火星探测窗口期

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.

火星或许并非养育下一代的合适地方，但是艾尔顿·约翰关于火星上无人养育孩子的著名歌词有朝一日很可能会被证明是错误的。现在，我们有了探测器、轨道器和着陆器。2020年，火星探测十分火热，有三个国家都要向火星发射火箭：美国要发射毅力号火星探测车，中国要发射天问一号火星探测器，阿拉伯联合酋长国则要发射希望号轨道器。

按计划这些发射都安排在7月，几乎让人们感觉进入了疯狂太空竞赛的新时代，各国都在奋勇争先。当然，某些人的自信心肯定会由此提升，但如此短的时间段内竞争加速，其实原因很简单：火星在2020年晚些时候会离地球很“近”——相对而言——从而提供了当前去往火星探测的理想机会。

事实上，2020年是相当长一段时间内的最好机会。下一次（几乎）同样合适的时机将出现在2033年。因此，各国航天局想抓住这次机会的急切心情就不难理解了。这并不是说火星探測在随后的13年内不能进行——毕竟，2022年已经规划了数个探测任务，包括延迟发射的罗莎琳德·富兰克林号火星车。只是发射条件没有2020年这么理想。

火星探测窗口期

哪怕最简单的太阳系模型也会清楚显示地球和火星围绕太阳的转动并不相同，最明显的差异在于距离和速度。地球以29.78千米/秒的平均速度，在距太阳1天文单位的距离绕太阳旋转，而火星距太阳约是地球距太阳的1.523倍，平均速度24千米/秒。地球公转一圈约需365天，火星则约需668个火星日，大致等于687个地球日。

将这些数字整合在一起主要可以说明一点：地球和火星在这方面并无太多相似之处，因此它们并不经常出现在对方的附近。然而，它们毕竟还是围绕着同一个太阳公转，因此总会偶尔相遇。好吧，假如此处理解过于依照字面，那“相遇”可是个会造成致命后果的重词，但其实它们只是相对距离较近而已。准确的术语是它们偶尔会相“冲”。

2020年火星探测

阿拉伯联合酋长国

日程安排最靠前的是阿拉伯联合酋长国（阿联酋）。由于其位于日本的发射场地天气条件不佳，阿联酋不得不将原定7月16日的发射推迟了3天。好消息是其发射窗口刚刚于7月15日打开，并将一直持续到8月12日，因此时间上并无太大压力。但是也没有继续延迟的必要，天气状况改善，当地时间周一凌晨——协调世界时7月19日21：58：14——其HII-A火箭从日本种子岛航天发射中心成功发射。

这次发射将把希望号探测器送入火星轨道，探测器将在那里记录大气层的一切，主要目的是创建“火星大气层的首个完整图谱”。阿联酋的目标是研究火星的气候动态——基本上就是绘制首个完整的火星气象图——并探究逃逸的氢气和氧气如何影响火星的天气，以及它们究竟为何会逃逸。探测器本身配备了三个成像设备：红外光谱仪、紫外光谱仪和高清晰度成像仪。

这次探测任务自然主要聚焦于研究本身，但是阿联酋，尤其是作为一个较小的国家，也要借机展示其在太空探索方面新近取得的地位。它希望树立自己“地区进步标杆”的形象，证明一切皆有可能，从而激励未来一代代阿拉伯人继续在航天科学领域求索。鉴于伊斯兰世界曾在其黄金时代为人类做出过诸多贡献，阿联酋的希望号也是为了纪念——即使不是“复兴”——该地区在天文学领域的重要地位。

中国

第二个发射的火星探测器是中国的天问一号，定于7月23日发射。在2011年与俄罗斯合作探测之后，中国开始独立尝试其第二次火星之旅。

据了解和推测，天问一号是一个技术完全成熟的全方位探测器，由环绕器（轨道器）、着陆器和巡视器（火星车）组成。尽管该探测器的部分目标也包括研究火星的大气层，但据推测主要研究领域是火星的地表及地下。其主要任务似乎包括创建地质图谱、探究土壤特性及寻找水冰储存区，另外还希望找到火星上过去乃至现在可能有生命存在的证据。

美国

第三个，也是最后一个，是美国的“火星2020”探测任务，计划于7月30日发射其毅力号火星探测车及车载机智号火星直升机。那天是其窗口期的第一天，该窗口期持续到8月15日。尽管美国之前已经多次到达火星，但此次的火星之行远非常规探索，其重要程度堪比另两个国家的探测任务。

毅力号将继续火星探测项目的工作，搜集土壤和岩石样本，以寻找火星上的生命遗迹，尤其是微生物生命的痕迹。按照设想，未来的探测器可以将其他设备带往火星分析这些样本，也可以将样本带回地球，具体采取哪种方案将取决于当时条件下哪种更为可行。      □

（译者单位：北京外国语大学）