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SpaceX's Falcon 9 rocket and Crew Dragon spacecraft are headed back to the launch pad, kicking off the second astronaut mission for SpaceX in just one month. On board will be four professional astronauts, destined for the International Space Station, .

This mission, called Crew-4, will mark a return to the crewed launches that SpaceX conducts in partnership with NASA after the company concluded the first for wealthy thrill seekers on Monday. Crew-4 is slated to take off in the early hours of Wednesday morning — at 3:52 am ET, to be exact — before arriving at the ISS around 8 pm ET that same day.

On board Crew-4 will be NASA astronauts Kjell Lindgren (who will command the mission), Robert Hines, and Jessica Watkins, and Italian astronaut Samantha Cristoforetti, who is flying on behalf of the European Space Agency.

The less than 40-hour turnaround between the conclusion of SpaceX's all-private mission, called , and the launch of Crew-4 is unprecedented for the company. NASA's human spaceflight lead, Kathy Leuders, said during a press conference Tuesday that the safety teams recognized they needed to take things "one step at a time."

"This is the time where you've got to rest and get ready to fly and still be watching the hardware and making sure you're making the right decision," Leuders said.

Here's everything you need to know about Crew-4.

WHAT MAKES THIS FLIGHT HISTORIC?

Watkins will become the first Black woman to complete such a mission.

Though more than a dozen Black Americans — including four Black women — have traveled to space since Guion Bluford became the first to do so in 1983, no Black woman has had the opportunity to live and work in space for an extended period, as the ISS has enabled more than 200 astronauts to do since 2000.

"This is certainly an important milestone, I think, both for our [space] agency and for the country," Watkins said during a press conference last month. "I think it really is just a tribute to the legacy of the Black women astronauts that have come before me as well as to the exciting future ahead."

She has a long history with NASA, having begun her career there as an intern, and she previously held roles at NASA's Ames Research Center in Mountain View, California, and at the Jet Propulsion Laboratory in Pasadena, California, where she worked with the. A trained geologist, she's studied the surface of the red planet.

Watkins' crew mates refer to her by the nickname "Watty."

WHO ELSE IS ON THIS MISSION?

This mission's crew is among the first to include as many women as men. Cristoforetti, who previously traveled to the ISS in 2014, is also the sole woman in ESA's astronaut corps. But Cristoforetti told reporters last month that the situation was "bound to end very soon."

"We definitely expect to have some some great female [ESA] colleagues by the end of the year," she added.

Cristoforetti, a veteran of the Italian Air Force who's earned her fighter pilot wings, joined ESA in 2009.

Hines is a 22-year veteran of the US Air Force, and this will mark his first time traveling to space since he was selected for the NASA astronaut corps in 2017.

Lindgren, who will command this mission, is certified in emergency medicine, and used to work as a flight surgeon on the ground at NASA's Johnson Space Center, supporting other astronaut missions. Lindgren was born in Taiwan and spent much of his childhood in England before moving to the United States and attending the US Air Force Academy.

The four astronauts have spent months training together, and even took time to do some extracurricular bonding. Watkins noted they went on a kayaking trip in Eastern Washington "just to get to spend some time getting to know each other and understanding how we all function ... and what makes each of us tick, and I think that's going to be really crucial."

"We get along great. It is just such a joy to have these folks on this team," Lindgren added.

HOW ARE THEY GETTING TO SPACE?

The crew will travel to the ISS aboard a SpaceX Crew Dragon capsule, which has launched seven crewed missions since entering service in 2020.

Though SpaceX designed the Crew Dragon to be reusable and three capsules are already in service, Crew-4 will fly aboard a brand new spacecraft.

The astronauts get to select the name for their capsule, and this group chose Crew Dragon "Freedom."

The Crew Dragon was developed by SpaceX under a $2.6 billion contract with NASA as part of the "Commercial Crew Program." The idea behind the program was to move NASA into a customer role — allowing private companies to design, build and test a new spacecraft to serve NASA astronauts while still giving the company ownership over the vehicle.

Since SpaceX controls the vehicle, it has the ability to sell seats to whomever it wishes, hence the all-private mission that the company just concluded and one previous space tourism mission last September.

NASA has deemed the program a massive success, and the space agency is adopting the same contracting method for several vehicles involved in its efforts to explore the moon.

WHAT WILL THEY DO IN SPACE?

After arriving Wednesday evening, the crew will be greeted by the cohort of astronauts already aboard the ISS — including three NASA astronauts and an ESA astronaut who were part of SpaceX's Crew-3 mission — and three Russian cosmonauts.

There'll be a five-day handover period, during which the Crew-3 astronauts will help the Crew-4 astronauts settle in, before Crew-3 returns home aboard their own SpaceX Crew Dragon capsule.

Then the Crew-4 astronauts will set to work on all the science experiments and space station maintenance duties they have on their to-do list.

"Experiments will include studies on the aging of immune systems, organic material concrete alternatives, and cardiorespiratory effects during and after long-duration exposure to microgravity," according to NASA. "These are just some of the more than 200 science experiments and technology demonstrations that will take place during their mission."

Crew-3 is slated to return from space in September, shortly after SpaceX launches its Crew-5 mission.