Interplanetary Commerce
Terrestrial shipping provides something of a guide to orders of magnitude for a future “Expanse” style Kardeshev-1 Civilization.
In 2024 the total Global shipping fleet of more than 100 tonnes was about 112,500 vessels totaling 2.44 billion tonnes in Dryweight Tonnage (DWT), shipping 12.72 billion tonnes, for a total of 66,781 billion tonne-miles.
The Tonne-Mile is an odd mix of metric and Imperial. Some countries convert it to Tonne-Kilometre. Thus 110,000 billion tonne-km. Using SI units the kg-metre would be inconveniently small. In energy terms, a kg-metre is missing an acceleration unit to make it joules. A tonne-kilometre is thus a MJ/(m/s^2).
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From the above an average shipload conveys a tonne 5,250 tonne-miles. The average ship does about 5 trips per year and carries about 21,690 tonnes DWT.
5,250 tonne-miles is 8.45 billion kg-metres.
Total Global Shipping energy use is about 11 exajoules (11E+18 joules) per annum. Thus the average tonne-kilometre needs 0.1 MJ. An effective “acceleration” of 0.1 m/s^2. A average freight journey thus requires 845 MJ per tonne.
In Space the energy per trip is much higher. The 93 million miles (149.6 million km) of 1 AU means sending a tonne that far at 0.1 m/s^2 acceleration needs at least 14.96 terajoules. About 17,700 times the average shipping tonne’s trip on Earth.
How long would it take? 28 days roughly. Not too different to a shipping trip on Earth.
