“That’s difficult,” Jakosky added, especially considering the Starship vehicle runs on cryogenic fuels — essentially oxygen and methane that are kept at temperatures so cold they liquify. And in the microgravity environment of orbit, that fuel can float about in its tank rather than settling in one place. So, among myriad other technical difficulties, SpaceX will likely have to devise pumps or motors that can effectively funnel the fuel from one ship to another. Currently, it’s not even clear how many tankers SpaceX would need to launch to give one Starship vehicle enough gas for a trip to Mars. (In prior estimates, NASA personnel and third-party experts projected it may take roughly one dozen Starship tankers for a moon mission.) In his speech, Musk said that he believed in-space fuel transfer would be “technically feasible.” SpaceX will not attempt to carry out its first tanker flight test before next year, Musk added. Barriers to reentry Even after SpaceX sorts out the propellant transfer problem, they’ll face another significant technological question: How will Starship survive the trip down to the surface of Mars? Musk last month called this issue “one of the toughest problems to solve.” “No one has ever developed a truly reusable orbital heat shield so that is extremely difficult to do,” he said. “This will be something that we’ll be working on for a few years, I think, to keep honing.” Vehicles that need to safely land on planetary bodies while traveling at orbital speeds must have a component called a heat shield — a special coating on the vehicle’s exterior that serves as a buffer to the scorching temperatures generated by the process of entering a planet’s atmosphere. Workers survey the SpaceX Starship rocket on March 3 ahead of its eighth uncrewed test flight. Brandon Bell/Getty Images On Mars, one crucial problem is the air: It’s almost entirely made up of carbon dioxide. When Starship slams into Mars’ atmosphere, it will violently compress the air in front of it and create searing temperatures. And the conditions of reentry are so intense that the process literally rips electrons away from atoms and splits molecules, turning the carbon dioxide into carbon and oxygen — the latter of which may start to “oxidize” or essentially incinerate the spacecraft’s heat shield, Musk said. Reentry on Mars will actually produce more heat -shield-destroying oxygen than the process of returning to Earth, Musk noted. Starship’s heat shield will ultimately need to be durable enough to survive both types of reentry, potentially multiple times. The human problem While the odds of SpaceX solving all the necessary technical quandaries in time to send a cargo-filled Starship to Mars at the end of next year are likely small, even larger problems must be solved later down the road. If SpaceX wants to send humans to the red planet, for example, the company must figure out how to ensure Starship’s exterior can keep people safe from the deadly radiation that will shower down throughout the six-month journey. Life support systems with plenty of breathable air would need to be on board. As Musk put it, every single human need must be accounted for. “You can’t be missing even, like, the equivalent of vitamin C,” he said. Once a Starship vehicle reaches its destination, it would likely need to top off its fuel at a Martian depot before returning home — another feat that presents enormous technological challenges. Starship’s heat shield will need to be durable enough to survive a trip to the surface of Mars. NASA/JPL/Cornell The idea that enough infrastructure will exist on Mars by 2029 — or 2031, as Musk has said in prior social media posts — to make such a crewed mission possible is outlandish. Still, industry experts say SpaceX’s bold ambitions spark both excitement and skepticism. “I am a fan of what SpaceX is trying to do. I totally subscribe to this vision of a multi-planetary society,” said Olivier de Weck, the Apollo Program Professor of Astronautics and Engineering Systems at the Massachusetts Institute of Technology. “But it’s a logistical problem first and foremost. And what’s lacking to me is the thought about the cycling, the fuel production — and the return to Earth.” This depicts the satellite-filled sky that is now a reality and getting more crowded every week! This adds together exposures taken over just 30 minutes on an early June night when, from my latitude of 51° N satellites even in low Earth orbit are lit all night by sunlight. Many of the parallel streaks heading generally horizontal west to east (right to left) may be from groups of SpaceX Starlinks. Others traveling vertically north-south are more likely from Earth observation satellites. There is at least one natural streak in the image — a meteor at centre, caught by chance on one frame. It appears as a colored and tapered streak. Other uniform undashed streaks may be from high-altitude satellites moving much more slowly. By comparison, most satellites appear as dashed lines because the image is a blend of many 2-second-long exposures with a gap of one second between exposures when the camera shutter was closed. So the motion of the satellites and image stacking turns them into dashes. The longer the dashes, the faster the satellite is traveling, with the fastest satellites being the lowest. This is looking due south and all the trails disappear low in the south above the trees, as that's where the Earth's shadow is, even on this June night. So the satellites aren't lit when they are in that small part of the sky. They emerge from the shadow heading north and disappear into the shadow heading south. The shadow creates the obvious boundary of where satellite trails are visible. At other times of the year low-orbit satellites are visible only after sunset or before sunrise, especially from lower latitudes. But not near summer solstice, and from higher latitudes. The field of view is about 100° by 75°. (Photo by: Alan Dyer/VWPics/Universal Images Group via Getty Images) Alan Dyer/VWPics/Universal Images Group/Getty Images Related article What is ‘Kessler Syndrome’ — and why do some scientists think the space disaster scenario has already started?