Late Thursday night—Friday in Adelaide, Australia where the speech will occur—Elon Musk will give a presentation at the International Astronautical Congress (IAC) meeting about his "updated" plans for a rocket system that will take humans to Mars. On Twitter, Musk has promised to discuss the "planetary colonizer design" in detail.

This speech follows a similar talk at last year's IAC meeting, in which Musk unveiled the "Interplanetary Transport System" design, with a massive 42-engine rocket capable of hefting as much as 550 tons to low Earth orbit—or about four times the amount of NASA's Moon rocket, the Saturn V launch vehicle. This reusable launch system, he said, could begin taking humans to Mars by the mid-2020s.

It was an audacious claim. The proposed spaceship would stand 50 meters tall, atop its rocket, with a maximum diameter of 17 meters. Instead of departing Earth orbit at 4.5km/s, its six Raptor engines optimized for the vacuum of space would accelerate it to 6 km/s, cutting the journey to Mars from six months to about three. After launching and being fueled on orbit, the ITS could deliver 100 tons to the surface of Mars. The largest payload NASA—or anyone—has ever safely landed on the Martian surface is the Curiosity rover, which weighs less than a single ton.

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While the 2016 speech presented an ambitious and bold vision to send humans to inhabit the Red Planet, it was almost too fantastical. The rocket was too massive to be believable. There was no roadmap for raising the funds to build it (only some jokes, such as "sell underpants"). It all seemed a bit mad.

The update

For this year, we might expect more realism. Already, Musk has suggested the big rocket will be scaled down to a 9-meter diameter, which would fit inside SpaceX's existing rocket factory. By scaling back to 9 meters, Musk likely intends to remove the outer ring of 21 Raptor engines, leaving a vehicle with 21 engines instead of the original 42. While still complicated to manage during launch and flight, 21 engines seems more reasonable. Such a vehicle would also have about 50 percent less mass.

Not much else is known about other changes Musk will announce, other than that he believes the design "feels right." Perhaps we will see an architecture that starts with a rocket powered by 10 or so Raptor engines, that can scale up to a 21-engine Mars rocket. This smaller version would be commercially viable with other heavy lift rockets coming online in the next five years, including Blue Origin's New Glenn booster.

What to watch

For me, the interesting parts won't be the rocket design. Rather, I want to see how Musk and SpaceX get to Mars from here. It is easy to show dazzling graphics, videos of large rockets lifting off, and people smiling on Mars. It is quite another thing to design the rocket, obtain the funding to build it, and negotiate the regulatory hurdles of launching cargo and people to another world. (It's important to remember that no commercial company has ever sent humans into orbit, or cargo, beyond low-Earth orbit).

So how will Musk pay for development of the Mars rocket? My guess is that SpaceX will follow its tried and true path. There will be some private investment—SpaceX continues to trend upward in private market valuations thanks to its growing dominance in the commercial satellite launch market—but we can expect the company to continue to leverage government contracts as well.

NASA is building its own heavy lift vehicle, the Space Launch System, to power human missions to the Moon and Mars. How will Musk's vehicle fit into that equation? Perhaps most importantly, the US Air Force is looking to develop a new generation of boosters in the mid-2020s, and it has been awarding hundreds of millions of dollars in Rocket Propulsion System grants. Will Musk's hardware fulfill the military's needs for a powerful rocket to deliver heavy satellites to geostationary space?

These are the questions we hope to see answered by Musk on Thursday night. We plan to have a full report published by Friday morning, so check back then.