移民太空？先看看太空生活对身体的影响吧

今年3月，在将近一年的空间站生活之后，宇航员斯科特•凯利返回了地球。

斯科特在太空的使命已经完成，不过另一项使命仍在进行之中：他是美国国家航空和宇宙航行局（NASA）创新研究的两位研究对象之一，该研究旨在揭示“与地球生活相比，太空生活可能会产生哪些细微后果与变化”。

另一位研究对象是谁？是他的同卵双生兄弟马克——一名退休的NASA宇航员。斯科特在太空中遵循着严格的时间、饮食和运动安排，而马克在亚利桑那州过着普通人的生活。

如今，NASA的研究人员正在以马克为对照，进行多达10项的系列研究，借此确定在以1.7万英里时速环绕地球的空间站生活一年后，斯科特的身体发生了什么变化。在执行任务前、任务过程中和如今斯科特回家后，研究人员都从这对双胞胎身上提取了样本。

研究结果至少还需要一年才能揭晓，不过部分研究人员已经在Reddit AMA论坛开设了主题，回答用户关于此项研究及其潜在影响的问题。

到目前为止，他们知道的事情如下：

斯科特和马克是理想的研究对象。参与此次研究的科罗拉多州立大学（Colorado State University）的放射癌症生物学和肿瘤学教授苏珊•贝利表示，他们是“你能找到的先天与后天环境最类似的人——同卵双生，两个都是宇航员”。另一位参加研究的生理学和生物物理学副教授克里斯托弗•梅森表示，利用储存的基准数据，研究人员应该能发现“主要由宇宙航行导致的分子层面的变化，这样我们就能研究出缓解和防护的办法”。

长期在太空中停留会让身体更容易受到伤害。NASA宇航员谢尔•林德格伦在AMA上表示，“航空飞行与在地球上生活大不相同。除了失重之外，大气成分（二氧化碳浓度）、环境噪声等级、压力水平等都不一样。差异太大了。”

这些差异之处，尤其是微重力环境，已经被证明会增加身体负担，导致骨质疏松、肌肉松弛、心血管功能紊乱、前庭觉出现变化。由于斯科特在太空待了很长时间，他可能会产生更严重的永久性问题。林德格伦在AMA上表示，他跟斯科特谈过恢复问题，“斯科特说，他的恢复时间比预计的更长。”

参与此次研究的斯坦福大学（Stanford）遗传学教授迈克尔•斯奈德表示，研究人员将在基因层面“检查斯科特的DNA和包裹DNA的物质是否在太空旅行中发生了变化，以及发生了什么变化”。此外，他们会密切观察斯科特的染色体端粒——这是位于染色体终端，防止染色体退化的部分——在国际空间站的这一年中的表现。“我们会看看斯科特的端粒是否比马克的更短，”若真如此，就说明他的端粒老化速度比马克更快。

辐射被认为是未来前往火星的计划中最大的障碍。从某种程度上说，这次“双胞胎研究”就是为了帮助NASA为将来前往这颗红色星球做好规划。科学家已经知道，太空旅行会让宇航员暴露在宇宙射线的辐射中，而之前的研究证明，这会增加宇航员罹患恶性肿瘤的概率。

通过对斯科特和马克的研究，NASA可以更清楚地了解暴露在这种辐射下会带来怎样的健康风险，并想出降低这种风险的潜在方案。

斯科特和马克身上的辐射强度已经经过了检测。梅森表示，毫不令人意外，“斯科特的强度高得多。”

科罗拉多州立大学的贝利坦率地说：“暴露在宇宙射线之中的风险，可能会是我们前往火星的拦路虎——这段旅途的时间更长，也更加深入宇宙。”

译者：严匡正

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.”