distance of 1 astronomical unit （AU）3 from the sun at an average speed of 29.78 km/s， while Mars does the same at ～1.523 times the distance and an average speed of 24 km/s. It takes Earth ～365 days to end up in the same spot again， and Mars ～668 amounts of its own definition of a day， i.e. sols， which is roughly the equivalent of 687 Earth days.

Throwing around all these numbers shows mainly one thing： Earth and Mars dont have much in common here， and as a result， they dont hang around much in each others proximity. Still， they do revolve around the same sun， and are therefore bound to meet on occasion. Okay， “meet” is a strong word with fatal results if taken too literally here， but rather have close encounters with each other. The accurate terminology would be that they are in opposition4 on occasion.

2020 Mars Missions

United Arab Emirates

First up in the schedule is the United Arab Emirates （UAE）， which had to postpone their initial July 16th launch due to bad weather conditions in their launch site in Japan for three days. The good news is， their launch window had just opened on July 15th， and would have remained open until August 12th， so there wasnt too big time pressure yet. But there was no need for further delay， the weather conditions improved， and in the early Monday morning hours local time—July 19th， 21：58：14 UTC—their HII-A rocket successfully took off from the Tanegashima Space Center.

The mission will send their Hope probe into Marss orbit， where it will record everything about the atmosphere with the main objective to create “the first complete picture of the Martian atmosphere”. Their goal is to research the climate dynamics on Mars—essentially creating the first full weather map of Mars—and how the escaping hydrogen and oxygen play into that and why its escaping in the first place. The probe itself is equipped with three imaging instruments： an infrared spectrometer， an ultraviolet spectrometer， as well as a high resolution imager.

While the missions main focus is naturally on the research itself， the UAE takes this also as an opportunity to demonstrate their newly achieved position in space exploration， especially as a rather small nation. Aiming to inspire future Arab generations to pursue the field of space science， they want to establish themselves as “beacon of progress in the region” and show that nothing is impossible. Considering the achievements the Islamic world once contributed to humankind during its Golden Age5， the UAEs Hope is also to commemorate， if not revive， the regions importance within astronomy.

China

The second mission is Chinas Tianwen-1， scheduled to launch on 23. July. After the joint expedition with Russia in 20116， China conducts their second-ever attempt to travel to Mars on their own.

From what is known and speculated， Tianwen-1 is a full-blown， all-inclusive mission with orbiter， lander， and rover on board. While parts of its objective will also look into Marss atmosphere， its assumed that the main focus lies on and below its surface. The main objectives seem to include creating a geological map， exploring soil characteristics， and finding water-ice pockets—also in hopes to find evidence of past and possibly present life on Mars.

United States

Finally， the third mission， the USs Mars 2020 mission， is expected to launch the Perseverance rover with its Ingenuity drone on July 30th—the first day of its launch window that closes on August 15th. And even though the US has made it to Mars numerous times before， an endeavor like this is hardly ever a routine operation， and theres just as much at stake as for the other two countries.

Continuing the work of the Mars Exploration Program， Perseverance will look for past life on Mars， specifically microbial life， by collecting soil and rock samples. The idea is that a future mission could either bring further equipment to Mars to analyze those samples， or bring them back to Earth， whichever seems more feasible at that time.

火星或許并非養(yǎng)育下一代的合適地方，但是艾爾頓·約翰關(guān)于火星上無人養(yǎng)育孩子的著名歌詞有朝一日很可能會被證明是錯誤的。現(xiàn)在，我們有了探測器、軌道器和著陸器。2020年，火星探測十分火熱，有三個(gè)國家都要向火星發(fā)射火箭：美國要發(fā)射毅力號火星探測車，中國要發(fā)射天問一號火星探測器，阿拉伯聯(lián)合酋長國則要發(fā)射希望號軌道器。

按計(jì)劃這些發(fā)射都安排在7月，幾乎讓人們感覺進(jìn)入了瘋狂太空競賽的新時(shí)代，各國都在奮勇爭先。當(dāng)然，某些人的自信心肯定會由此提升，但如此短的時(shí)間段內(nèi)競爭加速，其實(shí)原因很簡單：火星在2020年晚些時(shí)候會離地球很“近”——相對而言——從而提供了當(dāng)前去往火星探測的理想機(jī)會。

事實(shí)上，2020年是相當(dāng)長一段時(shí)間內(nèi)的最好機(jī)會。下一次（幾乎）同樣合適的時(shí)機(jī)將出現(xiàn)在2033年。因此，各國航天局想抓住這次機(jī)會的急切心情就不難理解了。這并不是說火星探測在隨后的13年內(nèi)不能進(jìn)行——畢竟，2022年已經(jīng)規(guī)劃了數(shù)個(gè)探測任務(wù)，包括延遲發(fā)射的羅莎琳德·富蘭克林號火星車。只是發(fā)射條件沒有2020年這么理想。

火星探測窗口期

哪怕最簡單的太陽系模型也會清楚顯示地球和火星圍繞太陽的轉(zhuǎn)動并不相同，最明顯的差異在于距離和速度。地球以29.78千米/秒的平均速度，在距太陽1天文單位的距離繞太陽旋轉(zhuǎn)，而火星距太陽約是地球距太陽的1.523倍，平均速度24千米/秒。地球公轉(zhuǎn)一圈約需365天，火星則約需668個(gè)火星日，大致等于687個(gè)地球日。

將這些數(shù)字整合在一起主要可以說明一點(diǎn)：地球和火星在這方面并無太多相似之處，因此它們并不經(jīng)常出現(xiàn)在對方的附近。然而，它們畢竟還是圍繞著同一個(gè)太陽公轉(zhuǎn)，因此總會偶爾相遇。好吧，假如此處理解過于依照字面，那“相遇”可是個(gè)會造成致命后果的重詞，但其實(shí)它們只是相對距離較近而已。準(zhǔn)確的術(shù)語是它們偶爾會相“沖”。

2020年火星探測

阿拉伯聯(lián)合酋長國

日程安排最靠前的是阿拉伯聯(lián)合酋長國（阿聯(lián)酋）。由于其位于日本的發(fā)射場地天氣條件不佳，阿聯(lián)酋不得不將原定7月16日的發(fā)射推遲了3天。好消息是其發(fā)射窗口剛剛于7月15日打開，并將一直持續(xù)到8月12日，因此時(shí)間上并無太大壓力。但是也沒有繼續(xù)延遲的必要，天氣狀況改善，當(dāng)?shù)貢r(shí)間周一凌晨——協(xié)調(diào)世界時(shí)7月19日21：58：14——其HII-A火箭從日本種子島航天發(fā)射中心成功發(fā)射。

這次發(fā)射將把希望號探測器送入火星軌道，探測器將在那里記錄大氣層的一切，主要目的是創(chuàng)建“火星大氣層的首個(gè)完整圖譜”。阿聯(lián)酋的目標(biāo)是研究火星的氣候動態(tài)——基本上就是繪制首個(gè)完整的火星氣象圖——并探究逃逸的氫氣和氧氣如何影響火星的天氣，以及它們究竟為何會逃逸。探測器本身配備了三個(gè)成像設(shè)備：紅外光譜儀、紫外光譜儀和高清晰度成像儀。

這次探測任務(wù)自然主要聚焦于研究本身，但是阿聯(lián)酋，尤其是作為一個(gè)較小的國家，也要借機(jī)展示其在太空探索方面新近取得的地位。它希望樹立自己“地區(qū)進(jìn)步標(biāo)桿”的形象，證明一切皆有可能，從而激勵未來一代代阿拉伯人繼續(xù)在航天科學(xué)領(lǐng)域求索。鑒于伊斯蘭世界曾在其黃金時(shí)代為人類做出過諸多貢獻(xiàn)，阿聯(lián)酋的希望號也是為了紀(jì)念——即使不是“復(fù)興”——該地區(qū)在天文學(xué)領(lǐng)域的重要地位。

中國

第二個(gè)發(fā)射的火星探測器是中國的天問一號，定于7月23日發(fā)射。在2011年與俄羅斯合作探測之后，中國開始獨(dú)立嘗試其第二次火星之旅。

據(jù)了解和推測，天問一號是一個(gè)技術(shù)完全成熟的全方位探測器，由環(huán)繞器（軌道器）、著陸器和巡視器（火星車）組成。盡管該探測器的部分目標(biāo)也包括研究火星的大氣層，但據(jù)推測主要研究領(lǐng)域是火星的地表及地下。其主要任務(wù)似乎包括創(chuàng)建地質(zhì)圖譜、探究土壤特性及尋找水冰儲存區(qū)，另外還希望找到火星上過去乃至現(xiàn)在可能有生命存在的證據(jù)。

美國

第三個(gè)，也是最后一個(gè)，是美國的“火星2020”探測任務(wù)，計(jì)劃于7月30日發(fā)射其毅力號火星探測車及車載機(jī)智號火星直升機(jī)。那天是其窗口期的第一天，該窗口期持續(xù)到8月15日。盡管美國之前已經(jīng)多次到達(dá)火星，但此次的火星之行遠(yuǎn)非常規(guī)探索，其重要程度堪比另兩個(gè)國家的探測任務(wù)。

毅力號將繼續(xù)火星探測項(xiàng)目的工作，搜集土壤和巖石樣本，以尋找火星上的生命遺跡，尤其是微生物生命的痕跡。按照設(shè)想，未來的探測器可以將其他設(shè)備帶往火星分析這些樣本，也可以將樣本帶回地球，具體采取哪種方案將取決于當(dāng)時(shí)條件下哪種更為可行。? ? ? □

（譯者單位：北京外國語大學(xué)）