When we talk about space exploration, our minds usually immediately go to the stars. To Alpha Centauri or to the center of the Milky Way. To journeys of thousands of light years, where physics, technology and imagination begin to intertwine to a special extent. But the most interesting case is closer and currently more difficult: We are talking about the manned exploration of our own Solar System, which remains, with today’s technological data, science fiction.
Not because we cannot send robotic missions to our nearby planets. We have already done this, and with impressive success. But today the model of space exploration, as far as manned missions are concerned, is based on “slow speeds”. Where to go, for example On Mars, the plan is to gradually assemble a large spacecraft in Earth orbit, with cargo delivered in stages, followed by a multi-month journey at low speed (or even multi-year if we want to go even further, e.g. to Pluto). The problem? This model works great for robotic missions, but for humans it quickly starts to run into fundamental limits.
1. There is the biological dimension. A 6–9 month trip to Mars exposes the crew to microgravity and cosmic radiation for long periods of time. This translates into bone loss, muscle atrophy, increased risk of cancer, and a general significant decline in physical fitness. As the duration increases, these problems do not increase linearly, but accumulate.
2. The exponential increase in complexity. A journey of days requires manageable reserves of supplies and relatively simple life support systems. A journey of months or years requires closed ecosystems, complete recycling, very high redundancy, and the ability to repair any potential failure without external assistance. The system doesn’t just get bigger. It gets qualitatively different.
3. Reliability. As the mission time increases, the probability of failure increases dramatically. In missions lasting many months or years, even small failures can become catastrophic if there is no immediate return or support available.
4. Energy and operational efficiency. Transporting large masses in multiple stages, with repeated launches and assembly in orbit, is technically feasible but extremely complex and expensive. “Slow” is not necessarily easier. It is often just differently difficult.
So we need to shift to the pursuit of high accelerations for human space travel. And this is not driven by a need to impress, but by an effort to move exploration from the level of multi-year survival to the level of manageable human-scale travel. In other words, “fast” is not just more comfortable. It is what, in many cases, makes manned exploration practically feasible.
A thought experiment
So let’s do a thought experiment. Suppose we have a spacecraft capable of continuously accelerating at 1g, that is, with an acceleration equal to that felt on the surface of the Earth. For the crew, this would be almost ideal. The spacecraft would have artificial gravity without rotating rings, without huge centrifugal structures, and without the serious problems of long-term stay in microgravity.
The scenario is simple: accelerate at 1g halfway, then reverse so that the spacecraft decelerates at 1g until it reaches its destination at zero relative velocity.
With such an acceleration-deceleration scenario, the Moon would be about three and a half hours away. Mars, depending on its relative position to Earth, would be about 2 to 5 days away. Jupiter about 6 days. Saturn about 8 days. Neptune about 15 to 16 days. Even the heliopause, the outer region where the solar wind ceases to dominate the interstellar medium, would be about a month away.
This alone shows how misleading our everyday sense of space is. The distances are vast, but they are not the ones that make human exploration nearly impossible. The deeper problem is that we can’t accelerate at 1g all the time. And we’re not even close.
How heavy would such a spacecraft have to be?
In a first, oversimplified approach, one might assume a 100-ton spacecraft. But this is probably a very optimistic number for a fully manned 4-astronaut spacecraft with serious safety requirements.
For comparison, NASA’s Orion is designed for a 4-person crew and missions of up to 21 days, with a total launch mass of about 78,000 pounds, or about 35 tons. However, it is not a 1g continuous propulsion spacecraft. The International Space Station has a mass of about 420 tons and a habitable volume of 388 m³, but it does not have a high-power propulsion system for interplanetary travel.
So, for a real 1g spacecraft, the mass cannot be calculated as if we were just talking about a “four-person capsule”. It must include habitable space, life support, shielding, thermal management, propulsion, power, reserves, and possibly submersibles or approach systems.
| Subsystem | Optimistic mass estimate (t) | Realistic mass estimate (t) | Explanation |
| Habitable space / crew cabin | 40 t | 80–150 t | For 4 people, not just a return capsule, but a real living environment |
| Life support, air/water recycling | 20 t | 40–100 t | ncludes redundancy and failover capability |
| Food, water, consumables, medical supplies | 10 t | 30–100 t | Depends on shipment duration and level of recycling |
| Radiation shielding | 50 t | 100–300 t | Especially for solar particles and cosmic radiation |
| High-velocity particle shielding | 50 t | 100–500 t | At speeds of thousands of km/s, even small particles become dangerous |
| Structure, tanks, connections, mechanical systems | 50 t | 100–300 t | The main body of the vehicle |
| Navigation, communications, control, electronics | 10 t | 20–50 t | With high redundancy |
| Thermal management / radiant coolers | 50 t | 100–500+ t | Depends dramatically on power and losses |
| Power and propulsion system, without propellant | 200 t | 500–2,000+ t | Here lies the great unknown |
| Propellant / reaction mass Unknown | Unknown | Unknown – possibly huge | Depends on propulsion technology. With chemical propulsion it is practically impossible |
| Total, without full inclusion of promotional tool | 480 t | 1,170–4,000+ t | Remains generously optimistic |
So 100 tons is only useful as an extremely optimistic example. A more serious thought experiment would have to use at least 500 to 1,000 tons, while a truly operational vessel could easily be in the several thousand ton range, especially if propulsion, subs, reserves, and full operational safety are included.
The duration of journeys and their required energy
Let’s take a month-long trip as a reference: 15 days of acceleration at 1g and 15 days of deceleration at 1g. After the first 15 days, the spacecraft will have reached about 12,700 km/s, or about 4.2% of the speed of light. This sounds small compared to the speed of light, but for human technology it is extreme.
The ideal energy required for a complete acceleration and deceleration cycle is about 1.62 × 10¹⁴ joules per kilogram of spacecraft, or about 45 million kWh per kilogram. Applying this figure to various possible spacecraft masses, we get:
| Spacecraft mass (t) | Total ideal energy (J) | In electrical energy (TWh) | Average power in acceleration phase (TW) | Comparison with current global electricity generation |
| 100 | 1.62 × 10¹⁹ | ~4,500 | ~6.2 | ~14% of today’s annual |
| 500 | 8.1 × 10¹⁹ | ~22,500 | ~31 | ~71% of today’s annual |
| 1,000 | 1.62 × 10²⁰ | ~45,000 | ~62 | ~1.4 times the current annual |
| 4,000 | 6.48 × 10²⁰ | ~180,000 | ~250 | ~5.7 times the current annual |
For comparison, global electricity production is in the tens of thousands of TWh per year. According to Ember, renewables produced 10,730 TWh in 2025, accounting for 33.8% of global electricity generation. This equates to a total global production of around 31,700 TWh.
That means a 100-ton vessel, even in the ideal case, would require energy equivalent to around 14% of global annual electricity generation. A 1,000-ton vessel would require around 45,000 TWh, more than current global annual electricity generation.
And that’s before we factor in actual losses, engine efficiency, propellant mass, heat, cooling requirements, shielding and reserves. This is the point where the problem ceases to be merely difficult and becomes almost colossal in scale.
Why Today’s Technologies Are Not Enough
Chemical rockets are impressive feats of engineering. But they have a fundamental limitation: low specific thrust. They can deliver huge thrust for a short period of time, but not sustained 1g acceleration for days or weeks.
Electric propulsion, such as ion engines, is much more efficient in terms of propellant use. But it has extremely low thrust. It is ideal for long-duration robotic missions. It is not suitable for accelerating a manned craft of hundreds or thousands of tons through 1g. To accelerate a 1,000-ton craft through 1g, a force of F = m · a is required. That is 1,000,000 kg × 9.81 m/s² = about 9.81 MN. Not for minutes. For days. And then the same again, for deceleration.
Nuclear thermal propulsion could be better than chemical propulsion. Nuclear electric propulsion could help with long-duration missions. But none of these technologies, as we have them or plan them today, come close to the 1g continuous scenario for a large manned craft.
Fusion is perhaps the first technological idea that starts to look realistic. But we don’t yet have a working, net positive energy fusion reactor on Earth. Let alone a compact, lightweight, and reliable fusion system for space propulsion, with a high power-to-mass ratio, thermal management, shielding, and the ability to operate continuously.
There are also more exotic ideas, such as propulsion with an external power supply via laser or microwave. This would partially obviate the need for the craft to carry all the energy on board. However, it creates other huge problems: Required power of many TW, gigantic broadcasting facilities, extreme targeting accuracy, thermal resistance of the vessel and, above all, the issue of deceleration at the destination.
Space is not a vacuum
There is also the issue of shielding. At speeds of thousands of km/s, even tiny dust particles become dangerous. You don’t need to collide with anything big. At such speeds, the kinetic energy of even a very small mass is enormous. Space is very thin, but it is not a complete vacuum. For slow or moderate missions, this is manageable. For spacecraft moving at thousands of km/s, it becomes a design nightmare.
And then there is heat. Every real propulsion system has losses. Every loss becomes heat. In space, there is no air to remove heat by convection. Heat rejection is mainly by radiation, i.e. with large surfaces and strict thermal constraints. So the greater the power of the craft, the greater the cooling problem becomes.
Conclusion
Theoretically, if we could accelerate continuously at 1g and decelerate accordingly, the Solar System would become humanly accessible and capable of repeated and continuous trips (and not just one, for historical reasons). With such a perspective, Mars would not be a trip of months but of a few days. Jupiter and Saturn would be destinations of a week. Neptune would be closer in time than a sea cruise is today. But the technical difficulty brings us back to reality abruptly.
We do not have a propulsion system that can provide continuous 1g in a large manned spacecraft. We do not have the required power in a suitable space form. We do not have practical thermal management for such power levels. We do not have sufficient shielding for such speeds. And we do not yet have the industrial infrastructure to build, assemble, fuel and support such vehicles in space.
Today we can explore the Solar System mainly with robotic missions. And that is already a huge achievement. Voyager, the Mars rovers, Cassini, New Horizons and so many other missions are prime examples of human creativity.
But it is one thing to send a robotic craft into multi-year or multi-decadal orbit. It is another to send humans quickly, safely, survivably, with armor, with life support, with the ability to slow down and, ideally, with the ability to return. We are still a long way off.
That is why the exploration of our Solar System is a strange case. It is close on an astronomical scale, but distant on a technological scale. It is in our neighborhood, but not yet within our real capabilities. It is scientifically understandable, but mechanically inaccessible on a human scale.
The Solar System is not forbidden by the laws of physics. It is forbidden by the current energy and technological level of our civilization. And this is perhaps the most interesting conclusion.
What if we wanted to go to Alpha Centauri?
If we apply the same hypothetical flight profile, i.e. 1g acceleration until the midway point and 1g deceleration until arrival, then the nearest star system, Alpha Centauri, at a distance of about 4.37 light years, starts to look almost “accessible” from the crew’s perspective.
The crew would need about 3.6 years of their own time, while on Earth it would take about 6 years. By the midway point, the spacecraft would have reached about 95% of the speed of light! And here the picture gets extreme: the ideal energy required for a full cycle of acceleration and deceleration would be about 4.05 × 10¹⁷ joules per kilogram of spacecraft, or about 112 billion kWh per kilogram.
For a 1,000-ton ship, this equates to about 112 million TWh, thousands of times the annual global electricity production. And that remains just the ideal kinetic energy accounting, with no losses, no propellant, no thermal management, no shielding, and no actual propulsion system.
Could we do it with AGI or ASI?
So perhaps the real question is not when we will build a better rocket. It is when we will have a civilization capable of designing, building, and maintaining technologies beyond today’s human scale. At this point, Artificial General Intelligence (AGI) and even more so Artificial SuperIntelligence (ASI) inevitably enter the conversation.
Not as a magic wand, but as a potential catalyst for accelerating physics, engineering, and industry to levels we currently cannot achieve. If we ever travel to Alpha Centauri in human lifetimes, it is very likely that the spacecraft will not simply be the product of better aerospace engineering. It will be the product of a civilization enhanced by intelligence far beyond our own.
