Delay in the ignition of the Artemis 1 mission. The NASA rocket that will take off for lunar orbit this Wednesday will try again to leave the launch pad at Cape Canaveral (Florida). Once again, because this is the fourth attempt to launch this unmanned flight since this summer.
August 29, September 3, September 27 and then November 14, 2022. For almost three months, the US space agency has been looking for its ideal window of opportunity so as not to miss its great return to Earth’s only natural satellite. And while the first two reports are due to rocket fuel leaks, the last two are due to Hurricanes Ian and Nicole, which also hit Florida airports.
“They are not the same limitations as an airplane”
Still, in normal times, weather conditions are much less restrictive for planes than for rockets. In fact, space launchers are much more sensitive to weather, as Christophe Bonnal, a launcher expert at Cnes (National Center for Space Studies, editor’s note), explains to us.
“We could make a launcher that supports everything: it would just be called an ICBM. Except it’s freakishly heavy and would cost us a lot in performance,” he explains to BFMTV.com.
“It does not have the same limitations as an airplane, a rocket requires many more materials, it is incomparable,” says Olivier Mousis, astrophysicist and director of the Origins Institute. “Here we take the objects to the limit of the resistance of the materials”.
The mass of the rocket plays an important role in this, especially at takeoff where the thrust is weakest. Therefore, everything is a matter of choice and calculation for space agencies that seek at all costs to lighten their launchers to optimize their consumption.
“Ariane 5’s main stage wall, at its thinnest point, is 1.5 millimeters thick. It is proportionally thinner than cigarette paper”, stresses the engineer. “The other side of the coin is that they should not suffer, that we blow on them too much or that there are too many external aggressions.”
Wind and lightning, the two main risks
Not all “external assaults” are the same. Thus, rain is not a problem and does not prevent a rocket from leaving its launch pad. Due to the possible clouds present in the path of the rocket, it is more complex.
“Like airplanes, we don’t like to think that we are going to cross a cumulonimbus cloud: it is not possible to take off when there are big, dense clouds with potential hail in them,” explains Christophe Bonnal.
Space weather must also be taken into account. It can disrupt a rocket launch, especially if the Sun generates flares, because “this can affect the electronics on board.”
Space agencies are particularly afraid of one weather phenomenon: wind, which can knock the launcher off course. The risk is particularly high at liftoff, when the rocket is on its launch pad and during the first few meters of altitude, “the time when we are most sensitive to crosswinds.” Only once the separation of the floors is completed and 50 kilometers reached does it become zero.
To prevent any risk, the agencies send a weather balloon a few hours before takeoff to measure the wind variables in the rocket’s launch trajectory. In the same way, space centers have lightning detectors to analyze the electromagnetic variations of the atmosphere generated by possible lightning strikes.
“If there is a risk a few kilometers from the rocket, we don’t shoot,” says Christophe Bonnal. The expert recalls the case of the Saturn V launcher from the Apollo 12 mission, struck twice by lightning during takeoff. An unforeseen event that caused the launcher to malfunction but was later resolved by the crew.
Not too hot and not too cold
The weather conditions in the hours before takeoff are equally important. In 1986, the space shuttle Challenger disintegrated a few seconds after leaving Cape Canaveral, killing all seven of its crew. Investigators concluded that the seals suffered from cold the night before takeoff and broke during takeoff.
On the contrary, in case of very high temperatures, there is a risk that the hydrogen present in the engines will boil. Therefore, the temperature is controlled very closely: it must be between 5 and 35 °C.
“The worst can happen at any time during a rocket liftoff,” recalls Christophe Bonnal. “Usually we get the green light on the weather forecast an hour before launch, so we’re very confident.”
Time also scrutinized around
The sky is also rigorously observed during atmospheric reentries (when a natural or man-made object enters the atmosphere). Some are out of control, especially when space junk (rocket debris, obsolete satellite, etc.) or a meteor hits or returns to Earth.
Regarding the return of manned capsules or shuttles, the weather once again plays a crucial role. In 2021, the return of Thomas Pesquet and other astronauts from the International Space Station (ISS) had to be postponed several times, their arrival having to be made by landing. “Obviously, there is a very important meteorological criterion here, since it is necessary to ensure that the recovery boats are not too far away”, underlines Christophe Bonnal.
A precedent that goes back to 1976 had marked the spirits. The return to Earth of the Soyuz 23 spacecraft, with two cosmonauts on board whose mission had to be aborted, occurs in the middle of a snow storm. It was supposed to land on the steppes of Kazakhstan, but the capsule ends its run over a hundred kilometers away… landing in a lake. After several hours of searching and a dangerous rescue operation, the crew escaped unharmed.
As weather measurement tools have improved, so have rockets. However, they are subject to the same limitations as 40 years ago, explains Christophe Bonnal: “Launchers, which have become lighter and more efficient, are no more available than before, so it all comes down to the same thing.”
In the case of the Artemis mission, the first flight of the SLS rocket, the most powerful in the world, is scheduled for Wednesday at 01:04 local time (07:04 Paris time), with a possible launch window of two hours. If liftoff occurs, the mission should last 25 1/2 days, with a tour of the Moon followed by a landing in the Pacific Ocean on December 11.
Source: BFM TV
