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.

Starlink is one of the most ambitious space missions we've ever undertaken. The current plan is to put 12,000 communication satellites in low-Earth orbit, with the possibility of another 30,000 later. Just getting them into orbit is a huge engineering challenge, and with so many chunks of metal in orbit, some folks worry it could lead to a cascade of collisions that makes it impossible for satellites to survive. But suppose we solve these problems and Starlink is successful. What's the next step? What if we take it further, creating a mega-constellation of satellites and space stations? What if an alien civilization has already created such a mega-constellation around their world? Could we see it from Earth?

This is the idea behind a recent article posted on the arXiv. It's based on an idea about how civilizations might grow over time, known as the Kardashev scale. It's based on the level of energy a civilization can tap into; Type I uses energy on a global scale, type II a star's worth of energy, and so on.