A photo from SpaceX CEO Elon Musk of the company "adding the rear moving fins" to the first Starship rocket in Boca Chica, Texas.

SpaceX is deep into development of its Starship rocket, with recent updates from CEO Elon Musk showing the first of the company's vehicles coming together at its facility in Texas.

"Adding the rear moving fins to Starship Mk1 in Boca Chica, Texas," Musk said in a tweet on Sunday.

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The photo showed the stainless steel hull of the rocket as SpaceX attached two large fins to the base. Starship is a massive, next-generation vehicle that Musk's company is building to send cargo and as many as 100 people to the moon and Mars.

SpaceX has successfully reused parts of its Falcon 9 and Falcon Heavy rockets but Starship would take the company's ambitions much further. Starship is designed to be fully reusable, launching and landing in one piece so as to make spaceflight more akin to commercial air travel.

The company is deep into its development plans for Starship, according to a federal document outlining the company's plans in May. Recently, SpaceX flew a prototype called "Starhopper" on a short flight to about 500 feet altitude. Starhopper flew for about one minute, landing back on a concrete pad successfully.

The Starship test program is next expected to feature flights of the rocket to an altitude of about two miles and then to 62 miles, reaching the edge of space.

SpaceX has steadily raised over $1.3 billion in new equity this year, key funding for the Starship program as well as Starlink – the ambitious internet satellite project the company is simultaneously undertaking.

Musk shared that the top of Starship will feature large, moving fins, as well as several other pieces of hardware. While the Starhopper prototype featured only one of the company's Raptor engines, Musk said this Starship will have three engines.

@elonmusk: Bottom half of Starship at night. Top half with forward fins & header tanks probably stacks on Wednesday. Three Raptors already installed.

The company's founder also shared that SpaceX has adjusted one key element of Starship's design, with the rocket featuring two large fins at its base, rather than three. Musk expressed skepticism about this change but said that SpaceX will stick with this plan for the first two rockets.

"Current analysis, which I'm not fully bought into, suggests that 2 rear fins with separate airframe-mounted legs will be lighter, so this is the plan for Mk1/Mk2," Musk said.