SpaceX's newest Starship prototype is standing tall.

The latest test version of the Mars-colonizing Starship spacecraft , called the SN3, has been stacked at SpaceX's South Texas facilities, new photos tweeted out by company founder and CEO Elon Musk show.

Musk posted two images on Twitter about 45 minutes apart early Thursday morning (March 26). The first shows the SN3's tank and engine sections sitting side-by-side in an assembly facility, and the second shows them joined. (Musk didn't provide much in the way of context, writing simply "SN3." )

Related: SpaceX's Starship and Super Heavy rocket in pictures

SN3 is short for Serial No. 3; the prototype is the third iteration of the latest Starship design. Completion of the stacking milestone suggests that SpaceX, which has been moving very quickly on the Starship project, will begin testing the vehicle soon.

Such testing will begin with pressure trials, which have felled other Starship prototypes . But if the SN3 makes it through that gauntlet, it will likely get to spread its wings. Musk has said he aims to conduct short flights in Earth's atmosphere with the SN3, then fly higher and longer test missions with the SN4.

Starship is designed to launch atop a huge rocket called Super Heavy, which will have space for 37 of SpaceX's next-generation Raptor engines. Both elements will be fully and rapidly reusable; Super Heavy will come back to Earth for vertical landings shortly after delivering Starship to orbit, and the spacecraft will do the same when its work is done.

Some of this work will take Starship very far afield, if all goes according to Musk's plan. Starship will be capable of carrying 100 people to distant destinations such as the moon and Mars, he has said. And the Red Planet is the ultimate goal: Musk wants the Starship architecture to help humanity set up a million-person city on Mars in the next 50 to 100 years.

Starship won't need Super Heavy to get off the lunar or Martian surface, by the way. The final spacecraft will sport six Raptors of its own, which will provide enough power to lift off those planetary bodies. (Earth is much bigger than the moon or Mars and therefore has a much stronger gravitational pull.)

We could see Starship and Super Heavy flying operational missions very soon, if testing and development go well. SpaceX representatives have said the system could begin launching satellites to Earth orbit as early as next year. And Japanese billionaire Yusaku Maezawa has booked Starship for a crewed round-the-moon voyage , with launch targeted for 2023.