Which mission will fly first?
Studied and analyzed on both Block 1 and Block 1B vehicles, Exploration Mission-2 (EM-2) and launch of Europa Clipper are back on Block 1.
“Exploration Mission-2 (EM-2) will use the SLS Block 1 configuration and the current Block 1 Mobile Launcher (ML-1) with minor modifications to support crewed flight,” a May 18 memo officially formalizing the decision said, adding that Europa Clipper would also use Block 1.
The official memo from Deputy Associate Administrator for Exploration Systems Development (ESD) Bill Hill also formally cancels the early 2016 memo that had moved EM-2 to EUS and Block 1B.
Even before the second ML funding was approved, the payload manifest for the EM-2 crewed Orion flight had been stripped down to its Block 1 origins. The EUS upper stage is designed to work with SLS to lift around 35 metric tons or more to the Moon, enough to take Orion and an additional eight to ten ton payload.
At one time, EM-2 was assigned to an ICPS Block 1 vehicle which can fly Orion to the Moon by itself, as will be demonstrated on the EM-1 uncrewed test flight. When EM-2 moved to an EUS Block 1B vehicle, the trunk area behind Orion was available and eventually assigned to the Power Propulsion Element (PPE) of a proposed cislunar gateway.
When in early February the Trump Administration formally proposed starting the gateway in FY 2019 , it also assigned a dedicated commercial launch for the PPE, removing it from EM-2. This made EM-2 an Orion-only launch again, at the time effectively using the bigger EUS to fly an ICPS-sized payload.
The Europa Clipper spacecraft is being designed to orbit Jupiter and whose mission is to make dozens of flybys of the planet’s large, Galilean moon. Beginning with U.S. federal budget appropriations in FY 2016, language mandating when and how to launch the spacecraft was enacted into law, saying “the National Aeronautics and Space Administration shall use the Space Launch System as the launch vehicle for the Jupiter Europa mission, plan for a launch no later than 2022, and include in the fiscal year 2017 budget the 5-year funding profile necessary to achieve these goals.”
Although the Trump Administration proposed dropping both of these requirements in their early February FY 2019 request, Congress continued to retain them in the FY 2018 bill enacted in late March, and the House of Representatives is maintaining them in the draft FY 2019 CJS appropriations language recently published.
Which mission would fly first, the first crewed Orion or Europa Clipper, will be determined in the future.
“Right now we’re not picking which one goes first, it’s going to be who is ready and if Clipper is ready and from a risk standpoint we’re willing to fly it, we would do that,” Mr. Lightfoot said at the April 12 House subcommittee hearing.
The May 18 memo also reflects this: “Determination as to whether this launch will be the SLS/Orion crewed launch (EM-2) or the SLS/Europa Clipper mission will be made based on risk and readiness of the Europa Clipper project.”
Assuming the mandate to fly Europa Clipper remains law, a main consideration is likely to be the frequency of launch opportunities. The minimum amount of energy required to launch a spacecraft to Jupiter occurs in a short, three-week long period every thirteen months.
The first Jupiter launch window in the “no later than” mandate opens early in June, 2022; if the spacecraft is not launched then, it will have to wait until July, 2023 for the next chance to launch. In comparison, Orion has a week-long period every month where it can be launched to the Moon.
On top of these considerations, more Block 1 launches require a new contract to purchase more ICPS stages. The original contract to design and build a structural test article of the ICPS and a flight article for EM-1 included options that could be exercised for additional stages; however, the options period expired.
“The baseline contract with Boeing did include an option for a second ICPS,” Kathryn Hambleton, Public Affairs Officer at NASA Headquarters, wrote in an email. “However, the timeframe to exercise that option expired in August 2016, requiring new negotiations. Those negotiations are expected to begin this summer.”
The May 18 memo indicated that the current thinking is that “Mid-2022 is the target date for the next SLS Block 1 launch using ML-1,” whichever flight that would be.