Training of the crew for the first entirely private trip to the International Space Station (ISS) is to begin soon, Axiom Space, the company behind the flight, said Monday at a joint press conference with NASA.

Four astronauts are to be launched to the ISS in late January aboard a rocket built by another space company, Elon Musk's SpaceX.

Only one of the four—NASA veteran Michael Lopez-Alegria—has been in space before.

The other three are businessmen—Larry Conner, an American, Mark Pathy, a Canadian, and Eytan Stibbe, an Israeli.

The mission dubbed Ax-1 is to last around 10 days, said Axiom Space president and CEO Michael Suffredini.

The astronauts will work and live in the American section of the space station and plan to conduct a number of scientific experiments while in orbit.

"We'll be starting what I would call serious training next week," said Lopez-Alegria, the Ax-1 commander.

"From there the pace will pick up, and we'll all be immersed essentially full time in ISS systems and Crew Dragon training starting in the fall.

The US Defense Advanced Research Projects Agency (Darpa) has recently commissioned three private companies, Blue Origin, Lockheed Martin and General Atomics, to develop nuclear fission thermal rockets for use in lunar orbit.

Such a development, if flown, could usher in a new era of spaceflight. That said, it is only one of several exciting avenues in rocket propulsion. Here are some others.

Chemical rockets

The standard means of propulsion for spacecraft uses chemical rockets. There are two main types: solid fuelled (such as the solid rocket boosters on the Space Shuttle), and liquid fuelled (such as the Saturn V).

In both cases, a chemical reaction is employed to produce a very hot, highly pressurized gas inside a combustion chamber. The engine nozzle provides the only outlet for this gas which consequently expands out of it, providing thrust.

The chemical reaction requires a fuel, such as liquid hydrogen or powdered aluminum, and an oxidiser (an agent that produces chemical reactions) such as oxygen.

When Oscar Wilde said "conversation about the weather is the last refuge of the unimaginative" he was unaware of some of the more extreme weather on planets and moons other than Earth.

Since the discovery of the first exoplanet in 1992, more than 4,000 planets have been discovered orbiting stars other than our own.

The continuing research with exoplanets involves trying to identify their atmospheric composition, specifically to answer the question of whether life could exist there. In this search for life though, astronomers have found a huge variety of potential worlds out there.

Here are four examples of bizarre weather on other astronomical bodies—to show how varied an exoplanet atmosphere could be.

1. Iron rain on WASP-76b

WASP-76 is a large, hot exoplanet discovered in 2013. The surface of this monster planet—roughly twice the size of Jupiter—is about 2,200℃ (4,000℉). This means a lot of material that would be solid on Earth melts and vaporizes on WASP-76b.

As described in a particularly famous 2020 study, these materials include iron.