How do asteroids spin in space? The answer could help us prevent a catastrophic Earth impact

小行星如何在太空中自传？答案可以帮助我们防止地球遭受灾难性撞击

When it comes to saving Earth from a potential doomsday rock, knowing where to hit it and how it spins could make all the difference. Two new studies presented last month at the Europlanet Science Congress in Helsinki may have just given scientists both answers.

当谈到从潜在的末日小行星撞击中拯救地球时，知道撞击它的哪个位置以及它的自转方式，可能会决定成败。上个月在赫尔辛基举行的欧洲行星科学大会（Europlanet Science Congress）上发表的两项新研究，或许刚好给了科学家这两个答案。

In one study, researchers led by Wen-Han Zhou of the University of Tokyo used data from the European Space Agency's now-retired Gaia mission to study how an asteroid's spin depends on how often it's been hit by other space rocks. In another study, a team led by Rahil Makadia of the University of Illinois at Urbana-Champaign developed a method for identifying the safest regions on an asteroid to strike with a deflection mission, without accidentally steering it back toward Earth.

在一项研究中，由东京大学周文翰率领的研究团队利用欧洲航天局现已退役的Gaia任务数据，研究了小行星的自转如何受到其被其他太空岩石撞击的频率。在另一项研究中，由伊利诺伊大学厄巴纳-香槟分校的Rahil Makadia率领的团队提出了一种方法，用于在进行偏转时识别小行星上最安全的撞击区域，从而避免意外地将其轨道重新引向地球。

Together, the findings offer a new way to understand the structure and behavior of these ancient bodies — knowledge that could prove critical to deflecting a dangerous asteroid if it were ever on a collision course with our planet.

总之，这些发现为理解这些古老天体的结构和行为提供了一种新的方法——在未来若有危险小行星与地球相撞时，这些知识可能对偏转它至关重要。

"By leveraging Gaia's unique dataset, advanced modelling and AI tools, we've revealed the hidden physics shaping asteroid rotation, and opened a new window into the interiors of these ancient worlds," Zhou said in a statement.

周文翰在一份声明中表示：“通过利用Gaia独特的数据集、先进的建模技术和人工智能工具，我们揭示了影响小行星自转的潜在物理机制，并为了解这些古老天体的内部结构打开了一个新窗口。”

In recent decades, astronomers have been puzzled by why some asteroids rotate like spinning tops while others tumble through space in chaotic, unpredictable ways. Zhou's team set out to solve that mystery using Gaia's vast archive of asteroid light patterns and new computer models.

近几十年来，天文学家一直困惑于为什么一些小行星像陀螺一样自转，而另一些小行星以混乱、难以预测的方式在太空中翻滚。周文翰团队利用Gaia庞大的小行星光变数据档案和新的计算机模型来解开这个谜团。

Their analysis showed that an asteroid's spin pattern boils down to a cosmic "tug of war" between two forces: collisions that knock them into unstable motion and internal friction that gradually smooths their rotation.

他们的分析表明，小行星的自旋模式可以归结为一场两种力量之间的宇宙“拔河”：一方面是碰撞使它们陷入不稳定的运动状态，另一方面是内部摩擦使其自转趋于平稳。

"When these two effects balance, they create a natural dividing line in the asteroid population," Zhou said in the statement.

周文翰在声明中表示：“当这两种力量达到平衡时，就会在小行星群中形成一条天然的分界线。”

Machine learning revealed this "dividing line" in Gaia's data as a clear gap between fast-spinning asteroids and slow, tumbling ones. Slower rotators, the researchers found, are more easily jolted into a wobble by impacts, while faster ones resist those disturbances, according to the statement.

声明指出，机器学习在Gaia的数据中揭示了这条“分界线”，即快速自转的小行星和缓慢翻滚的小行星之间存在一个明显的空隙。研究人员发现，自转速度较慢的小行星更容易因碰撞而产生晃动，而速度较快的小行星可以抵抗这种扰动。

Sunlight also plays a subtle but important role, the study reports. As an asteroid's surface heats up during the day and cools at night, it emits tiny bursts of radiation that act like microscopic thrusts. For smoothly spinning asteroids, those pushes line up in the same direction and gradually change their spin rate. But for tumblers, the pushes more or less cancel each other out, trapping them in their slow, chaotic motion.

研究报告指出，阳光也在其中起着微妙但重要的作用。当小行星表面白天受热，晚上变冷时，会释放微弱的辐射脉冲，产生极小的推力。对于平稳自转的小行星，这些推力的方向大体一致，会逐渐改变其自转速度。但对于翻滚状态的小行星，这些推力大多相互抵消，使其陷入缓慢而混乱的运动中。

The results also indicate that many asteroids aren't solid chunks of rock, but loose clusters of rock and dust held together weakly by gravity, known to astronomers as "rubble piles." That distinction matters for planetary defense, scientists say, because a fragile, porous asteroid would react to a spacecraft's impact very differently than a dense, solid one.

研究结果还表明，许多小行星并非坚硬的岩石块，而是由岩石和灰尘松散聚集、仅靠微弱引力维系的“碎石堆”。科学家指出，这一区别对抵御行星撞击很重要，因为脆弱而多孔的小行星在与航天器相撞时，其反应方式与致密、坚固的小行星截然不同。

As more sky surveys come online, scientists will be able to apply this method to much larger samples, Zhou said in the statement. With upcoming observatories such as the Vera C. Rubin Observatory's Legacy Survey of Space and Time, "we'll be able to apply this method to millions more asteroids, refining our understanding of their evolution and make-up."

周文翰在声明中表示，随着更多的空间项目投入，科学家将能够将这种方法应用于更大规模的小行星样本。借助即将启用的天文台，如维拉·C·鲁宾天文台的“时空遗产调查”（Legacy Survey of Space and Time），我们将能够将这种方法应用于研究数百万颗小行星，完善我们对它们的演化过程和构成的认识。”

If knowing how an asteroid spins is the first step, the next is figuring out where to hit it.

了解小行星如何自转是第一步，下一步就是确定应该撞击它的哪个位置。

Makadia's team investigated what happens when a spacecraft slams into an asteroid, and found that not all impact sites are created equal. Striking the wrong spot could send an asteroid drifting into what scientists call a gravitational keyhole — a tiny region of space where a planet's gravity could subtly bend the asteroid's orbit and make it swing back toward Earth on a future pass decades or centuries later.

马卡迪亚团队研究了航天器撞上小行星时会发生什么，发现并非所有撞击结果都相同。撞击错误的位置可能会使小行星漂移到科学家所说的“引力钥匙孔”——这是太空中一个微小区域，行星的引力可能会轻微地改变小行星的轨道，使其在几十年或几个世纪后的下一次经过时重新朝向地球。

"Even if we intentionally push an asteroid away from Earth with a space mission, we must make sure it doesn't drift into one of these keyholes afterwards," Makadia said in a different statement. "Otherwise, we'd be facing the same impact threat again down the line."

马卡迪亚在另一份声明中表示：“即使我们通过太空任务有意将小行星推离地球，也必须确保它之后不会漂移到这些钥匙孔中。否则，未来我们将再次面临同样的撞击威胁。”

To avoid such a cosmic boomerang effect, Makadia's team created probability maps of asteroid surfaces that could guide mission planners. Using lessons from NASA's DART mission, which slammed into the asteroid Dimorphos in September 2022, and realistic spacecraft trajectories, they simulated hundreds of millions of kinetic-impact missions, each varying slightly in speed, angle and timing.

为了避免这种宇宙回旋镖效应，马卡迪亚团队制作了撞击小行星表面的概率图，为任务规划者提供参考。他们借鉴NASA2022年9月撞击小行星Dimorphos的DART任务经验，并结合真实的航天器轨迹，模拟了数亿次动能撞击任务，每次任务的速度、角度和时间略有不同。

For each simulation, they calculated how an asteroid's motion would change, and whether it might drift into one of these gravitational keyholes. Repeating this process for a range of impact points and rotation angles allowed them to pinpoint the safest and most effective strike zones, according to the second statement.

根据第二份声明，他们在每次模拟中都计算了小行星运动的变化，以及它是否可能漂移到这些引力钥匙孔中。通过对一系列不同撞击点和自转角度的重复，团队得以确定最安全且最有效的撞击区域。

"With these probability maps, we can push asteroids away while preventing them from returning on an impact trajectory, protecting the Earth in the long run," said Makadia.

马卡迪亚说：“有了这些概率图，我们可以在推离小行星的同时防止它们返回撞击轨道，从长远来保护地球。”

To test their model, the researchers applied it to the near-Earth asteroid Bennu, one of the best-studied near-Earth objects thanks to NASA's OSIRIS-REx mission, which precisely mapped its surface, studied its orbit and brought a sample of the space rock home to Earth.

为了验证他们的模型，研究人员将其应用于近地小行星Bennu。这颗小行星是研究最透彻的近地天体之一。得益于NASA的OSIRIS-REx任务，该任务精确地绘制了其表面地图，研究了其轨道，并将其岩石样本带回地球。

Earlier models had shown that Bennu's path included several potential gravitational keyholes that could, in theory, redirect it toward Earth sometime in the 22nd century. But data from OSIRIS-REx dramatically reduced those uncertainties, ruling out many potential keyholes and future impact scenarios.

早期模型表明，Bennu的轨道上存在若干潜在的引力钥匙孔，理论上可能会在22世纪的某个时候将其引向地球。但来自OSIRIS-REx的数据大幅降低了这些不确定性，排除了许多潜在的钥匙孔和未来的撞击场景。

Using that precise orbital data, Makadia's team simulated spacecraft impacts under a variety of conditions and projected Bennu's potential keyhole encounters, creating detailed impact-probability maps.

利用这些精确的轨道数据，马卡迪亚团队模拟了各种条件下的航天器撞击，并预测了Bennu可能经过的引力钥匙孔，从而制作出详细的撞击概率图。

These maps show which areas on an asteroid would make the safest targets, and which regions could increase the long-term risk of a future encounter, the study notes. The optimal strike zones, marked as bright crosshairs in the model, show where a spacecraft could nudge an asteroid's orbit away from Earth without triggering a dangerous return later on.

研究指出，这些概率图显示了小行星上的哪些区域是最安全的撞击位置，以及哪些区域可能会增加未来碰撞的长期风险。模型中得以明亮十字标记的最佳撞击区，显示了航天器可将小行星的轨道偏离地球，而不会在未来引发危险的位置。

According to the study, this kind of analysis could help design safer deflection missions even using only ground-based observations, when time doesn't allow for a dedicated rendezvous spacecraft.

根据这项研究，即便在没有时间发射专门的会合航天器时，仅利用地面观测数据，这种分析也可以帮助设计更安全的小行星偏转任务。

"Fortunately, this entire analysis, at least at a preliminary level, is possible using ground-based observations alone, although a rendezvous mission is preferred," Makadia said in the statement.

马卡迪亚在声明中表示：“幸运的是，至少在初步阶段，这整套分析仅依靠地面观测就可以完成，尽管优选方案仍是发射会合航天器。”

As next-generation telescopes and missions uncover millions more asteroids, studies like these are helping scientists write a planetary defense blueprint to safeguard Earth.

随着下一代望远镜和航天任务发现数百万颗小行星，类似的研究正帮助科学家制定行星防御蓝图，以保护地球安全。