NASA's upcoming Europa Clipper mission will send a spacecraft to explore Jupiter's icy moon Europa. Over the course of 45 flybys, ranging from 2,700 to just 25 kilometers, the mission will explore whether or not the moon is habitable, and pave the way for a potential future lander that will directly search for the life.

The spacecraft itself is on track to launch as early as 2022, providing the White House and Congress continue funding the mission at its required pace. But whether or not Clipper actually launches in 2022, and what rocket it uses, is another matter.

NASA recently announced the first flight of the Space Launch System, which is the current rocket of choice, won't happen until at least December 2019, and more likely, closer to June 2020. This further complicates what was already a messy tangle of politics, programmatic challenges and orbital mechanics.

When and how?

NASA has debated the priority of Clipper with Congress since before the mission's inception. With an estimated price tag between $3 and $4 billion, according to a GAO analysis, the agency worries how Clipper will impact other planetary science programs. A line in NASA's 2018 budget request states the agency's current position as such: "NASA does not recommend acceleration of the launch to 2022, given potential impacts to the rest of the Science portfolio."

Congress—particularly, Texas House Representative John Culberson—has tried to alleviate those concerns by feeding more money to planetary science, earmarking $275 million directly to the Clipper mission for 2017 alone. That money came with a mandate, written into law, that NASA will launch the mission in 2022 using the Space Launch System.

Just how much wiggle room NASA has on that mandate is tricky to decipher. Policy experts told me the only enforcement mechanism is pulling the project's funding, and if SLS isn't ready to fly, the directive could be a moot point, anyway.

Furthermore, the requirement did not come with any additional money to make it happen; who would pay for an extra SLS flight remains unclear. NASA, perhaps skeptical that it can afford to fly Clipper on SLS without additional funding, also addressed the situation in the 2018 budget: "The Europa [funding estimates] above assumes an Evolved Expendable Launch Vehicle [specifically, a Delta IV Heavy or Falcon Heavy], as the cost of an SLS flight is not yet known." NASA's official Clipper mission materials have also not adjusted to Congress's mandate, merely estimating a launch in the 2020s.

Any other rocket besides SLS—including SpaceX's upcoming Falcon Heavy—lacks the power to blast Clipper directly from Earth to Jupiter. A conventional rocket would rely on three gravity assists from Earth and one from Venus, increasing the transit time from about 2.7 years to 7.5 years.

Because it's hotter at Venus, Clipper would have to carry thermal baffles and use materials that withstand higher temperatures, increasing the spacecraft's complexity. At the moment, the Clipper program is still designing the spacecraft with those Venus requirements in mind.

SLS delay impact

A Clipper launch aboard SLS in 2022 is even more uncertain in the wake of NASA's announcement that the first flight, Exploration Mission 1 (EM-1), won't launch until at least December 2019, and as late as June 2020. After that, there will be a 33-month pause while the rocket's ground systems are upgraded to handle a new upper stage.

Direct-to-Jupiter launch windows have tight, 3-week constraints, and occur about every 13 months. The first launch window runs from June 4 through June 24, 2022. Even if SLS embarks on its first mission in December 2019, it won't be ready again by the end of the first window on June 24, 2022, and the next Clipper launch opportunity isn't until July 2023. That means NASA can't currently meet Congress's requirement that Clipper launch on SLS in 2022.

This runs counter to recent statements made by William Gerstenmaier, NASA's associate administrator for human spaceflight exploration and operations, during a recent hearing held by the House space subcommittee. When asked if the SLS launch delay would impact Clipper timing, Gerstenmaier said, "There's really no impact there. We can support pretty much whatever the science mission directorate needs for that mission, and we'll figure out whether it occurs after the first flight or after the second flight to meet their needs."

Why the 33-month delay?

At the heart of this messy situation is the 33-month stand-down after EM-1. Here's the background: SLS was such an expensive, ambitious rocket, it couldn't be built all at once. NASA opted to make the vehicle design evolvable, getting more powerful over time with an advanced upper stage, and eventually—so the plan goes—liquid-fueled boosters.

For EM-1, NASA is using an off-the-shelf Delta IV upper stage. The new upper stage, which will be introduced for the crewed, second flight (EM-2), will make the vehicle 13 meters taller. A 13-meter height change triggers a staggering amount of modifications to the mobile launcher, or ML—the tower holding all the rocket's support equipment, such as propellant, power, and access for pad workers.