为了庆祝 全球天文学月, we’re posting a few articles about the satellite side of things. 你要多少钱? 真的 了解卫星和它们称之为家的轨道? Here are five interesting facts about satellites that just might surprise you.
1: Isaac Newton first proved that orbits existed centuries before humans could actually deliver objects into orbit.
牛顿在1687年首次提出, the first law of physics states that an object in motion remains in motion unless acted upon by another force. 用于轨道上的卫星, this makes life a constant balancing act between momentum (which wants to keep the satellite moving) and gravity (which wants to drag it down). 牛顿使用了 关于炮弹的思想实验 描述他的第一定律如何应用于轨道.
A cannonball shot with a small amount of gunpowder will travel a short distance before gravity takes over and brings it back to the ground. Use too much gunpowder and your ball goes flying off into space. But use just the right amount of gunpowder and you can overcome the force of gravity and send your cannonball into an orbit. Safety notice: Don’t try to prove Newton’s first law at home.
卫星轨道有三种常见的类型.
These range from around 300 miles above the Earth’s surface to exactly 22,236英里高, and satellite operators rely on different orbits for payloads with different purposes. In 近地轨道, satellites are generally smaller and less complex than the larger satellites in higher orbits. 把卫星送到这个轨道只需要500-1,500 km (300–900 miles) is easier than attaining those higher orbits, 而且通常一次火箭发射会携带多个leo.
接下来，我们来看看 中地球轨道, which is typically in the range of 5,000–12,000 km (3,100–7,500 miles) up. This is high enough that a constellation of seven to 10 satellites can cover most of the globe.
最后, 地球静止轨道 is at a precise altitude of 35,786 km (22,236 miles), directly above the equator. This altitude allows satellites to cover very large portions of the earth, and just three can cover most of the globe (as our ViaSat-3 constellation is planned to do). 地球同步轨道卫星的另一个优点是, because they orbit at exactly the same speed as the Earth’s rotation, 它们在天空中看起来是静止的. This allows them to cover a fixed position on the ground with a relatively simple antenna.
3: New satellites in upper orbits need to stay in very specific “slots” to avoid conflict with other satellites.
Because GEO satellites must be exactly over the equator at a specific altitude, the number of available orbital positions is somewhat limited. All new GEO satellites are given precise orbital “slots” assigned by the national space agency of the satellite operator and regulated by the 国际 Telecommunications Union (ITU). The process takes years and there’s a lot of paperwork involved, but it’s all necessary to keep the peace in the geostationary neighborhood.
4: Satellites “wobble” slightly and need to be periodically flown back into their allocated slot in orbit.
The process of keeping geostationary satellites in their proper orbit is called “station keeping.“任其自生自灭, a geostationary satellite will drift north and south in a figure-8 pattern, losing its alignment with antennas on the ground and rendering it useless. GEO satellites are equipped with thrusters that allow operators to periodically correct for this drift. Relying on a constant stream of information flowing between satellites and ground networks, satellite operators engage these engines and carefully nudge the satellite back toward the center of its slot.
5: When satellites reach the end of their mission, they end their service life in different ways.
Satellites have widely varying lifespans — from just a couple years to many decades. But since there’s no requirement or incentive for satellite operators to retrieve them once their operating days are over, 大多数退役的卫星只是留在轨道上. 通常, an abandoned satellite in a lower orbit will lose its fight with gravity and burn up in the atmosphere within a few years.
对于一颗地球同步轨道卫星来说, the operators use the last of the thruster fuel to push it up a few hundred kilometers into what’s called a “graveyard orbit.” This keeps it out of the way of other satellites, and there it stays for likely millions of years!
But sometimes, satellites assumed to be washed up have a little surprise left for their operators. So-called “zombie satellites” have been known to spontaneously begin transmitting again decades after being declared “dead.” The phenomenon first gained attention in 2013, when British amateur radio astronomer Phil Williams 收到LES-1的信号, a United States 空气 Force communications satellite launched in 1965 and assumed lost in 1967. 最近，加拿大人斯科特·蒂利 与实验卫星LES-5取得联系 将于2020年进入地球静止轨道.
Looking for zombies in graveyards is sure to keep a lot of amateur astronomers up at night!