今年3月，在將近一年的空間站生活之后，宇航員斯科特?凱利返回了地球。

斯科特在太空的使命已經(jīng)完成，不過(guò)另一項(xiàng)使命仍在進(jìn)行之中：他是美國(guó)國(guó)家航空和宇宙航行局（NASA）創(chuàng)新研究的兩位研究對(duì)象之一，該研究旨在揭示“與地球生活相比，太空生活可能會(huì)產(chǎn)生哪些細(xì)微后果與變化”。

另一位研究對(duì)象是誰(shuí)？是他的同卵雙生兄弟馬克——一名退休的NASA宇航員。斯科特在太空中遵循著嚴(yán)格的時(shí)間、飲食和運(yùn)動(dòng)安排，而馬克在亞利桑那州過(guò)著普通人的生活。

如今，NASA的研究人員正在以馬克為對(duì)照，進(jìn)行多達(dá)10項(xiàng)的系列研究，借此確定在以1.7萬(wàn)英里時(shí)速環(huán)繞地球的空間站生活一年后，斯科特的身體發(fā)生了什么變化。在執(zhí)行任務(wù)前、任務(wù)過(guò)程中和如今斯科特回家后，研究人員都從這對(duì)雙胞胎身上提取了樣本。

研究結(jié)果至少還需要一年才能揭曉，不過(guò)部分研究人員已經(jīng)在Reddit AMA論壇開(kāi)設(shè)了主題，回答用戶關(guān)于此項(xiàng)研究及其潛在影響的問(wèn)題。

到目前為止，他們知道的事情如下：

斯科特和馬克是理想的研究對(duì)象。參與此次研究的科羅拉多州立大學(xué)（Colorado State University）的放射癌癥生物學(xué)和腫瘤學(xué)教授蘇珊?貝利表示，他們是“你能找到的先天與后天環(huán)境最類似的人——同卵雙生，兩個(gè)都是宇航員”。另一位參加研究的生理學(xué)和生物物理學(xué)副教授克里斯托弗?梅森表示，利用儲(chǔ)存的基準(zhǔn)數(shù)據(jù)，研究人員應(yīng)該能發(fā)現(xiàn)“主要由宇宙航行導(dǎo)致的分子層面的變化，這樣我們就能研究出緩解和防護(hù)的辦法”。

長(zhǎng)期在太空中停留會(huì)讓身體更容易受到傷害。NASA宇航員謝爾?林德格倫在AMA上表示，“航空飛行與在地球上生活大不相同。除了失重之外，大氣成分（二氧化碳濃度）、環(huán)境噪聲等級(jí)、壓力水平等都不一樣。差異太大了。”

這些差異之處，尤其是微重力環(huán)境，已經(jīng)被證明會(huì)增加身體負(fù)擔(dān)，導(dǎo)致骨質(zhì)疏松、肌肉松弛、心血管功能紊亂、前庭覺(jué)出現(xiàn)變化。由于斯科特在太空待了很長(zhǎng)時(shí)間，他可能會(huì)產(chǎn)生更嚴(yán)重的永久性問(wèn)題。林德格倫在AMA上表示，他跟斯科特談過(guò)恢復(fù)問(wèn)題，“斯科特說(shuō)，他的恢復(fù)時(shí)間比預(yù)計(jì)的更長(zhǎng)。”

參與此次研究的斯坦福大學(xué)（Stanford）遺傳學(xué)教授邁克爾?斯奈德表示，研究人員將在基因?qū)用妗皺z查斯科特的DNA和包裹DNA的物質(zhì)是否在太空旅行中發(fā)生了變化，以及發(fā)生了什么變化”。此外，他們會(huì)密切觀察斯科特的染色體端粒——這是位于染色體終端，防止染色體退化的部分——在國(guó)際空間站的這一年中的表現(xiàn)。“我們會(huì)看看斯科特的端粒是否比馬克的更短，”若真如此，就說(shuō)明他的端粒老化速度比馬克更快。

輻射被認(rèn)為是未來(lái)前往火星的計(jì)劃中最大的障礙。從某種程度上說(shuō)，這次“雙胞胎研究”就是為了幫助NASA為將來(lái)前往這顆紅色星球做好規(guī)劃。科學(xué)家已經(jīng)知道，太空旅行會(huì)讓宇航員暴露在宇宙射線的輻射中，而之前的研究證明，這會(huì)增加宇航員罹患惡性腫瘤的概率。

通過(guò)對(duì)斯科特和馬克的研究，NASA可以更清楚地了解暴露在這種輻射下會(huì)帶來(lái)怎樣的健康風(fēng)險(xiǎn)，并想出降低這種風(fēng)險(xiǎn)的潛在方案。

斯科特和馬克身上的輻射強(qiáng)度已經(jīng)經(jīng)過(guò)了檢測(cè)。梅森表示，毫不令人意外，“斯科特的強(qiáng)度高得多。”

科羅拉多州立大學(xué)的貝利坦率地說(shuō)：“暴露在宇宙射線之中的風(fēng)險(xiǎn)，可能會(huì)是我們前往火星的攔路虎——這段旅途的時(shí)間更長(zhǎng)，也更加深入宇宙。”

譯者：嚴(yán)匡正

After living for nearly a year aboard the International Space Station, astronaut Scott Kelly returned to Earth in March.

His mission to space is now complete, but another one is still underway: Scott is half of a novel NASA study designed to reveal “the subtle effects and changes that may occur in spaceflight as compared to Earth.”

The study’s other half? His identical twin brother Mark, himself a retired NASA astronaut. While Scott was in space following a strict schedule, diet and exercise routine, Mark was leading a normal life in Arizona.

Now, using Mark as a control, NASA researchers are performing a series of 10 studies to determine how Scott’s biology was altered from a year spent circling the Earth at 17,000 miles per hour. Samples from the twins’ were taken before and during the mission, and are being collected now that Scott has returned home.

While the results won’t be in for at least a year, some of the researchers’ involved hosted a Reddit AMA to answer users’ questions about the study and its possible implications.

Here’s what they know so far:

Scott and Mark are ideal test subjects. They’re about “as similar in nature and nurture as you can get – identical twins, both astronauts,” said Susan Bailey, a professor of radiation cancer biology and oncology at Colorado State University, and one of the study’s investigators. Using a stockpile of baseline data, the researchers should be able to establish “molecular changes that are most impacted by space travel, so we can plan for ways to mitigate and protect against them,” added Christopher Mason, an associate professor of physiology and biophysics and another investigator on the project.

Sustained time in space leaves the body vulnerable. “Spaceflight is altogether different from living on Earth. In addition to weightlessness, the atmospheric composition (CO2 levels), ambient noise levels, stress levels, etc. There are A LOT of variables,” Kjell Lindgren, a NASA astronaut, said in the AMA.

These variables, particularly microgravity, have been shown to put added stress on the body, leading to bone loss, muscle loss, cardiovascular deconditioning and vestibular changes. Because Scott spent an extended period of time in space, he may exhibit more serious, permanent effects. In the AMA, Lindgren said he’d spoken to Scott about his recovery, “and he would say that [it’s] taking longer.”

On a genetic level, the researchers will “examine whether and how Scott’s DNA and his DNA packaging material gets modified during space travel,” said Michael Snyder, a professor of genetics at Stanford and one of the study’s investigators. In addition, they will take a close look at how Scott’s telomeres — the end portion of a chromosome that protects it against deterioration — reacted to his year aboard the ISS. “We will see if telomeres get shorter in Scott relative to Mark,” a sign his chromosomes have aged in comparison to his brother’s.

Radiation is primed to be the largest obstacle in future missions to Mars. In part, the “Twin Study” is intended to help NASA strategize for a future voyage to the Red Planet. Scientists already know that space travel exposes astronauts to cosmic ray radiation, which previous research suggests could increase the likelihood of developing aggressive tumors.

By studying Scott and Mark, NASA should be able to paint a more detailed picture of the specific health risks associated with this type of radiation exposure. And, potentially, strategies to reduce its impact.

Radiation levels in Scott and Mark have already been tested. Unsurprisingly, they’re “much higher in Scott,” said Mason.

Bailey, of Colorado State University, put it bluntly: “Exposure to space radiation has the potential to be a showstopper as we make our way to Mars – longer periods of time deeper and deeper in space.”