Do you ever wonder how a space probe like Voyager 2 can travel over 17 billion km and visiting the four outer planets and over a space of 40 years with perfection? So how do spacecrafts navigate deep in space? Voyager 2 had reached Neptune within only 100km from the target area after travelling 7 billion km and after being swung by Jupiter and Saturn and all with mid 70’s tech. In the Sci-Fi movies, spacecraft seems to move into space where they want in no time at all. However, in reality, it is more complicated and takes much longer to get around. The New Horizons probe, one of the fastest spacecraft, took nine years to go from Earth to Pluto, a distance of about five billion km. It might sound like a difficult challenge, but when you learn how space and physics work, it is a series of processes, scientific facts rather than science fiction. Moreover, the secret to all of them is to know how gravity works and how it influences not only you and me but also everything in the world. So let’s find out how do spacecraft navigate deep in space.
About 400 years ago, Johannes Kepler first worked out the laws of planetary motion. Isaac Newton then worked out newton’s laws of motion and the creation of classical mechanics based on planetary motion. With classical mechanics, we were able to predict the movement of everything perfectly. Newton’s first law states that an object at rest or travelling a straight line will stay that way unless a force act upon it. A rock, for example, on the ground, will not move by self unless something else pushes it. This case will also be similar if it were in space. However, Sun’s gravity or the gravity of any other thing with mass will always put a force on that rock. The larger mass will exert a larger amount of force.
The speed is the other crucial component. According to Newton’s second law, an object’s speed will change when a force is applied to it, and this is also reversible as changing the speed will generate force. Because of this reason, asteroids can release huge force as kinetic energy upon hitting the Earth when the incredible speed suddenly changes to zero. When you shoot a bullet on Earth parallel to the Earth, it will inevitably crash down to the ground because of gravity. However, if you throw it with fast enough speed and it maintains the speed, it will still be travelling along a straight line. But, Earth’s gravity will continuously act upon it, and when the curvature of the trajectory matches that of the Earth then it is in orbit around the Earth. Basically, in this case, the force of a projectile trying to go on a straight line matches with the force of pulling by Earth’s gravity. This is how satellites and space-stations stay in orbit, but they experience a little amount of drag of the very thin atmosphere. This gradually slows them, and the balance of force shifts towards gravity and they will be eventually brought down to the ground unless there are periodic boosts in the speed.
If indeed, a spacecraft increases its velocity, the orbit will become wider and elliptical, but it will still return to the point where the velocity was initially boosted. The speed of the spacecraft needs to be increased to escape velocity such that it will escape Earth’s gravity and enter in an orbit around the Sun. More increase in speed will increase the size of its orbit, and if we get the speed boost correctly timed, we can get the orbit of our spacecraft to intersect the orbit of a planet.
The most common method for moving from one moving body to another is the method known as the Hohmann transfer approach. Though there are now more effective but much longer methods, such as the method of low thrust transfer and the method of interplanetary network transport. From there, our spacecraft can reach a planet’s orbit, or we can use the gravity of the planet to slingshot around it, or we can use gravity assist as it is called to raise the speed of craft concerning the Sun. When a spacecraft flying closely, gravity of that planet can assist it to increase or decrease speed or change its direction. When a spacecraft move in the direction of the planet’s direction its speed increases and decreases if it move oppositely. The course of the spacecraft can be changed dramatically depending upon how it approaches the planet.
Nevertheless, there is nothing without cost; the energy conservation must happen in the universe, so the amount of energy the spacecraft gains slows down the planet. Jupiter’s orbit around the Sun slowed down but only about one foot per trillion years when the voyagers used it’s gravity assist. The gravity assists can be used to move from planet to planet, but the speed can be increased up to escape velocity. At this point, it will be travelling fast enough to escape the pull of the Sun and leave the solar system just like the voyager 1. However, the Sun’s gravity will still pull on the craft and slow it down; the Sun’s gravitational effect extends out about 2.5 light-years, and it will take voyagers travelling at 60000km/h about 40000years to reach the point where the Sun’s gravity no longer dominates.
Newton’s third law states that every action has an equal and opposite reaction. The thrust from an engine moves a craft forward. Few people might think that the thrust pushes against the ground or air, and thus they cannot work in space. But, this is not the case as even when there is nothing to push against, our thrusters do not stop working. The thrust is nothing but the burst of gases that releases with huge force. The spacecraft use this thrust to increase or decrease speed, to change its orbit, as well as move it in its X and Y planes and to point their camera toward the target or to orientate its antenna towards Earth.
Again, the most important thing to move in space is to know how gravity works. Apart from that, an accurate model of the solar system is the next most crucial thing. This will let us know where the planets will be with respect to the Sun, other planets, asteroids and comets. This model comes from a time table which is based on the positions of all the planets and other objects relative to the Sun, this time table is known as planetary ephemera. The time table is created based on the data over the centuries. The Babylonians created the first ones in 1200BC. Without these ephemerides, space missions will probably become impossible. However unknown planets or objects and the gravity can distort this ephemeris greatly. NASA continuously updates their ephemerides each year for 20 years. It can give space missions incredible accuracy like the voyagers.
In 1964, Gary Flandro, while working at JPL found out an amazing once in a 175 years alignment of Jupiter, Saturn, Uranus and Neptune that allowed the voyager 1 to make the grand tour. This allowed the spacecraft to complete the mission in 10 years instead of 40 years. Voyager 2 was the first to set out on another grand tour of the four outer planets in 1977 and finally flew out of the plane of the solar system. The Galileo, Cassini, and the new Horizons used this same gravity assist principle to move. Voyager 1 follow a quicker route to visit Jupiter, Saturn and Saturn’s moon Titan. When it was flying out of the solar system, it turned around so that the camera could point toward Earth and took one last set of photos. They were the farthest images ever taken of the solar system. One of them showed the position Earth in it, covering just 0.12 pixels in size in the middle of a lens flare the famous “Pale Blue Dot” as Carl Sagan called it was taken 6.4 billion KM away looking down at a 32-degree angle onto the solar system.
However, knowing only the gravity assists or the ephemeris does not solve all the problem, we need something to guide the craft to remain in its planned trajectory. To overcome this problem, scientists developed an inertial navigation system, comprises a highly accurate gyroscope, accelerometer and other sensors. These sensors process pieces of information and send them to the navigator to see if the spacecraft is on its course.
However, all mechanical devices suffer from integration drift, tiny errors sensors and the inertial navigation system is no exception. It is compounded over time, as they measure their location as they pass from the last previously measured position so that the longer, they go, the more errors build up. A good system has the error of less than 1.1km per hours, so if a journey to mars lasted eight months, then the error would be around 6300km, and because of this when it reaches Mars, it drifted so much that it cannot enter its orbit. To enter the orbit, you have to have an accuracy of about a few kilometers. To minimize the error, we need another fixed reference system other than the Sun like the marine navigators who used sextants to work out their position. Optical sensors and cameras are used by spacecraft to assess their location and reset inertial navigation systems. We used a star tracker on the voyager’s probes that could search for a very bright guide star, Canopus. This also had a Sun tracker, which could be used in combination with an Earth radio signal. Newer spacecraft have increasingly advanced cameras systems to search for observed bodies such as stars, comets and asteroids, or the target itself.
Though how perfectly you have planned the course, stuff will vary the way they go. Certain other forces also can impact a craft deep in space. For example, the solar wind, the flow of charged particles from the Sun, will slowly change a spacecraft’s path over time and needs to be corrected, and timing is everything. Within a tiny window of time, our spacecraft must arrive at specific space points along the journey. This might cause the spacecraft either sucked in the planet or undershoot the expected path if you are out by more than a few minutes or so. NASA uses the Deep Space Network to connect and to figure out the speed and distance of a craft. This is a network of radio telescopes scattered across the globe so that at least one of them can still be in contact with a spaceship. By transmitting a radio signal to the craft and by measuring how much time the signal took to return and by using a highly precise atomic clock and Doppler effect we can now measure the speed with 180 millimeters per hour accuracy and the speed within 3 meters.
Combining all that above information, we are now able to send space probes with incredible accuracy, so much so that we can now land on a comet as we did with the Rosetta probe. It’s now possible for Philae lander and to take a close-up picture of Pluto within a two-hour time window, nine years after launch and 5 billion km away, and when we only had one-third of Pluto’s orbit mapped. Five spacecraft have now achieved escaped velocity using these methods we have spoken about and are now the farthest objects created by man. Pioneer 10,11, Voyager 1 &2 and new Horizons. It’s crazy to know that this all was done based on hypotheses formed through observation developed hundreds of years ago and the drive to find out how the universe operated way before we even realized it was possible to get into space by using gravity as the main engines.
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