SpaceX has announced it’s Interplanetary Transport System (ITS), with the goal of sending humans to Mars. While there remains many questions about how such a mission will be achieved, one thing that’s very clear is that the ITS will be the biggest rocket ever constructed. It has to be. Basic physics requires it.
The ITS is designed to have more than 13 million Newtons of thrust at sea level, compared to the 3.5 million Newtons of the Saturn V rockets used to send Americans to the Moon. All this while having only about 10% heavier. Such a big increase in thrust vs weight is necessary, because it determines not only how much mass you can lift into Earth orbit, but whether you can get that mass all the way to Mars.
It all comes down to delta-V, or how much you can change the velocity of your rocket. When it comes to reaching Earth orbit, bigger is better. The SpaceX ITS should be capable of lifting up to 550 tonnes of payload into low Earth orbit, compared to the 140 tonnes of the Saturn V. This is necessary because a trip to Mars isn’t a few-day trip to the Moon. It will require a larger crew and significantly more food and resources.
Once in Earth orbit, getting to Mars will require even more rocket power to overcome what is known as delta-V. This is the amount of speed a spacecraft needs to gain or lose to reach your destination. It takes much more delta-v to reach the surface of Mars than it does the surface of the Moon. To reach Mars you not only have to overcome Earth’s gravity, you have to overcome the Sun’s pull as you travel toward Mars. You also have to account for the fact that the orbital speed of Mars is slower than the orbital speed of Earth. Finally you have to overcome the gravity of Mars to land softly on its surface. All of this adds to the total amount of needed delta-V. To meet this need the SpaceX plans to refuel the ITS in Earth orbit with a second launch.
There are ways to minimize your delta-V requirements for an interplanetary mission. One way is to make a close flyby of a different planet. Basically, if you approach a planet in the direction of its orbit (coming up from behind, if you will), then the gravity between the planet and your spacecraft will cause the spacecraft to speed up at the cost of slowing down the planet by a tiny, tiny amount. Making a flyby in the opposite direction can cause your spacecraft to slow down. This costs you nothing in terms of fuel, but takes time because you need to orbit the Sun in just the right way. It’s a common trick used for robotic spacecraft, where we use a flyby of Earth to reach Mars or Jupiter, or a flyby of Jupiter to reach the outer solar system.
Flybys are cheap and easy for space probes, but they can add years to the time it takes to reach your destination. That’s a big problem for a crewed mission. So the alternative is to look at optimized orbital trajectories. For example, about every two years the positions of Earth and Mars are ideally suited so that a trip needs much less delta-V. This was actually discovered in 1925 by Walter Hohmann, who proposed a trajectory now known as the Hohmann transfer orbit. You could, for example, build a large spacecraft in such an orbit and use it as a shuttle between Earth and Mars. Such an idea was used in the book and movie The Martian.
There are other useful tricks, such as using a planets atmosphere to “aerobrake” a spacecraft, significantly reducing its delta-v once it reaches the planet. Since both Earth and Mars have atmospheres this can be used for landing spacecraft. You can also modify the flyby method by thrusting your spacecraft just as it makes its closest approach, in what is known as an Oberth maneuver (another trick used in The Martian). But these will only take you so far. To reach the surface of Mars in a reasonable time, any rocket will require more delta-v than we’ve ever had, which is why the ITS has to be so big.
The one up-side of all this is that once SpaceX, Blue Origin, or NASA builds a rocket with enough power to send humans to Mars, lots of other destinations open up as well. The delta-V requirements to reach the asteroids, Jupiter or Saturn aren’t significantly different. If we can land on Mars, we can reach the moons of Jupiter, or even start mining asteroids.
Mars is not only an awesome destination, it is also a gateway to the solar system.