If a rocket stage crashes into the moon and no one sees it, did it really crash?

如果一枚火箭的某一级撞上月球却没有人观测到，那它真的撞上月球了吗？

A discarded upper stage of a SpaceX Falcon 9 rocket slammed into the surface of the moon around 2:35 a.m. Eastern time on Wednesday at a speed of 5,400 miles per hour.

美国东部时间周三凌晨2点35分左右，SpaceX公司“猎鹰9号”运载火箭的一个废弃上面级以每小时5400英里（约合8690公里）的速度猛烈撞击了月球表面。

However, it appears no one has yet reported any obvious signs of the impact.

然而，目前似乎还没有人报告发现任何明显的撞击迹象。

Astronomers hoped that the collision would create a bright enough flash of light and a big enough cloud of dust and debris that it could be observed through telescopes from Earth. But at the appointed time, the moon looked like the moon.

天文学家曾希望，这次碰撞会产生足够明亮的闪光以及足够庞大的尘埃和碎片云，可以通过地球上的望远镜进行观测。但在预定时间，月球看起来依然如常。

More subtle observations taken at the Lowell Discovery Telescope in Arizona and the Very Large Telescope in Chile could reveal evidence of the collision, like specific wavelengths of light emitted or absorbed by the plume of debris, but those data need to be analyzed.

在亚利桑那州的“洛厄尔发现望远镜”和智利的“甚大望远镜”进行的更精细观测可能会揭示此次碰撞的证据，例如碎片羽流发射或吸收的特定波长光，但这些数据还需要进行分析。

“We were able to observe despite intermittent thick clouds, but need to do some quick reduction on the data to know if we saw anything,” said Patrick Lierle, a Boston University astronomer involved with the Lowell Discovery observations. “So the answer is nothing yet.”

“尽管有间歇性的厚云层，我们还是能够进行观测，但需要对数据进行一些快速处理，才能确定我们是否观测到了什么，”参与“洛厄尔发现望远镜”观测的波士顿大学天文学家帕特里克·利尔说。“所以答案是，目前还没有发现。”

The most promising observations so far come from Chile.

迄今为止，最有希望的观测结果来自智利。

Another Boston University scientist, Carl Schmidt, used the Very Large Telescope to measure sodium and lithium in a plume a few tens of kilometers in size that lasted five to 10 minutes after impact.

波士顿大学的另一位科学家卡尔·施密特利用“甚大望远镜”测量了撞击后持续五到10分钟、规模达数十公里的羽流中的钠和锂。

Dr. Schmidt said he was “100 percent confident” that the readings came from the impact.

施密特表示，他“100%确信”这些测量读数来自于此次撞击。

However, it seems that no one was able to directly see or photograph the collision.

然而，似乎没有人能够直接看到或拍摄到这次碰撞。

“I’ve so far only heard negatives from imaging folks,” Mr. Lierle said.

“到目前为止，我从从事成像工作的人那里听到的都是否定的结果，”利尔说。

The anticlimactic demise of the rocket stage was not a surprise.

火箭残骸以这种平淡的方式收场并不令人意外。

Computer simulations calculated the effect of the rocket stage striking the moon in a vertical orientation, which would have produced the most debris, but the stage would more likely have belly-flopped at an angle instead. (The vertical case was the only one simple enough to be computed.)

计算机模拟计算了这一上面级以垂直方向撞击月球的效果——这种撞击会产生最多的碎片，但它更有可能是以一定倾斜角度摔落的。（垂直撞击是唯一简单到足以进行计算的情况。）

But even in the case of a vertical impact, the predicted flash and plume were at the edge of what might be visible, and the dynamics depended on other unknown factors like whether the stage hit solid bedrock or loose lunar soil.

但即使是在垂直撞击的情况下，预测的闪光和羽流也处于可能被看见的极限边缘，而且动力学过程还取决于其他未知因素，比如它撞击的是坚硬的基岩还是松散的月壤。

The Korean spacecraft Danuri passed over the expected crash site about eight hours after impact. Even if it successfully captured an image of the crater, it could take days or weeks to identify it. That is because the process of locating the newly disrupted moonscape is like finding a needle in a haystack. Confirmation will require comparing the gouge with an earlier photograph that shows a pristine surface at that location before impact.

韩国航天器“赏月号”在撞击发生约八小时后飞越了预期的坠毁地点。即使它成功拍到了撞击坑的图像，也可能需要几天或几周的时间才能识别。这是因为，定位新近被破坏的月球地貌，过程就像大海捞针一样困难。要确认这一撞击坑，需要将其与一张显示撞击前该位置原始地表的早期照片进行对比。

In six days, NASA’s Lunar Reconnaissance Orbiter will also pass over the site, providing additional photographs to aid the search.

六天后，美国宇航局的月球勘测轨道飞行器也将飞越该地点，提供更多照片协助搜寻。

Is it possible that the predictions were wrong, and that the Falcon 9 stage missed the moon?

预测是否可能有误，“猎鹰9号”的废弃上面级有没有可能并未击中月球？

Scientists are confident that the collision happened. Like everything flying around in the solar system, the rocket stage moved according to Newton’s laws of motion with only slight deviations arising from additional forces like the solar wind.

科学家们确信碰撞已经发生。就像在太阳系中飞行的所有物体一样，这枚火箭的残骸按照牛顿运动定律运行，只有受到太阳风等额外作用力时才会产生轻微偏差。

The uncertainty in where the impact would occur was just a few miles.

撞击发生位置的不确定范围只有几公里。