What next for space?

Speculations on the future in space, plus a solution to the Fermi paradox

Basics of space travel and why interstellar travel will always take a long time
I start with a basic of space travel, the rocket equation:

1.        Δv = g I_SP ln(m_S/m_F)

Here Δv is the change in rocket velocity in km/s, g is the acceleration of gravity which is 0.0098 km/s², I_SP is the specific impulse in seconds, and m_S and m_F are the masses of the rocket plus propellant at the start and end of the acceleration period, respectively. Specific impulse is a property of the type of spaceship drive used.

The largest rocket humanity has ever built is SpaceX’s Starship rocket, which has a launch mass (m_L) of 5000 tons and payload (m_PL) of 150 tons. To travel a distance L, the rocket would accelerate by Δv, coast to the destination, and then decelerate by Δv. The total change in mass (m_L/m_PL) will be equal to the change in mass during acceleration multiplied by the change in mass during deceleration. These are the same so the change in mass for one is simply the square root of the total change:

2.       m_S/m_F = (m_L/m_PL)^½

 Substitution into equation 1 gives

3.       Δv = 0.0049 I_SP ln(m_L/m_PL)

Starship engines (in space) have a specific impulse of 350 seconds. With this value and the launch mass and payload values given above, equation 3 gives a Δv value of 6 km/s. A fly-by trip that uses all the fuel for the initial acceleration would achieve twice this velocity or 12 km/s. For Starship to leave the solar system it must first escape the solar system, which requires a Δv of about 16.5 km/s, which cannot be achieved with a 150-ton payload. For this article, I assume fully fueled interstellar flights will launch from the outer solar system rather than Earth, when the Δv need to leave the solar system is small and can be ignored.

With these assumptions Starship could send a 150-ton interstellar spacecraft at a speed of 6 km/s (0.020 light year/millennium) to a nearby star and then decelerate to enter orbit. The nearest star to our sun is Proxima Centauri, which is 4.25 light years distant. Such a journey would take more than 200 thousand years. The trip length could be shortened considerable by changing the mission to a fly-by with a Voyager sized payload. In this case travel time would be reduced to 42 thousand years.

This travel time can be reduced using a more advanced propulsion technology. Direct fusion drive has a specific impulse around 10000 seconds. Plugging this value into the rocket equation with the Starship values for the other parameters gives a Δv of 0.57 light years/millennium. The voyage to Proxima would take about 7500 years. The fly-by mission would take about 1500 years.

The energy produced from fusion, if directly used like the chemical energy in conventional rockets could give specific impulse as high as 130,000 seconds. This is 13 times more than the direct fusion drive and would correspondingly reduce the travel time to Proxima to 570 years (116 years for the fly-by).

Why there can be no interstellar travel and the consequences for the Fermi Paradox
We have seen that trips to the stars take a very long time. This means the people on board the ship would be living their lives on the ship. They would not be passengers on a ship traveling from one to another destination, but rather, people living their lives in a moving home. This is an important point. The concept of travel implies that it is the starting point and the destination that are the main things, not what happens in between. Travelers live their lives in one place and are going to a new place where they will live their lives. The trip itself is a disruption in their lives of relatively short duration, in which the travelers put up with a poor quality of life for the promise of the destination. But in the case of interstellar voyages, the people will have never left home, their home is simply moving through space.

This means that before any humans set out for other stars, we will first learn how to live in space in some sort of habitat such as an O’Neil cylinder. Humans who learn to do this will be space dwellers; a “cultural species” distinct from their Earth-dwelling relatives. In order to make an interstellar trip a space-dwelling people must be completely independent of planetary resources. Once such a people exist, it will be possible for them to leave our solar system for another.

But this raises the question, why would they want to do that? They are space dwellers who have no use for terrestrial planets other than Earth (for sentimental reasons). But if they were to leave the planet Earth, they would have no use for (and no reason to go to) any other planet. And this gives rise to the true Fermi paradox. Although interstellar travel would certainly be possible for a sufficiently advanced species, those can make the trip (space dwellers) have no interest in planets and no reason to do so. Conversely, those interested in going (planet dwellers) aren’t space dwellers and can’t go.

Future directions for space utilization
Elon Musk wants to colonize Mars. Colonizing Mars would require a lot of infrastructure, some of which would have to be invented. This means a lot of real capital (both physical and intellectual) would be needed. To acquire this capital, he founded SpaceX. Its initial goal was to develop a low-cost launch capability that could be used for putting commercial satellites and government payloads into orbit. SpaceX has launched and landed over 250 Falcon 9 boosters over the last five years, taking many payloads into space. In principle, a similar launch rate should be achievable with Starship.

Such a launch rate with Starship could loft more than a dozen International Space Stations into low earth orbit (LEO) each year. Rather large orbital constructs become possible with this kind of lift capacity. Also possible is sending large payloads to the moon or Mars. Saturn V put into LEO a two-man moon lander, 3-man return capsule and the booster to get them to the moon and back. A series of Starship launches could put a large ship, lander, and a very large (empty) booster into space. Additional launches could lift fuel and oxidizer to fill the booster. This trip could be preceded by a series of robot spacecraft sent to Mars to land equipment and supplies required for an extended stay by those coming on the manned flight. Many Starship launches would be necessary to put all this into space, which is why a Starship capability analogous to that of Falcon 9 is necessary.

Moving from a satellite-launching service to interplanetary travel strikes me as too big a step. The revenue from serving an existing market for satellite launches generated the cash flow required to create the Falcon 9 launch capability SpaceX possesses. The cash flow to fund the development of Starship will come from revenue from Starlink, their high-speed satellite internet service. In order to finance the next stage of space-faring evolution, a third market is needed to generate the necessary cash flow. I see no Mars-related market that can generate the cash flow needed to fund the technology development and physical capital accumulation needed to establish a permanent Mars colony.

Space settlements as a starting point
A space hotel would seem a more reasonable intermediate goal. It would start small, offering relatively spartan accommodations for adventurous moguls willing to pay a large sum to spend a week in space. Assuming the business model works the cash flow generated from early adopters would be used to enlarge the hotel adding additional amenities such as regions with artificial gravity for restaurants and zero-g regions for space sports. Construction would lead to more efficient space construction methods and possibly lunar mining of construction materials. As the hotel becomes able to handle more guests, the price would come down, increasing the size of the customer base. And as the size of the facility increases more amusements can be added to increase the allure of space vacations once the novelty wears off. The growth of the space tourism business would proceed along the S-curve typical of new product introduction.

To the Moon
The growth of such a leading sector would see the development of new space capabilities to exploit new market niches that would become possible once a large space presence has been established in earth orbit. Excursions to the moon from the orbital hotel/spaceport might be followed by moon resorts featuring large gyms in which lunar versions of terrestrial sports would be played in “good weather” (when radiation from solar flares was not present).

On to Mars
Colonization of Mars would be a natural extension of this trend. It is different in that the development of such a colony would have to make use of Martian resources and funded by cash flow from Martian business enterprise. The beginnings of a space tourism industry in Earth orbit could probably be built today once Starship comes online. Assuming use of lunar materials for expansion of orbital facilities makes financial sense, the capability for operating on the moon would exist once the orbital tourist business is becoming mature, creating the opening for lunar tourism. Such a development would create the technology needed for space mining, refining and construction that would be transferable to a Martian settlement.

Such a development is necessary to make realistic assessments of the economics of Martian settlements. Given some Martian enterprise that can generate the necessary cash flow, colonization of Mars can commence. This enterprise does not have to be a business serving Earth-based customers like space and lunar tourism. It could be the desire of people to live free from political restrictions. The Pilgrim settlement at Plymouth, unlike the Jamestown colony, was not a business proposition. They simply wanted to live their lives free of religious interference. They were willing to pay for the opportunity to live subsistence-level lifestyles in a place where they would be free to practice their religion.

Similarly, wealthy libertarians desiring to “live free or die” (on Mars) could invest their Earthly fortunes into the Martian colony project. If the project was successful, later immigrants would pay for their passage and a fee to join the colony. This fee would be used to pay for the shipment of supplies from Earth in high demand on Mars. Such a colony would be largely autarkic, though mining, tourism, and services could in time become sources of the foreign exchange required to pay for imports from Earth.