UPDATE: Apr. 6, 2022, 3:21 p.m. EDT NASA called off another attempt to fill the mega moon rocket's tanks on Tuesday, after encountering more problems. The team discovered a malfunctioning vent valve, which relieves pressure on the rocket's large core while the fuel loads. NASA hasn't yet announced a date for when the simulation test will resume.

First, thunderstorms delayed the dress rehearsal for NASA's megarocket over the weekend, the final crucial step before launching it to the moon.

Then, lightning "struck twice" for the simulation test — figuratively speaking.

Mission operators said they needed to stop mid-rehearsal on Sunday to sort out a problem in the mobile tower that supports the rocket, known as the Space Launch System, on the ground. A fan that keeps hazardous gases out of the launcher wasn’t working. When it failed, technicians tried to use a backup fan to proceed, only to discover that it wasn’t working either. Each one was having a different issue, officials said.

The fans are intended to prevent gases that could cause a fire from entering the launchpad during the fueling process, said Charlie Blackwell-Thompson, launch director at Kennedy Space Center in Cape Canaveral, Fla. This is called "positive pressurization."

Without the fan system, the NASA team didn’t believe it was safe to put the propellant into the rocket.

"We decided that we wanted to really understand that, given it was the first time loading the vehicle, and we made the decision to stand down to get in a configuration to go troubleshoot that and then be ready to make another run at it," she said during a briefing with reporters on Sunday.

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By Monday, it appeared the space agency had resolved the fan issue and was prepared to continue the dress rehearsal.

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NASA is readying the 32-story, 5.75 million-pound rocket for a mission to the moon, known as Artemis I. It's the first in a series of deep space exploration voyages that could blast off as early as May. The upcoming launch won't include astronauts, but the monthlong flight will allow the United States to send a crew on the next, more complex mission, Artemis II.

The rocket is thought to be the most expensive ever built with each launch estimated to cost over $4 billion.

It's been a long time since NASA had a rocket of this magnitude, capable of sending large payloads — astronauts and cargo — into deep space. Not only is the Space Launch System, or SLS, built to travel to the moon, it's expected to one day put millions of miles on the odometer during the first crewed flight to Mars. Robotic scientific journeys to Saturn and Jupiter also could be in its future.

At its Florida launchpad, the fully assembled rocket, capped with the Orion spacecraft, is undergoing a two-day demonstration that involves filling the rocket with liquid fuel, practicing a countdown that stops right before ignition, and draining the tanks.

Credit: NASA/Ben Smegelsky

The four main engines, fueled with 700,000 gallons of super-cold fuel, will produce a thrust powerful enough to keep eight Boeing 747s aloft. Filling the humongous tanks will take about eight hours.

Only after successfully completing this so-called "wet dress rehearsal" will NASA set a date for the first lunar mission, agency leaders have said. Officials have said Artemis could lift off as early as May, though it’s more likely it will happen this summer, given the tight schedule.

As of Sunday evening, NASA didn’t believe the storms and lightning in the vicinity of the launch pad had anything to do with the fans malfunctioning. Mission representatives said the system ran normally during the foul weather.

When NASA said it was "go" for tanking fuel, it wasn’t immediately clear to the public how the agency addressed the fan problem overnight.

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Mike Sarafin, Artemis mission manager, told reporters on Sunday the events surrounding the rocket rehearsal, including four lightning strikes inside the launch pad perimeter the day before, have been unusual.

"It has been one of the more interesting 48 hours that I've had in the context of working missions leading up to a launch, and, in this case, leading up to a key test," he said.

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