One of the most challenging questions a space enthusiast can pose to an expert is how long it would take to travel from Earth to another planet in the Solar System and how to make that journey faster. The answer depends on numerous factors.
Traveling to other planets
How long does it take to fly to Mars? And to Saturn? And what if you could fly twice as fast? All these questions are not as simple as they seem. On Earth, we are used to the idea that if we have two cities connected by road or rail and know the average speed of a car or train, we can divide the distance by the speed and get an approximate travel time.
If you look at a simplified model of the Solar System, where the planets are arranged one after the other, you may get the impression that everything is the same in space. If the average distance from the Earth to the Sun is 149.6 million km, and Mars is 227.9 million km, then 227.9 – 149.6 = 78.3 million km. Then you can divide it by the spacecraft’s speed in km/s and get the flight time in seconds.
But let’s take a look at the minimum time that a spacecraft spent to reach a particular celestial body. We will see that it is much more complicated than that, because the record flight to Mercury was slightly longer than the one to Mars, although the average distance to it is slightly shorter. And the only flight to Vesta took much longer than the record-breaking flight to Jupiter. What does the matter? Let’s figure it out together!
Movement in orbits
The first thing to understand is that, unlike cities on Earth, planets do not stand still, but move in orbits. An orbit has a near point (pericenter), a far point (apocenter), and an eccentricity, a characteristic that determines its elongation. That is, even the distance of a celestial body from the center of rotation is constantly changing according to a rather complex law.
In the case of the Moon, things are not so complicated, since only its motion needs to be taken into account, and the Earth is considered stationary. Therefore, everything here is relatively simple to a first approximation: the flight distance varies from 362 to 405 thousand kilometers.
The case of other large and dwarf planets is much more complicated, as both they and the Earth simultaneously move in their orbits, each with its speed, aphelion, perihelion, eccentricity, and even inclination to the ecliptic.
At any given time, the distance between two bodies is different, and their position in the coordinate system tied to the Sun is constantly changing. That is, when launching a spacecraft to a planet, one must remember that it should meet it not at the point where it is now, but at some other point where it will be in a certain time.
The distances between the planets and their orbital velocities are large enough to be taken into account even in the first, most crude calculations of flight times. In fact, at this level of understanding, things are not so bad because there is a minimum distance between the Earth and a planet. For example, between the Earth and Mars, it varies from 55.76 to 401 million kilometers, and no matter what time you take, it will remain within these limits, no more and no less.
Forces in space
It would seem that, based on these estimates, it is possible to determine at least the range within which the flight time between planets varies. Just divide the two values by the speed of the spacecraft. And this would be a mistake again. After all, as soon as we think about how fast a spacecraft can move, we go beyond the simple “path – speed – time” scheme and move to the concept of acceleration, and therefore to the force that this acceleration provides.
On Earth, we can ignore them for such simple calculations. A car, a train, or even an airplane moves every second, pushing off from something in the external environment: the road, the rails, or the air. Each time, they have to exert a certain amount of force to overcome the effects of gravity and friction and provide acceleration to balance the braking.
There is no environment in space. So the only way to accelerate there is in a much more costly way: by ejecting a certain mass from the engine at a certain speed. But the absence of friction also means that if a spacecraft has gained a certain speed, then, unlike a train or airplane, it will practically not lose it for days, weeks, or even years of flight.
The problem is that, having reached the target, you need to not just fly past it, but enter its orbit. To do this, you need to slow down. And this will have to be done in the same way as during acceleration – by shedding mass using a rocket engine. This, in turn, requires significant energy and resources.
Moreover, all this mass, which at the end of the trajectory will simply be thrown into space for the sake of braking, must first be accelerated along with the payload of the vehicle. Our ability to accelerate spacecraft is quite limited. In particular, all the records for the speed of flight between planets shown in the infographic, except for the Dawn spacecraft that explored Vesta, were set by flyby missions that did not have to brake near the celestial body itself.
In addition, there is no universal “spacecraft speed”. It is determined individually each time, depending on the specific mission. Even the usual notion that a spacecraft first accelerates, then flies straight to the point of contact with another celestial body, and slows down there is rather conventional and simplistic.
Because all this time, the main force that operates in the Solar System and holds it together in general, the Sun’s gravity, was not taken into account. It also affects the spacecraft itself, giving it an acceleration towards our star, and here everything becomes quite complicated, because depending on the specific trajectory, it can accelerate, slow down the spacecraft, or change its flight direction.
Therefore, in reality, the trajectories of spacecraft do not resemble straight lines, but curves, and in the case of long-term flights, even spirals. If the probe leaves the Earth’s gravity at a speed of less than 16.6 km/s, it is currently in its near-solar orbit, which simply intersects at a certain point with the target’s orbit. And if it does not slow down at this point, it will continue to circle our star.
This explains some of the strange things associated with the speed records for flights between planets. For example, to reach Mercury, a spacecraft essentially needs to decelerate very strongly relative to the Sun and first enter a comet-like orbit, and then decelerate again to reduce its aphelion. This is exactly what the Mariner 10 probe did in 1973-74, and it required incredible fuel consumption for such a small spacecraft.
That is why this approach is no longer used. In space missions, it is more important to deliver more payloads to the planet’s orbit than to reach it as quickly as possible. Therefore, modern spacecraft use complex, extended trajectories that allow precise calculations and gravity maneuvers to direct the vehicle in the right direction with minimal fuel consumption.
Gravity maneuvers and aerodynamic braking
The main trick used by engineers to make spacecraft travel more lively is the gravity maneuver. If the gravity of a celestial body can give a vehicle acceleration on its own, why not use it to change the trajectory without significant fuel consumption?
To do this, it is only necessary that the flight path passes close to the celestial body and is calculated so that the resulting acceleration does not lead to a collision with it, but to the desired change in the velocity vector. If necessary, the gravity maneuver can be repeated several times, and its effect can be enhanced by turning on the engine at the right time, which can further reduce travel time. The Voyager-2 spacecraft, for example, did this four times before leaving the Solar System.
Another trick is called aerodynamic braking. It hasn’t been used that often so far, but it is seen as very promising. Its essence lies in the fact that it is possible to greatly reduce the fuel consumption for braking when entering the planet’s orbit or to get rid of it altogether. To do this, it is only necessary to fly tangentially into the outer layers of the planet’s atmosphere, and the friction force, which was so lacking in space, will slow down the vehicle. However, it should be remembered that the spacecraft is affected as if it were being melted, filed, and crushed at the same time.
It is necessary to ensure the structural strength of the vehicle and equip it with a heat shield. Even so, there is still a chance that some minor defect could manifest itself, and the ship could be destroyed. Nevertheless, this method may well become commonplace in the future, especially for unmanned probes.
Is it possible to fly faster?
Due to all these difficulties, launches from Earth to other celestial bodies almost always take place at clearly defined periods when the relative position of the planets makes the flight as efficient as possible. These periods are called launch windows. For example, for a flight to Mars, the launch window opens approximately once every two years.
However, even if we use launch windows, the travel time to the inner planets will be measured in months, and to Jupiter, Saturn, or Uranus, in years. The use of gravity maneuvers and aerodynamic braking can greatly reduce the time of travel, but not more than two or three times.
So, what needs to change to significantly reduce travel time between planets in the Solar System?
First of all, the efficiency of the acceleration and deceleration of spacecraft must be significantly improved. This efficiency is determined by how much acceleration the spacecraft receives when a unit mass of propellant is ejected through the nozzle of a jet engine. This parameter is called specific impulse. For chemical jet engines, it is relatively small, which significantly limits their effectiveness in interplanetary flights.
Ion engines, which were used, for example, on the Dawn probe, are much more efficient. Magnetoplasma engines should be even better, followed by nuclear and fusion engines. How much their specific impulse can be increased remains a matter of debate. It may be tens or hundreds of times. And in this case, everything can change significantly.
After all, the complexity of all modern orbital dynamics is largely determined by the fact that the speeds at which spacecraft fly are still within the third space velocity or comparable to it. And this means the same 16.6 km/s for the Earth. If they are much higher than that – 100, 200, or 300 km/s – the rules of the game will change significantly, because the total change in speed that the spacecraft receives during the flight will be very small and can be taken into account less.
The launch windows will remain, but they will become much wider. The trajectories of the vehicles will become straighter and straighter as the speed increases, and at some point, the situation will come close to that simplified scheme where there are simply three flight phases: acceleration, free flight, and braking.
In this case, the flight to the inner planets will take days and weeks, and the journey to the outer planets will take weeks and months. It would seem that this is also a huge amount of time compared to traveling around the Earth, when we are used to the fact that it takes less than a day to get anywhere in the world by air.
However, if we want to explore space, we have to accept it as it is. And it is incredibly big. So much so that even light takes several hours to travel to the limits of the Solar System.
