🚗太阳‘小区‘的到访者🏡Visitors of the Solar Residency
近些年，太阳系可是个好地方，我们这个小小的社区也会时常会有太空‘游客’的到访呢。
Recently our little Solar community has been a popular place with space ‘travellers’ coming to visit every so often.
四年前，天文学家们第一次在太阳系中发现了个从未见过的东西——来自另一个星系的访客！由于当时是被夏威夷天文台第一个发现的，这颗星际天体(ISO)被命名为了 ʻOumuamua - 夏威夷语中的“第一个远方的信使”。两年后，天文学家们又发现了另一颗，这次是一颗彗星，后被命名为Borisov。
Four years ago, astronomers discovered something in the Solar system that has never been seen before – a visitor from another solar systems! Having been discovered by observatories in Hawai’i, the interstellar object (ISO) was given the name ʻOumuamua - “the first distant messenger” in Hawaiian language. Two years later, another one was seen, this time a cometary, called Borisov.
图为ʻOumuamua 的一个艺术效果图
An artist impression of 'Oumuamua
虽然我们此前早就知道了这些物种的存在，但为什么它们是最近才被发现的呢？他们来自哪里？我们又多久能见到它们一次呢？
Though we knew the exitance of these creatures before, why were they only recently observed? Where have they come from? How often are we expecting to see them?
今天，让我们跟随一群来自美国和欧洲的天文学家的足迹，看看我们的太阳系到底有多受太空旅行者们的欢迎吧。
Today, let us follow the path of a group of astronomers from the US and across Europe, to find out how popular our Solar system is for those space travellers.
满足你的好奇心
Satisfying the Curiosities
以上我们对于这些外来天体的好奇其实可以归结于它们的一个性质：差分到达率(Γ)，意思是，每年在太阳的特定区域内经过的天体数量。作为一个函数，Γ不仅会包括关于访问者速度数据，还会有近日点（最接近太阳的距离）的成分。
Our curiosity of those foreigners can be summed up with just one property of them: the differential arrival rate (Γ), which simply means the number of objects that will pass within a certain area of the Sun every year. Γ, as a function, would include all the facts on the visitor’s velocity and perihelion (distance of closest approach to the Sun) .
对Γ的计算，我们将需要三个因素的数据:
1. 每单位体积有多少个ISO - 数量密度
2. 在一定体积内能观察到的某一类型ISO的速率 - 容积采样率
3. ISO具有一定速度的概率 - 概率分布函数
However, the modelling of Γ would requires the knowledge of the three factors:
1. How many ISOs there are per unit volume - number density
2. The rate that a given volume sees ISOs of a certain type - volume sampling rate
3. The probability of an ISO having a certain velocity - probability distribution function
图为NASA的哈勃望远镜在7小时内对Borisov的观测结果的延时压缩视频。当时，这颗彗星正以每小时18万公里的极快速度移动着。
A time-lapse sequence compressing Hubble Space Telescope observations of Borisov, spanning a seven-hour period. The comet was moving along at a breakneck speed of 180,000 km/hour at the time.
请问您几位？
How Many of You?
首先，了解物种的总数是很关键的一步，然后我们才能计算出它们中有多少会选择来拜访我们。这里，研究小组引用了之前的数据，用 ʻOumuamua的发现作为一定体积内ISO的数量上限。
Firstly, it is key to know these creatures’ population, and then we can find out how many of them would choose to come and visit us. Here, the research team referred to previous data, there astronomers used the discovery of ʻOumuamua as a limit the number of ISOs within a given volume.
最初，在2017年，这一估计是根据PanSTARRS天文台每年观测到的总体积除以探测率得出的。但随着时间的推移，现在的天文台观测的质量也得到了提高，研究小组在这里就使用了原始数据的一半探测率 (之前观察到的ISO数量的一半)。
Originally, in 2017, the estimate was from dividing the detection rate by volume PanSTARRS observatory sees every year. Now that more time has passed and the quality of the observatory has improved, the team assumed the detection rate here to be only half of the original data (i.e. ½ as many ISOs per volume as predicted previously).
您常来吗？
Coming Often?
虽然听起来很简单，因为我们只需要计算空中特定区域内经过的物体数量，但在现实中操作起来并没有那么简单。由于太阳的质量要比经过的ISO大得多，我们必须考虑到“引力聚焦”，也就是太阳引力对ISO轨道的影响。
Though it sounds easy, as we just need to count the objects passing by in a set area in space, it is actually more difficult to practice in reality. Since the Sun is a lot more massive than the ISOs passing by, we would have to take into account for ‘gravitational focusing’, which is the effect of the Sun’s gravity on the ISOs’ trajectories.
物体移动得越慢，“聚焦”就越强，因此对这些物体的采样率就会变高。不过，话虽如此，这里的计算只需要大约一个典型的近日点，可以忽略部分计算中的细节。
The slower the objects move, the stronger the ‘focusing’ is, hence higher sampling rate for those objects. However, having said that, the calculation here would only need the assumption of a typical perihelion, hence some details in the calculations can be ignored.
谁的速度更快点？
Who’s Faster?
在所有这三种成分中，概率分布数据属最难取的了，因为我们只观察到过两个ISO，而且它们截然不同。仅用两个样本数据是不可能分析出一个概率分布的。所以，是时候我们向星星们求助了。
Out of all three ingredients, the probability distribution is the most difficult to get, since there were only ever two ISOs observed and they are substantially different. It would be impossible to determine a distribution with two samples. So, it’s time we ask the stars for some help.
假设在太阳系中，ISO的速度分布与它们在家乡恒星周围的相似，除了有些极端情况下，ISO会被恒星一脚踢到超高的弹射速度，大多数ISO的弹射速度都会相对较低。有了这个假设，我们只需要找出附近恒星上具有代表性样本的三维运动就可以进行下一步分析啦。
Assuming the velocity distribution of ISOs here, in the Solar system, is similar to what it was like back around their hometown stars, most of the ISOs are seen to have a relatively low ejection velocity, with some extreme cases of objects having super high from the star’s kicks. We just need to find out the three-dimensional motion of a representative sample from a star nearby.
正好，欧洲航天局于2013年发射的盖亚飞船已经收集了数百万颗恒星的精确运动数据，其中可供我们挑选的有几十万个样本，都在距离太阳的100parsec (~3千万亿公里)以内。经过选拔，研究小组最终挑选了附近的70000多颗恒星作为我们的ISO速度分布样本。
Lucky, the Gaia spacecraft, launched in 2013 by the ESA, has collected precise motion data from millions of stars, we are able to choose from several hundred thousand samples all within 100 parsec (~3 quadrillion km) from the Sun. After some mass-audition, the team picked out >70000 nearby stars as the samples for our ISO velocity distribution.
用这70000多个样本进行建模后，数据显示，大部分进入太阳系的ISO速度都小于60km/s。不出所料，ʻOumumua和Borisov在到达地球时的速度也都接近模型中的平均速度。这也再一次证明了我们的太阳系在宇宙中是多么的平平无奇，我们只不过是另一颗普通的恒星的普通行星罢了。
After modeling with the 70000+ samples, distribution shows that the majority of ISOs entering the Solar system would have a velocity < 60km/s. Unsurprisingly, both ʻOumumua and Borisov had a velocity near the average velocity when coming to us. Again, showing how common our Solar community is in the universe, just another average star with its average planets.
好奇心的结晶
Crystalizing the Curiosities
现在，用于Γ计算的必要成分都已经收集完毕，研究小组的最后一步就是找到预计每年通过太阳系的ISO的数量了，也就是相加所有不同的速度的到达频率。
Now the necessary ingredients for the calculation of Γ all gathered, the team had a final step to finding the number of ISOs expected to pass through the Solar system each year. That is summing up all possible velocities with different arrival rates.
最终结果显示，平均每年在距离太阳系1个天文单位的范围内，会有6.9个ISO经过。92%旅行者们的速度小于100km/s，最普遍的速度是38km/s左右。
Final results show that on average, 6.9 ISOs should pass within 1AU of the Solar system each year. The 92% of these travellers having velocities <100km/s, the most popular velocity is ~38km/s.
图为经过太阳1AU范围内的ISO的数量和速度的关系，显示了大多数ISO会以低于60km /s的速度进入太阳系
The number of ISOs that pass within 1AU of the Sun as a function of their velocity, showing that the majority of ISOs will enter the solar system with velocities < 60 km/s
海纳百川
All Over the Galaxy
另外，研究小组还分析了银河系中不同速度的ISO的可能起源。结果表明，根据恒星在星系中不同的位置，它周围的物体相对于太阳的速度也会有所不同。ISO最终被分为5个类型，随着类型编号数的增加，出现的概率逐渐减小(如下表所示)。
Additionally, the team analyzed the probable origins of the ISOs with different velocities in the Milky Way. Result show that, depending on the location of the star in the galaxy, objects around it would have different velocities relative to the Sun. Divided into five types, ISOs have decreasing likeliness of appearing with increasing type numbers (as shown in the table below).
该表为根据ISO的类型，总结的可探测到的的ISO速度范围、到达率和占总数量的比例
According to the type of interstellar object, this table summarises the velocity range, arrival rate and fraction of the total population of detectable ISOs
在银河系最外层的薄圆盘中，是第1类ISO的发源地，也是恒星相对于的速度是最低的地带。向内移动，盘面较厚的地方是第2类ISO的家乡，那里恒星的轨道会变得更倾斜，移动速度也会变得更快。
In the thin disk of Milky Way, which is the most outer areas of the galaxy, the stars there are likely to have smaller velocities relative to us, home to type 1 ISOs. Moving inwards, within the thick disk, stars have orbits that are more inclined and eccentric and also moving faster, providing ultimate nest for type 2 ISOs.
在星系最中心部分的光晕，是主要由过去的吸积事件产生的碎片组成的。作为第3类ISO诞生地，那里的恒星的轨道会受到了更大的扰动，运行速度也更快。但是，这还没完，培育第4类ISO的恒星才是运动速度最快的，而它们其实并不属于我们的星系。
In the most central part of the galaxy, the Halo, is mostly the debris from past accretion events. Being the place where type 3 ISOs are born, stars there have even more disturbed and faster orbits. However, that’s not it, the fastest stars providing type 4 ISOs are actually the ones that are not bound to our galaxy.
最后，在另一个极端，第5类ISO是相对于太阳速度最慢的那一类。这些物种被我们观察到的概率只有0.2%。严格来说他们也并不属于太阳系，很有可能是被我们的引力而吸引，从奥尔特云而来。
Finally, on the other side of the spectrum, type 5 ISOs are the slowest in their velocity relative to the Sun. With only 0.2% likeliness of these creatures appearing, they technically do not belong to the Solar system, but were likely be nudged here from the Oort Clouds by our gravitational attractions.
跟其他许多与太阳系平行的结构不同，奥尔特云被认为是围绕在太阳系的由极冷太空碎片组成的一堵巨大的球形墙体。其中一些天体能达到山脉的大小，甚至更大，据预测，它将包含数十亿，甚至数万亿的各色天体。
Unlike other structures that lie almost flat with the Solar system, the Oort Cloud is believed to be a giant spherical thick wall of icy space debris pieces surrounding the rest of the solar system. With some of the objects the sizes of mountains or even larger, it is predicted to contain billions, or even trillions, of objects.
图为奥尔特云可能的外观的模拟。由于那里彗星的体积过小，暂时没有直接证据表明它的存在及其与我们的距离，不过它很有可能是一个类似于得多惠伯带，但远得多的系统。方框中显示了我们的太阳系与奥尔特云相比是多么的小
A simulation of the possible look of the Oort Cloud. Due to the small sizes of the comets, there is no direct evidence of showing the existence of it and its distance from us, how every it is likely to be a similar system to the Huiper Belt, but much further away. The box shows how small out Solar system is compared to the Oort Cloud
下个访客会是谁？
Who’s Next?
ISO的研究无限可能性，使它成为一个充满惊喜的研究领域。在之前没有发现他们可能只是因为这些物体移动得太快或太远，或因为时间而错过遇见的机遇。
When it comes to the studies of ISOs, there are just so many possibilities, making it a field full of surprises. The lack of discoveries before might just have been the objects moving too fast or too far or at the wrong time for us to see.
今天的研究不仅告诉我们访客出现的频率，
还解释了这些访客可能的家乡。
随着科技的进步，
以及我们对不同天体的不断了解，
在不久的将来，
我们一定会再次遇见这些太空旅行者的！
Today’s study not only taught us
the frequency we should expect a visitor
but also where they might be traveling from.
With so much on offer today,
we have reason to suspect such an event may
take place in our own lifetimes soon!
图为拍摄于2013年11月19日的洛夫乔伊彗星，位于大熊座的它，虽然肉眼几乎看不见，但一副好望远镜还是能清楚的看到的
Comet Lovejoy taken on Nov. 19, 2013, though barely visible to the unaided eye in the constellation of Ursa Major, it still really stands out in a good pair of binoculars.
图片来自 NASA 官网以及以下文章
文中部分英文信息参考来自 T. M. Eubanks, A. M. Hein, M. Lingam, A. Hibberd, D. Fries, N. Perakis, R. Kennedy, W. P. Blase Jean Schneider 的 ‘KE_5.eps Interstellar Objects in the Solar System: 1. Isotropic Kinematics from the Gaia Early Data Release 3' 文章
其余中英文内容为原创
Images from official website of Forbs, ITIF, Bloomberg, Wikipedia and the below articles
Parts are sited from ‘KE_5.eps Interstellar Objects in the Solar System: 1. Isotropic Kinematics from the Gaia Early Data Release 3' from T. M. Eubanks, A. M. Hein, M. Lingam, A. Hibberd, D. Fries, N. Perakis, R. Kennedy, W. P. Blase Jean Schneider
The rest of the Chinese and English content are original
