Landing Starship on Mars

SCANALYZER Tracking with Closeups
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This week, instead of the usual summary of space news, Marcus House takes a deep dive—alongside SpaceX’s Starship—into the atmosphere of Mars, exploring the issues involved in landing Starship on the Martian surface.

Mars is one of the most difficult places to soft land in the solar system. It has an atmosphere, which means any spacecraft arriving at orbital or interplanetary velocity must have a thermal protection system (heat shield) to dissipate kinetic energy, but the atmosphere is less than 1% as dense as Earth’s and, although it does permit scrubbing off more than 99% of the entry velocity, it’s insufficient to allow parachutes to brake objects to a survivable velocity. This requires propulsive (rocket-assisted) landing for final descent, which raises issues with interaction between rocket plumes and the dusty Martian surface.

All of this makes landing SpaceX’s Starship on the Martian surface a very different matter than on Earth, where the atmosphere can be made to do most of the work via the “belly flop” maneuver, or on the Moon, where the lack of an atmosphere and gravity only 1/6 of Earth’s makes a pure propulsive landing of a ship that doesn’t require a heat shield relatively straightforward.

Crunching the numbers seems to indicate Mars-bound Starships will require modifications to meet the challenges of landing. SpaceX’s rapid development cycle and flexible assembly techniques may allow tailoring Starship to the mission for which it is intended.

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As a devotee of Gerard K. O’Neill, I am still unclear as to why Musk is so intent on a Mars project rather than an asteroid project - mining and/or establishing a permanent base without a gravity well to overcome. The economic and future spacefaring value of such an undertaking is very large. The only difference I can see is the public relations value of landing on a planet (for little return). Given the mountains of media ‘disinformation’ about the importance of landing on an uninhabitable planet which lacks the essential spacefaring resources of many accessible asteroids, I just don’t get it. I would have thought Musk was more savvy.

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I have not heard Musk address this directly, but if I had to make the case for Mars it would be based upon his goal of making human civilisation multiplanetary as soon as possible. A planet like Mars (probably) has all of the primary resources needed to make a settlement self-sufficient, so that if the ships from Earth ceased to come, it could carry on humanity’s endowment to populate the solar system and bring the galaxy to life.

Space colonies and asteroid exploitation that can operate entirely independent of Earth will require the development of a full-fledged inner solar system spacefaring economy, exploiting resources from the Moon to those in the main asteroid belt. This will take a great deal of time and investment, and will be fragile until it is running entirely independent of Earth. Humans already know how to establish pioneer settlements in resource-rich but hostile environments, and that’s what Mars is, basically.

But once you have the infrastructure to build a city on Mars, you automatically have the capability to build O’Neill colonies from lunar and asteroid material, so there’s no reason both approaches can’t compete and, ideally, both succeed at what they do best.

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Another advantage of Mars as an initial settlement destination that occurred to me after I posted the previous comment is that on the Martian surface, with access to water ice and an atmosphere which is 90% CO₂, all you need is energy input (solar or nuclear) to manufacture as much rocket fuel (liquid oxygen and methane) as you wish. These happen to be the propellants used by Starship and, for that matter, Blue Origin’s New Glenn. They could be used not only to launch Starship from Mars (the weaker gravity means there would be no need for a Super Heavy booster—Starship could achieve orbit in one stage) but to establish a propellant depot in orbit around Mars or on one of the Martian moons. This would be an ideal “gas station” for expeditions to explore and exploit resources in the main asteroid belt, as the delta-v to get into and out of Martian orbit is small compared to landing on the surface.

The Martian atmosphere is 2.8% nitrogen, which means that element is easily available for “free”, and can be used along with oxygen extracted from the CO₂ for breathing air for habitats, and via the Haber-Bosch process with hydrogen from water ice to make fertiliser for agriculture.

We now know that all of these elements are available in some inner solar system asteroids, but they’re going to be harder to find and more work to extract than on Mars, where they’re essentially just sitting there waiting to be used.

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As regards changing orbits of objects like asteroids, I have a basic absence of understand as to the direction of the forces which must be applied. Let’s say there is a juicy asteroid in the belt beyond Mars which we want to move to a near-Earth orbit. I assume (maybe incorrectly?) the asteroids’ speed of revolution around the Sun is slower than the orbital velocity of Earth.

Intuitively, it seems that slowing it would allow the Sun’s gravity to overcome the centripetal force of its velocity and cause it to move closer to the Sun. On the other hand, an orbit nearer the Sun requires a higher velocity to balance the greater gravitational centrifugal force - so it must have a higher orbital velocity; but if one simply accelerates it from its present orbit, would it not move to an orbit farther from the Sun?

So, in my confusion, I ask in what direction must a force be exerted on the object to dislodge it from its present orbit with the result that it assumes an orbit similar to Earth’s? Does a single application of the correct force suffice, or would it require more than one application of force and in what direction. (I suppose this is a heuristic suggesting purchase and use of Kerbal Space Program, but I lack time to “play”).

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As is often the case in engineering, the easiest way to think about this matter is in terms of energy. Start with an asteroid in a circular orbit outside the orbit of Mars. It has two relevant forms of energy: kinetic energy from its motion along its orbit and potential energy due to its distance from the Sun in the Sun’s gravitational field. The kinetic energy increases as the square of the velocity, while the potential energy increases with the distance from the Sun (actually, the latter is a bit more complicated because the Sun’s gravity falls off with distance, but we can ignore this for small changes in distance). Since energy is conserved, the sum of these two energies will be absolutely constant as the asteroid orbits the Sun.

Now attach a rocket to the asteroid and fire it in the direction opposite its motion in orbit. This will reduce the kinetic energy of the asteroid in its orbit. Now, to maintain the energy balance, when it reaches the other side of the orbit (opposite from the impulse imparted by the rocket), it will be closer to the Sun, since it has lost the kinetic energy it needed to balance the potential energy at a greater distance. It will now be in an elliptical orbit with aphelion outside the orbit of Mars and perihelion touching Earth’s orbit. As it approaches the Sun in each orbit, its speed increases so the kinetic energy makes up for the decrease in potential energy from being closer to the Sun, according to Kepler’s equal area rule. (From conservation of energy and a little calculus, this rule can be deduced straightforwardly.)

Wait until the asteroid reaches perihelion at Earth’s orbit, then fire the rocket again opposing the direction of orbital motion. This will further reduce kinetic energy, which means that when it next reaches aphelion, it must have less potential energy to compensate, so the aphelion will shrink. If the impulse imparted by the rocket is correctly computed, the asteroid will now be in a circular orbit with the same radius as Earth’s.

I’ve assumed here that the asteroid and Earth are orbiting in the same plane (the ecliptic) and that we don’t care about rendezvousing with the Earth (arriving at the location of the Earth at the same time). These are additional complexities, but the same principles apply.

In short, to raise an orbit at the point opposite to the burn, fire the rocket in the direction of orbital motion. To lower it at the opposite point, fire opposed to orbital motion. To change from one circular orbit to another in the same plane, it takes two burns, first to lower one side, then a second at periapsis to circularise at the desired radius.

As you note, there is no better way to learn this intuitively than to play Kerbal Space Program. For the gory details of how this is done in the real world, see the 1963 MIT Ph.D. thesis “Line-of-sight guidance techniques for manned orbital rendezvous” by some guy named Buzz Aldrin.

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Thank you! This is a perfectly clear explanation, comprehensible to a non-engineer.

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