NASA is now officially accepting proposals for lunar lander designs that can carry humans to the surface of the Moon. The space agency issued a final call to the commercial space industry today, with proposals due November 1st.

Human lunar landers are a critical component of NASA’s Artemis program, an initiative to send people back to the Moon’s surface in less than five years. These landers are meant to live at a new space station that NASA wants to build in orbit around the Moon called the Gateway. Astronauts will supposedly travel to the Gateway in NASA’s future rocket — the Space Launch System, or SLS — and from there, they’ll travel in the landers down to the Moon.

Specifically, NASA is looking for landers that consist of two to three main pieces. First, the descent stage — hardware that will lower the vehicle down to the surface of the Moon. Second, the ascent stage, which is what astronauts will ride in when they take off from the lunar surface and return back to the Gateway. A third component is known as the transfer stage; this piece is basically a tug that can transport the other two elements from the Gateway to an orbit that’s closer to the Moon, making it easier to get everything to and from the surface.

NASA is looking for landers that consist of two to three main pieces

Companies will also need to specify how they’ll get their lunar landers to the Gateway. They can use commercial vehicles, such as SpaceX’s Falcon Heavy or Blue Origin’s future New Glenn rocket, or they can use NASA’s future SLS rocket, slated to fly in 2021 at the earliest.

From the proposals it receives, NASA eventually plans to select two of those companies to actually go through the process of making the landers and sending them to the Gateway. One company’s lander will be tasked with doing the first coveted landing with people on board in 2024, while the second will do another crewed landing in 2025. NASA does not plan on doing any uncrewed test landings with these designs beforehand, though.

A few companies have already given the public a sneak peek of their plans. Lockheed Martin unveiled a lunar lander design that’s derived from the company’s Orion capsule, which NASA astronauts will ride in on top of NASA’s SLS. Jeff Bezos also showed off Blue Origin’s Blue Moon lunar lander concept in May, which he claims the company has been working on for the last three years.

This finalized call to industry comes a few months after NASA posted two draft documents detailing what kind of landers and contracts it was looking for from the commercial space sector. During those months, companies provided NASA with feedback as it edited its request. NASA claims it removed language and requirements that companies thought might slow down the development schedule. The entire process — from drafts to final call — has been much speedier than many of NASA’s other contract selections, which can take many months to years to be awarded. The expedited schedule has been necessary in order to meet Vice President Mike Pence’s challenge to NASA from March of this year — to send people back to the Moon four years earlier than the agency had planned.

While NASA has been speedy in getting out this call to industry, these lunar lander selections all hinge on what kind of budget the space agency is given next year. In order to get a jump-start on the Artemis program, the Trump administration requested an extra $1.6 billion for NASA for next year in a budget amendment, with $1 billion going toward the development of new lunar landers. However, it’s not clear whether NASA will actually receive those additional funds. The Senate Appropriations Committee recently passed a funding bill for next year that gives NASA a big boost in its budget, but only provides $744.1 million for developing new lunar technologies for Artemis.

The final budget for 2020 is still being decided, but NASA says that money is key. “For this year, what we need is that budget amendment so that we can get the landing systems awarded — get those contracts out,” Ken Bowersox, the acting head of NASA’s human exploration office, testified before the House Science Committee this September. “Because that’s our long pole for right now for getting to the lunar surface.”