Gravity, What a Hold You Have on Us!
Why Planetary Mass Determines the Limits of Space Exploration
In science fiction novels, we visit other planets without ever worrying about their mass. We imagine that we could land on super-Earths as naturally as on a twin of our own world, or even on Mars. The implicit assumption is often “the bigger, the better”, as if a planet’s size were a guarantee of wealth or at least Earth-like living conditions, and that this would be enough to make it not only tempting but easy to go there.
Physical reality tells a very different story. Super-Earths are mirages or decoys — worlds where landing would not only be impossible under current conditions, but from which we could never leave. Planets with the same mass as Earth, “Earth twins”, also present insurmountable return problems. In fact, only planets less massive than Earth represent realistic targets for human exploration — and among them, only one is within our reach today: Mars.
All of this comes down to a single factor (apart from the time factor): gravity, which depends directly on the planet’s mass. It is gravity that determines the escape velocity — the minimum speed a spacecraft must reach to break free from a celestial body’s gravitational pull.
Escape velocity, the barrier that can be crossed…or not
Escape velocity is the minimum speed required for an object to permanently escape the gravitational pull of a celestial body. It is calculated by the formula:
v_lib = √(2GM/R)
where G is the gravitational constant, M the planet’s mass and R its radius. The more massive a planet, the higher its escape velocity — and the harder and more energy-costly it becomes to leave.
| Body | Masse (M⊕) | g (m/s²) | Escape velocity | Weight 80 kg | Spacesuit |
| Mars | 0,107 | 3,72 | 5,03 km/s | 30 kg | 48 kg ✅ |
| Terre | 1,000 | 9,81 | 11,2 km/s | 80 kg | 130 kg ⚠️ |
| Super-Terre | 1,500 | 12,4 | 13,4 km/s | 101 kg | 164 kg ❌ |
Table 1 — Comparison of gravity conditions and their practical consequences
The table above illustrates the problem. On a Super-Earth of 1.5 Earth masses (to take an example close to the mass of the Earth), an 80 kg astronaut would feel a weight of 101 kg. His EVA spacesuit, which already weighs 130 kg on Earth, would weigh 164 kg and be impossible to carry. On Mars, by contrast, the same astronaut would feel only a weight of 30 kg and his spacesuit would weigh 48 kg: full mobility would be preserved.
The Super-Earth, an inaccessible world
1. Landing, an impossible deceleration
L’EDL (Entry, Descent, Landing) on a Super-Earth would face a twofold insurmountable constraint. On one hand, the kinetic energy to be dissipated during atmospheric entry being proportional to the square of the entry speed, the heat flux on the shield at 13.4 km/s (versus 11.2 km/s for Earth) would be approximately 1,4 times more intense. And we know from the Starship test flights that we already are at the limit of what is possible to bear. With this higher heat flux we would go beyond the current material strength capabilities. On the other hand, the powerful deceleration experienced by the crew could reach lethal values.
In practice, a Super-Earth could only be studied from afar, following an aerobraking orbital capture or a propulsive braking (use of engines at perigee to reduce orbital energy) — as with the Juno probe around Jupiter or Cassini around Saturn. A human presence in orbit would be conceivable, but it could not be considered on the surface.
2. Liftoff, a physical impossibility
Even assuming a successful landing, escaping the gravitational pull would be out of reach with current technologies. The comparison of Δv required for liftoff is telling:
| Corps | Δv for liftoff | Propellants (~Starship) | Feasibility |
| Mars | ~4,1 km/s | ~300 t (ISRU) | ✅ Feasible |
| Terre | ~9,4 km/s | ~4,600 t (~3,400 t SuperHeavy + ~1,200 t Starship) | ⚠️ Requires launcher + ~4,600 t |
| Super-Terre | ~12,0 km/s | > 30,000 t | ❌ Out of reach |
Table 2 — Energy required for liftoff by planet
To leave a Super-Earth, one would need to carry more than 30,000 tonnes of propellant — approximately seven times the total mass of the complete Starship/SuperHeavy system (~4,600 t). No known or currently developed launch vehicle comes anywhere close.
NB : Important note on the “impossible landing”
Technically, the word “impossible” deserves a qualifier: under favorable atmospheric conditions, a robotic landing or one using very powerful propulsive braking could theoretically be attempted. It is first of all the return journey that is impossible; with current technologies. This is what, anyway, makes a crewed mission a one-way trap.
The Earth-Twin, limitation by the launcher
A planet of identical mass to Earth raises a different but equally fundamental problem: that of the return launch vehicle. To leave Earth, Starship relies on SuperHeavy, a 70-metre first stage consuming approximately 3,400 tonnes of propellants (liquid methane and liquid oxygen) at liftoff. This booster, once its job is done, falls back to the planet of departure.
During the first missions to an Earth-twin, no launch vehicle would be available on site. One would have to be manufactured locally — which means: extracting raw materials, developing a metallurgical industry, producing and storing propellants in very large quantities (the 3,400 tonnes, of the SuperHeavy plus 1200 tonnes for the Starship), and keeping those cryogenic propellants stable for several years in an unknown environment.
These constraints make human exploration of an Earth-twin out of reach for the coming decades, even if it remains theoretically conceivable in the very long term.
4. The less massive planets, an accessible opportunity
Planets with less than half the mass of Earth — Mars being the archetype — are the only category for which human exploration is feasible in the near future. Several factors converge:
Liftoff without a dedicated launch vehicle: the low escape velocity (~5 km/s) allows the spacecraft’s own engines to do the job, with no additional first stage.
Realistic ISRU: on Mars, methane/oxygen propellant production via electrolysis of atmospheric CO₂ and subsurface water has been technically demonstrated. ~300 tonnes suffice for the return trip, produced over 18 months by a portable nuclear reactor.
Tolerable physiological conditions: at 3.72 m/s², Martian gravity (~38% of Earth’s) is demanding but not incapacitating for the human being. Spacesuits remain wearable, and blood circulation is not compromised.
5. Time and radiation constraint
Beyond gravity, a second major physical constraint (linked to time) limits our reach: space radiation. Outside Earth’s magnetosphere, two sources threaten astronauts:
SePs (Solar energetic Particles):particles emitted during solar flares, dangerous but partly predictable and reducible with adequate shielding (hydrogen rich).
GCRs (Galactic Cosmic Rays): galactic cosmic radiation, and in particular HZE nuclei (high-energy heavy atomic nuclei) that penetrate all known shielding and generate secondary gamma rays through spallation on the spacecraft walls.
NB: Radiation doses by destination
Mars (7-month transit): up to ~300 mSv in a single leg — half the NASA “career limit” (600 mSv under the ALARA principle), not counting the surface stay and return. Fortunately, this dose could be reduced by improving SeP shielding, for example by placing water reserves or food supplies around the astronauts’ sleeping quarters. But GCRs HZE will keep crossing all known shielding.
Jupiter moons (2-year transit): ~1,000 mSv — serious cancer risk, fatal in the long term. Saturn/Titan (6–7-year transit): dose incompatible with survival.
These figures make any human mission beyond the asteroid belt impossible with current shielding technologies.
The combination of these two constraints — radiation and travel time — successively eliminates all other conceivable targets:
Venus: too close to the Sun (intense SeP flux), atmospheric pressure of 90 bar at the surface, temperature of 450°C — hostile to any form of human surface exploration.
Mercury: even closer to the Sun, with no protective atmosphere and extreme temperatures.
Jupiter’s moons and beyond:distances incompatible with radiation dose limits, regardless of any other criterion.
Conclusion: Mars the only target within our reach
The analysis of gravitational and radiation constraints leads to a clear conclusion: Mars is not merely the default target — it is the only truly viable target. Its mass, precisely in the window that makes liftoff feasible without a dedicated launch vehicle, its distance compatible with dose limits, and the availability of water and CO₂ for local propellant production make it a unique opportunity.
Gravity, often perceived as an abstract constraint, is in reality the fundamental discriminating factor of human solar system exploration. Too strong, and it traps you. Too weak (like the Moon), and it fails to retain a sufficient atmosphere. It is in the narrow window that Mars represents — at 38% of Earth’s gravity — that the key to humans’ next step in space is offered.
The Moon, accessible at any time and at near-instantaneous communication distance (almost no time lag), holds a special place in this analysis: it is an integral part of our system and does not really belong to Deep Space. It is the stepping stone to its exploration but it would be very difficult to live on. Mars is THE destination.
Title illustration: The three types of planets: super-Earths with 1.5 times the mass of Earth, Earth-Twins, and Mars. They are shown here to scale. It can be deduced that both other-Earths and super-Earths are decoys.
Methodological note:
The figures in this document are derived from orbital mechanics calculations (vis-viva equation – at every point in an orbit, the sum of kinetic and potential energies is constant -, Tsiolkovsky rocket equation) and calibrated against real mission data (NASA, ESA, SpaceX). Radiation dose values are taken from measurements by the RAD instrument aboard Curiosity (2012–2013).
Copyright Pierre Brisson
Computation have been made with the help of claude.ai
