Spacecraft flights involve huge energy consumption. For example, the Soyuz launch vehicle, standing on the launch pad and ready to launch, weighs 307 tons, of which more than 270 tons is fuel, that is, the lion's share. The need to spend a crazy amount of energy on movement in outer space is largely related to the difficulties of mastering the far reaches of the solar system.
Unfortunately, a technical breakthrough in this direction is not yet expected. The mass of propellant remains one of the key factors in planning space missions, and engineers take every opportunity to save fuel in order to prolong the operation of the device. Gravity maneuvers are one way to save money.
How to fly in space and what is gravity
The principle of moving the device in a vacuum (an environment from which it is impossible to push off either with a propeller, or wheels, or anything else) is the same for all types of rocket engines made on Earth. This is jet thrust. Gravity opposes the power of a jet engine. This battle against the laws of physics has been wonSoviet scientists in 1957. For the first time in history, an apparatus made by human hands, having acquired the first cosmic speed (about 8 km / s), became an artificial satellite of the planet Earth.
It took about 170 tons of iron, electronics, purified kerosene and liquid oxygen to launch a device weighing just over 80 kg into low Earth orbit.
Of all the laws and principles of the universe, gravity is, perhaps, one of the main ones. It governs everything, starting with the arrangement of elementary particles, atoms, molecules and ending with the movement of galaxies. It is also an obstacle to space exploration.
Not just fuel
Even before the launch of the first artificial Earth satellite, scientists clearly understood that not only increasing the size of rockets and the power of their engines could be the key to success. The researchers were prompted to search for such tricks by the results of calculations and practical tests, which showed how fuel-consuming flights outside the earth's atmosphere are. The first such decision for Soviet designers was the choice of the site for the construction of the cosmodrome.
Let's explain. To become an artificial satellite of the Earth, the rocket needs to accelerate to 8 km/s. But our planet itself is in constant motion. Any point located on the equator rotates at a speed of more than 460 meters per second. Thus, a rocket launched into airless space in the area of the zero parallel will in itself behave free almost half a kilometer per second.
That is why, in the wide expanses of the USSR, a place to the south was chosen (the speed of daily rotation in Baikonur is about 280 m/s). An even more ambitious project aimed at reducing the effect of gravity on the launch vehicle appeared in 1964. It was the first marine cosmodrome "San Marco", assembled by the Italians from two drilling platforms and located on the equator. Later, this principle formed the basis of the international Sea Launch project, which successfully launches commercial satellites to this day.
Who was the first
What about deep space missions? Scientists from the USSR were pioneers in using the gravity of cosmic bodies to change the flight path. The reverse side of our natural satellite, as you know, was first photographed by the Soviet Luna-1 apparatus. It was important that after flying around the moon, the device had time to return to the Earth so that it would be turned to it by the northern hemisphere. After all, the information (the received photographic images) had to be transmitted to people, and the tracking stations, radio antenna dishes were located precisely in the northern hemisphere.
No less successful was the use of gravitational maneuvers to change the trajectory of the spacecraft by American scientists. The interplanetary automatic spacecraft "Mariner 10" after a flyby near Venus had to reduce the speed in order to go into a lower circumsolar orbit andexplore Mercury. Instead of using the jet thrust of the engines for this maneuver, the speed of the vehicle was slowed down by the gravitational field of Venus.
How it works
According to the law of universal gravitation, discovered and confirmed experimentally by Isaac Newton, all bodies with mass attract each other. The strength of this attraction is easily measured and calculated. It depends both on the mass of both bodies and on the distance between them. The closer, the stronger. Moreover, as bodies approach each other, the force of attraction grows exponentially.
The figure shows how spacecraft, flying near a large cosmic body (some planet), change their trajectory. Moreover, the course of movement of the device under number 1, flying farthest from the massive object, changes very slightly. What can not be said about the device number 6. The planetoid changes its direction of flight dramatically.
What is a gravity sling. How it works
The use of gravity maneuvers allows not only to change the direction of the spacecraft, but also to adjust its speed.
The figure shows the trajectory of a spacecraft, usually used to accelerate it. The principle of operation of such a maneuver is simple: in the section of the trajectory highlighted in red, the device seems to be catching up with the planet running away from it. A much more massive body pulls a smaller body with its force of gravity, dispersing it.
By the way, not only spaceships are accelerated this way. It is known that celestial bodies that are not tied to the stars roam the galaxy with might and main. These can be both relatively small asteroids (one of which, by the way, is now visiting the solar system), and planetoids of decent size. Astronomers believe that it is the gravitational sling, i.e. the impact of a larger cosmic body, that throws less massive objects out of their systems, dooming them to eternal wanderings in the icy cold of empty space.
How to slow down
But, using the gravitational maneuvers of spacecraft, you can not only accelerate, but also slow down their movement. The scheme of such braking is shown in the figure.
On the section of the trajectory highlighted in red, the attraction of the planet, in contrast to the variant with a gravitational sling, will slow down the movement of the apparatus. After all, the vector of gravity and the direction of flight of the ship are opposite.
When is it used? Mainly for launching automatic interplanetary stations into the orbits of the studied planets, as well as for studying near-solar regions. The fact is that when moving towards the Sun or, for example, towards the planet Mercury closest to the star, any device, if you do not apply measures for braking, willy-nilly accelerate. Our star has an incredible mass and an enormous force of attraction. A spacecraft that has gained excessive speed will not be able to enter the orbit of Mercury, the smallest planet of the solar family. The ship will just slip throughby, little Mercury can't pull it hard enough. Motors can be used for braking. But a gravitational trajectory to the Sun, say at the Moon and then Venus, would minimize the use of rocket propulsion. This means that less fuel will be needed, and the freed weight can be used to accommodate additional research equipment.
Get in the eye of a needle
While early gravitational maneuvers were conducted timidly and hesitantly, the routes of the latest interplanetary space missions are almost always planned with gravitational adjustments. The thing is that now astrophysicists, thanks to the development of computer technology, as well as the availability of the most accurate data on the bodies of the solar system, primarily their mass and density, have more accurate calculations available. And it is necessary to calculate the gravity maneuver extremely accurately.
Thus, laying a trajectory further from the planet than necessary is fraught with the fact that expensive equipment will fly not at all where it was planned. And underestimation of the mass can even threaten the collision of the ship with the surface.
Champion in maneuvers
This, of course, can be considered the second spacecraft of the Voyager mission. Launched in 1977, the device is currently leaving its native star system, retiring into the unknown.
During its operation, the apparatus visited Saturn, Jupiter, Uranus and Neptune. Throughout the flight, the attraction of the Sun acted on it, from which the ship gradually moved away. But, thanks to well-calculated gravitationalmaneuvers, for each of the planets, its speed did not decrease, but grew. For each planet explored, the route was built on the principle of a gravitational sling. Without the application of gravitational correction, Voyager would not have been able to send it this far.
Besides the Voyagers, gravity maneuvers have been used to launch such well-known missions as Rosetta or New Horizons. So, Rosetta, before going in search of the Churyumov-Gerasimenko comet, made as many as 4 accelerating gravitational maneuvers near the Earth and Mars.
