What is a satellite?

什么是卫星？

A satellite is an object in space that orbits or circles around a bigger object. There are two kinds of satellites: natural (such as the moon orbiting the Earth) or artificial (such as the International Space Station orbiting the Earth).

卫星是太空中环绕比它更大物体的物体。卫星分为两种：自然卫星（如环绕地球运行的月球）或人造卫星（如环绕地球运行的国际空间站）。

There are dozens upon dozens of natural satellites in the solar system, with almost every planet having at least one moon. Saturn, for example, has at least 53 natural satellites, and between 2004 and 2017, it also had an artificial one — the Cassini spacecraft, which explored the ringed planet and its moons.

太阳系中有几十颗天然卫星，几乎每一颗行星都至少有一颗卫星。以土星为例，它至少有53颗天然卫星围绕着它。在2004年至2017年间，它还拥有一颗人造卫星——卡西尼号宇宙飞船，它负责探测这颗环状行星及其卫星。

Artificial satellites, however, did not become a reality until the mid-20th century. The first artificial satellite was Sputnik, a Russian beach-ball-size space probe that lifted off on Oct. 4, 1957. That act shocked much of the western world, as it was believed the Soviets did not have the capability to send satellites into space.

然而，人造卫星直到20世纪中期才成为现实。第一颗人造卫星是苏联于1957年10月4号发射升空的“斯普特尼克”，它是一颗沙滩球大小的太空探测器。斯普特尼克的发射震惊了西方世界，因为人们认为苏联并没有能力将卫星送入太空。

Following that feat, on Nov. 3, 1957 the Soviets launched an even more massive satellite — Sputnik 2 — which carried a dog, Laika. The United States' first satellite was Explorer 1 on Jan. 31, 1958. The satellite was only 2 percent the mass of Sputnik 2, however, at 30 pounds (13 kg).

在实现这一壮举之后，苏联又于1957年11月3日发射了一颗更大的卫星——斯普特尼克2号，它搭载了一只名叫莱卡的狗。美国的第一颗卫星是于1958年1月31日发射的“探索者1号”。然而，这颗卫星的质量只有斯普特尼克2质量的2%，重30磅(13公斤)

The Sputniks and Explorer 1 became the opening shots in a space race between the United States and the Soviet Union that lasted until at least the late 1960s. The focus on satellites as political tools began to give way to people as both countries sent humans into space in 1961. Later in the decade, however, the aims of both countries began to split. While the United States went on to land people on the moon and create the space shuttle, the Soviet Union constructed the world's first space station, Salyut 1, which launched in 1971. (Other stations followed, such as the United States' Skylab and the Soviet Union's Mir.)

“斯普特尼克”和“探索者1号”成为美苏太空竞赛开幕，这场竞赛至少持续到20世纪60年代末。随着1961年两国都将人类送入太空，把卫星作为政治工具的焦点开始偏移到了人身上。十年后，两国的目标开始出现分歧。当美国继续将人类送上月球并制造航天飞机时，苏联建造了世界上第一个空间站——礼炮1号，并于1971年发射升空。（其他空间站也紧随其后，比如美国的天空实验室和苏联的和平号。）

Other countries began to send their own satellites into space as the benefits rippled through society. Weather satellites improved forecasts, even for remote areas. Land-watching satellites such as the Landsat series (on its ninth generation now) tracked changes in forests, water and other parts of Earth's surface over time. Telecommunications satellites made long-distance telephone calls and eventually, live television broadcasts from across the world a normal part of life. Later generations helped with Internet connections.

其他国家开始将自己的卫星送入太空，因为卫星带来的收益已经波及到了社会上下。气象卫星改善了预报，甚至偏远地区也同样有所改善。陆地卫星系列（现在已经是第九代了）等陆地观测卫星跟踪了森林，水和地球表面其他部分随着时间的变化。通信卫星拨打长途电话，并最终使得来自世界各地的电视直播成为了人们生活的一个部分，为后来的几代人建立了互联网连接。

With the miniaturization of computers and other hardware, it's now possible to send up much smaller satellites that can do science, telecommunications or other functions in orbit. It's common now for companies and universities to create "CubeSats", or cube-shaped satellites that frequently populate low-Earth orbit.

随着计算机和其他硬件的小型化，现在可以发射更小的卫星，这些卫星可以在轨道上执行科研，通信或其他功能。现在，公司和大学都在制造“立方体卫星”，也就是经常出现在近地轨道上的立方体卫星。

These can be lofted on a rocket along with a bigger payload, or sent from a mobile launcher on the International Space Station (ISS). NASA is now considering sending CubeSats to Mars or to the moon Europa (near Jupiter) for future missions, although the CubeSats aren't confirmed for inclusion.

这些卫星可以通过火箭和更大的有效载荷一起发射，也可以通过国际空间站(ISS)的移动发射装置发射到太空。美国国家航空航天局现在正在考虑将立方体卫星送往火星或（木星附近的）月球木卫二，以完成未来的任务，尽管立方体卫星还没有被确认包括在内。

The ISS is the biggest satellite in orbit, and took over a decade to construct. Piece by piece, 15 nations contributed financial and physical infrastructure to the orbiting complex, which was put together between 1998 and 2011. Program officials expect the ISS to keep running until at least 2024.

国际空间站是轨道上最大的卫星，花费了十多年时间才建成。在1998年到2011年之间，有15个国家一点一点为这个轨道的综合设施提供了财政和物质基础设施。据项目官员预计，国际空间站至少会运行到2024年。

Every usable artificial satellite — whether it's a human or robotic one — has four main parts to it: a power system (which could be solar or nuclear, for example), a way to control its attitude, an antenna to transmit and receive information, and a payload to collect information (such as a camera or particle detector).

每一个可用的人造卫星——无论它是一个人类或机器人——都由四个主要部分:电力系统(例如,可以是太阳能和核能),来控制它的姿态；天线，负责发送和接收信息；和负载，用来收集信息(如相机或粒子探测器)

As will be seen below, however, not all artificial satellites are necessarily workable ones. Even a screw or a bit of paint is considered an "artificial" satellite, even though these are missing these parts.

然而，正如下面将看到的那样，并不是所有的人造卫星都一定是可行的。即使是一颗螺丝钉或一点油漆也被认为是一颗“人造”卫星，尽管它们缺少这些必要的部分。

A satellite is best understood as a projectile, or an object that has only one force acting on it — gravity. Technically speaking, anything that crosses the Karman Line at an altitude of 100 kilometers (62 miles) is considered in space. However, a satellite needs to be going fast — at least 8 km (5 miles) a second — to stop from falling back down to Earth immediately.

卫星可以被理解为一种抛射物，即只有一个力作用于它的物体——重力。从技术上讲，任何在100公里(62英里)的高度越过卡门线的东西都被认为是在太空中。然而，一颗卫星需要速度快——至少每秒8公里(5英里)——才能防止立即停止掉落回地球。

If a satellite is traveling fast enough, it will perpetually "fall" toward Earth, but the Earth's curvature means that the satellite will fall around our planet instead of crashing back on the surface. Satellites that travel closer to Earth are at risk of falling because the drag of atmospheric molecules will slow the satellites down. Those that orbit farther away from Earth have fewer molecules to contend with.

如果一颗卫星的速度足够快，它将永远“落”向地球，但地球的曲率意味着卫星将落在地球周围，而不是撞回地球表面。因为大气分子的阻力会使卫星减速，距离地球较近的卫星有坠落的危险，那些离地球较远的卫星需要处理的分子较少。

There are several accepted "zones" of orbits around the Earth. One is called low-Earth-orbit, which extends from about 160 to 2,000 km (about 100 to 1,250 miles). This is the zone where the ISS orbits and where the space shuttle used to do its work. In fact, all human missions except for the Apollo flights to the moon took place in this zone. Most satellites also work in this zone.

围绕地球的轨道有几个公认的“区域”。一种是低地球轨道，长度从160公里到2000公里(约100到1250英里)不等。这是国际空间站运行的区域，也是航天飞机过去工作的地方。事实上，除了阿波罗登月计划，所有的人类任务都发生在这个区域。大多数卫星也在这个区域工作。

Geostationary or geosynchronous orbit is the best spot for communications satellites to use, however. This is a zone above Earth's equator at an altitude of 35,786 km (22,236 mi). At this altitude, the rate of "fall" around the Earth is about the same as Earth's rotation, which allows the satellite to stay above the same spot on Earth almost constantly. The satellite thus keeps a perpetual connection with a fixed antenna on the ground, allowing for reliable communications. When geostationary satellites reach the end of their life, protocol dictates they're moved out of the way for a new satellite to take their place. That's because there is only so much room, or so many "slots" in that orbit, to allow the satellites to operate without interference.

然而，地球静止轨道或地球同步轨道是通信卫星运行的最佳地点。这个区域位于地球赤道上方，海拔35786千米(22236英里)。在这个高度，绕地球的“下落”速率与地球自转速率大致相同，这使得卫星几乎一直保持在地球上同一地点的上方。因此，卫星与地面上的固定天线保持永久的连接，从而实现稳定的通信。当地球同步卫星到达其寿命结束时，协议规定它们将被移开，以让一颗新卫星取代它们的位置。这是因为在那个轨道上只有那么多的空间，或者说只有那么多的“插槽”，可以让卫星不受干扰地运行。

While some satellites are best used around the equator, others are better suited to more polar orbits — those that circle the Earth from pole to pole so that their coverage zones include the north and south poles. Examples of polar-orbiting satellites include weather satellites and reconnaissance satellites.

一些卫星最适合在赤道附近使用，而另一些则更适合在极地轨道上使用——那些从极点到极点环绕地球的轨道，因此它们的覆盖区域包括北极和南极。极地轨道卫星的例子包括气象卫星和侦察卫星。

There are an estimated half-million artificial objects in Earth orbit today, ranging in size from paint flecks up to full-fledged satellites — each traveling at speeds of thousands of miles an hour. Only a fraction of these satellites are useable, meaning that there is a lot of "space junk" floating around out there. With everything that is lobbed into orbit, the chance of a collision increases.

据估计，目前地球轨道上有50万个人造卫星，从油漆斑点的小卫星到成熟的卫星，每个卫星的飞行速度都高达每小时数千英里。这些卫星中只有一小部分是可用的，这意味着有很多“太空垃圾”漂浮在那里。随着所有物体都被抛入轨道，碰撞的几率就会增加。

Space agencies have to consider orbital trajectories carefully when launching something into space. Agencies such as the United States Space Surveillance Network keep an eye on orbital debris from the ground, and alert NASA and other entities if an errant piece is in danger of hitting something vital. This means that from time to time, the ISS needs to perform evasive maneuvers to get out of the way.

航天机构在向太空发射物体时必须仔细考虑轨道轨道。像美国太空监视网络这样的机构会从地面监视轨道上的碎片，如果有碎片有可能撞到重要的东西，就会向NASA和其他机构发出警报。这意味着，国际空间站需要不时地进行规避操作，以避开这些障碍。

Collisions still occur, however. One of the biggest culprits of space debris was the leftovers of a 2007 anti-satellite test performed by the Chinese, which generated debris that destroyed a Russian satellite in 2013. Also that year, the Iridium 33 and Cosmos 2251 satellites smashed into each other, generating a cloud of debris.

然而，碰撞仍然会发生。太空碎片的最大元凶之一是2007年中国进行的反卫星试验的残余物，该试验产生的碎片在2013年摧毁了一颗俄罗斯卫星。同年，铱星33号和宇宙2251号卫星相撞，产生了一片碎片云

NASA, the European Space Agency and many other entities are considering measures to reduce the amount of orbital debris. Some suggest bringing down dead satellites in some way, perhaps using a net or air bursts to disturb the debris from its orbit and bring it closer to Earth. Others are thinking about refueling dead satellites for reuse, a technology that has been demonstrated robotically on the ISS.

美国国家航空航天局(NASA)、欧洲航天局(European Space Agency)和许多其他实体正在考虑采取措施减少轨道碎片的数量。一些人建议用某种方式将报废的卫星击落，比如用网或空气爆炸来扰乱轨道上的碎片，使其更接近地球。其他人则在考虑给报废的卫星补充燃料，以便重新使用，这项技术已经在国际空间站上进行了机器人演示。

Most planets in our solar system have natural satellites, which we also call moons. For the inner planets: Mercury and Venus each have no moons. Earth has one relatively large moon, while Mars has two asteroid-sized small moons called Phobos and Deimos. (Phobos is slowly spiralling into Mars and will likely break apart or fall into the surface in a few thousand years.)

我们太阳系中的大多数行星都有天然卫星，我们也称它们为卫星。内行星:水星和金星都没有卫星。地球有一个相对较大的卫星，而火星有两个小行星大小的小卫星，分别是火卫一和火卫二。(火卫一正在缓慢盘旋进入火星，可能会在几千年后解体或落入火星表面。)

Beyond the asteroid belt, are four gas giant planets that each have a pantheon of moons. As of late 2018, Jupiter has 79 confirmed moons, Saturn has 53, Uranus has 27 and Neptune has 14. New moons are occasionally discovered – mainly by missions (either past or present, as we can analyze old pictures) or by performing fresh observations by telescope.

在小行星带之外，有四颗巨大的气态行星，每颗行星都有一万神殿般的卫星群。截至2018年底，木星有79颗已确认的卫星，土星有53颗，天王星有27颗，海王星有14颗。新月偶尔会被发现——主要是通过任务(过去的或现在的，因为我们可以分析旧照片)或通过望远镜进行新的观测

Saturn is a special example because it is surrounded by thousands of small objects that form a ring visible even in small telescopes from Earth. Scientists watching the rings close-up over 13 years, during the Cassini mission, saw conditions in which new moons might be born. Scientists were particularly interested in propellers, which are wakes in the rings created by fragments in the rings. Just after Cassini's mission ended in 2017, NASA said it's possible the propellers share elements of planet formation that takes place around young stars' gassy discs.

土星是一个特殊的例子，因为它被数千个小物体包围，形成一个环，即使从地球上的小型望远镜也能看到。在卡西尼号任务期间，科学家们对土星环进行了13年的近距离观察，发现了可能孕育新卫星的条件。科学家们对螺旋桨特别感兴趣，它是由环上的碎片形成的环上的尾迹。卡西尼号的任务于2017年结束后不久，美国宇航局表示，这两个推进器有可能共享在年轻恒星气体盘周围的行星形成的元素。

Even smaller objects have moons, however. Pluto is technically a dwarf planet. However, the people behind the New Horizons mission, which flew by Pluto in 2015, argue its diverse geography makes it more planet-like. One thing that isn't argued, however, is the number of moons around Pluto. Pluto has five known moons, most of which were discovered when New Horizons was in development or en route to the dwarf planet.

然而，更小的物体也有卫星。严格来说，冥王星是一颗矮行星。然而，2015年飞越冥王星的“新视野号”(New Horizons)任务的幕后人员认为，冥王星的地理多样性让它更像一个行星。然而冥王星周围有多少颗卫星是没有争议的。冥王星有5颗已知的卫星，其中大多数是在“新视野号”开发前或前往这颗矮行星的途中发现的。

A lot of asteroids have moons, too. These small worlds sometimes fly close to the Earth, and the moons pop out in observations with radar. A few famous examples of asteroids with moons include 4 Vesta (which was visited by NASA's Dawn mission), 243 Ida, 433 Eros, and 951 Gaspra. There are also examples of asteroids with rings, such as 10199 Chariklo and 2060 Chiron.

很多小行星也有卫星。这些小星球有时会靠近地球飞行，卫星会在雷达的观测中突然出现。几个著名的带有卫星的小行星例子包括4号灶神星（NASA的黎明号访问过它），243号艾达，433号爱神星和951号加斯普拉。也有小行星带有光环的例子，如10199夏里克洛和2060夏隆。

Many planets and worlds in our solar system have human-made "moons" as well, particularly around Mars — where several probes orbit the planet doing observations of its surface and environment. The planets Mercury, Venus, Mars, Jupiter and Saturn all had artificial satellites observing them at some point in history. Other objects had artificial satellites as well, such as Comet 67P/Churyumov–Gerasimenko (visited by the European Space Agency's Rosetta mission) or Vesta and Ceres (both visited by NASA's Dawn mission.) Technically speaking, during the Apollo missions, humans flew in artificial "moons" (spacecraft) around our own moon between 1968 and 1972. NASA may even build a "Deep Space Gateway" space station near the moon in the coming decades, as a launching point for human Mars missions.

太阳系中的许多行星和世界也都有人造的“卫星”，特别是在火星周围，有几个探测器围绕着这颗行星运行，对其表面和环境进行观测。水星，金星，火星，木星和土星这些行星在历史上的某个时刻都有人造卫星观测过它们。其他天体也有人造卫星，如67P/Churyumov-Gerasimenko彗星（欧洲航天局的罗塞塔任务访问过）或灶神星和谷神星（美国宇航局的黎明任务访问过这两颗彗星） 从技术上讲，在阿波罗任务期间，人类在1968年至1972年间乘坐人造“卫星”（宇宙飞船）绕着我们自己的月球飞行。NASA甚至可能在未来几十年内在月球附近建造一个“深空门户”空间站，作为人类火星任务的发射点。

Fans of the movie "Avatar" (2009) will remember that the humans visited Pandora, the habitable moon of a gas giant named Polyphemus. We don't know yet if there are moons for exoplanets, but we suspect — given that the solar system planets have so many moons — that exoplanets have moons as well. In 2014, scientists made an observation of an object that could be interpreted as an exomoon circling an exoplanet, but the observation can't be repeated as it took place as the object moved in front of a star. However, a second exomoon might have been found very recently.

看过电影《阿凡达》(2009)的粉丝们应该还会记得，人类造访了潘多拉星球，那是一颗名叫波利菲莫斯的气态巨星的宜居卫星。我们还不知道系外行星是否有卫星，但我们怀疑——鉴于太阳系行星有如此多的卫星——系外行星也有卫星。2014年，科学家们观测了一个物体，该物体可以被解释为一个围绕着一颗系外行星的系外卫星，但观测结果不能重复，因为它是在该物体在一颗恒星前移动时发生的。然而，第二颗系外卫星可能是最近才被发现的。

Read about some of the satellites that NASA has in orbit around Earth.

阅读美国宇航局在绕地轨道上的一些卫星。

Learn about the types of orbit that we put satellites in.

了解我们放置卫星的轨道类型。

Discover how satellites can prepare us for the increasing frequency of flood events around the world.

探索卫星如何让我们为世界各地日益频繁的洪水事件做好准备。

Joukowsky Institute, Brown University, "13 Things - space"

布朗大学茹科夫斯基研究所，《13件事--空间》

Amanda Barnett, NASA’s Jet Propulsion Laboratory for NASA’s Science Mission Directorate, "Basics of Space Flight - Section 1: Environment, Chapter 5: Planetary Orbits"

Amanda Barnett，NASA科学任务局喷气推进实验室，“空间飞行基础--第1节：环境，第5章：行星轨道”

Astromaterials Research & Exploration Science, NASA, "The Orbital Debris Issue"

天体材料研究与探索科学，NASA，“轨道碎片问题”