Between ferrying NASA researchers to the ISS and seeding the night skies with its Starlink satellites, SpaceX especially will have its hands full. But it’s not like we can launch rockets into space whenever we feel like it. In fact, NASA has developed a strict set of criteria that must be met before they can light the fuse on a Falcon 9 rocket. Here’s how they decide whether or not we can safely reach for the stars today.

Space, as you may have heard, is hard. Just figuring out when to set a launch window depends on any number of factors including the goals and objectives of the mission, the Earth’s position relative to other celestial bodies, even the type of rocket used and its required trajectory to reach its target. The two biggest factors, however, are the spacecraft’s destination and its solar power. Some spacecraft may need full exposure to our local star to power themselves or may need to avoid the bright solar rays in order to study deep space. The launch window — the span of time that a rocket can successfully launch during — will therefore be different for putting an Earth-observing satellite into LEO versus launching a Dragon Capsule to precisely rendezvous with the ISS. The launch window for Mars missions, for example, is only open once every 26 months.

The reason we only get a shot at Mars once every two plus years is due to their relative orbital positions in the solar system. We want Mars’ orbit to be as close to the Earth’s as possible so that we can huck a spacecraft to intercept it using as little fuel as possible, since the more fuel you have to carry to get there decreases the amount of capacity you have for cargo and supplies. Instead, we use Mars’ gravitation to help pull us along while the spacecraft “coasts” into orbit. This is known as a Hohmann transfer orbit and while it is considered the most efficient means of moving between two planets, the timing has to be precise. If the spacecraft is launched too early or too late, it’ll miss the rendezvous point with its target.

Adding to that challenge is the fact that both planets are not only moving through space, they’re revolving as well. This combination of forward and rotational momentum is why rocket launches don’t go straight up, but rather arc (below). It’s like throwing a football. The ball gets all of its energy with the initial throw and it follows a curved flight path leading the receiver (i.e. heading to where the receiver will be, not where they currently are) so that both the ball and its target arrive at the same point in space at the same moment in time.

NASA

“You’ve got this object that’s going to go flying out into the air and you’ve got to shoot it,” Eric Haddox, lead flight design engineer in NASA’s Launch Services Program (LSP), said in a 2012 press statement. “You have to be able to judge how far away your target is and how fast it’s moving, and make sure you reach the same point at the same time.”

“To launch at any time other than that optimal time, you’re going to have to alter the trajectory, steer the rocket to get back to that point,” Haddox continued. “So that’s where it becomes a trade of, ‘Okay, if my window were a half hour long, how much performance would I need to fly at any time within a half hour? Or, if my window were an hour long, how much performance would I be able to get out of the rocket to fly at any time within that one hour?'”

For example, let’s say NASA wants to deliver supplies to the ISS. Flight engineers will calculate the ISS’ orbital trajectory to figure out when it will pass as close to directly overhead of the Kennedy Space Center in Florida as possible. This is known as the plane window. “If it passes directly over Kennedy Space Center, that’s our optimal time for launch,” NASA Flight Dynamics Officer, Richard Jones told Air and Space in 2002. “If it passes a few miles to the east or west, that’s okay but it will require some additional steering. And that takes additional fuel and adds stress to the external tank, which we want to minimize.”

The problem is that while the Earth is rotating at just over a thousand miles an hour, the ISS’ orbit is fixed in space. This means that on one of its 90 minute orbits of the Earth, the ISS might fly directly over Kennedy Space Center, but when it passes by again 90 minutes later, the Earth will have rotated and the ISS will fly over a point about 1000 miles west of Florida. As such, ISS resupply missions have a single launch window per day, since it takes 24 hours for the Earth to complete a rotation. 

This is why SpaceX’s successful second attempt to deliver astronauts to the ISS occurred at roughly the same time of day as the scrubbed first attempt — it’s when the station’s orbital trajectory would bring it closest to the coordinates of the Dragon Capsule’s launch site. Flight officers must also contend with the phase window. That’s the span of time that a rocket has to catch up and match speed with its orbiting target before it runs out of fuel. 

This of course presumes that conditions on the ground won’t interfere with the launch or pose a safety threat to the crew. But when has Mother Nature ever cooperated? In the lead-up to a launch, NASA monitors weather both at the launch site and the surrounding region and reacts to adverse conditions in accordance with regulations set by the 12 points of the lightning launch commit criteria (LLCC). Atmospheric data is provided by the 45 Weather Squadron operating out of the nearby Patrick Air Force Base which keeps an eye out for thunderstorms, high winds, and low cloud cover. “Weather is the single largest source of launch delays and scrubs,” notes a study out of Patrick AFB.

For example, during the SpaceX crewed launch in May NASA kept an eye on storms up and down the Eastern seaboard since, if the astronauts had to bail, they’d splash down somewhere along there.

“We have a really complicated way of weighting different locations, depending on how much risk they have in terms of an escape,” Hans Koenigsmann, SpaceX’s vice president of build and flight reliability, told Spaceflight Now in May. “For the landing, there are also special parameters: winds, waves and wave direction.”

“Some of these individually have to go ‘green’ and be a ‘go,’ and then on the entire launch corridor we have a common risk number that we use to basically make an assessment, and then get to a go/no go decision,” he continued. 

The nine conditions that will ground a prospective Falcon 9 launch outright are:

• Sustained winds at the 162-foot level of the launch pad in excess of 35 mph.

• The presence of wind shear conditions, which could lead to control problems for the launch vehicle.

• Having to launch through a cloud layer greater than 4,500 feet thick that extends into freezing temperatures — the Challenger was lost in 1986 due to a broken o-ring which cracked in chilly conditions.

• Having to launch within 10 nautical miles of cumulus clouds with tops that extend into freezing temperatures.

• Having to launch within 10 nmi of the edge of an active thunderstorm within 30 minutes of the last observed lightning strike.

• Having to launch within 10 nmi of anvil clouds, which often generate strong winds and lighting.

• Having to launch within 5 nmi of “weather clouds that extend into freezing temperatures and contain moderate or greater precipitation,” as they tend to produce damaging hail.

• Having to launch within 3 nmi of a thunderstorm’s debris cloud.

• Having to launch through cumulus clouds either formed from or currently attached to a smoke plume. Again due to the possibility of lightning strikes. 

 Two weather conditions will delay the launch for either 15 or 30 minutes:

• If field mill instruments (they measure the strength of magnetic fields in the atmosphere) within 5 nmi of the launch pad exceed +/- 1,500 volts per meter, or +/- 1,000 volts per meter, the launch is delayed 15 minutes.

• If lightning of any kind is observed within 10 nmi of the launch pad or flight path, that’s a 30 minute delay.

You’ll notice that most of these conditions are designed to mitigate lightning strikes, especially rocket triggered lightning strikes. This phenomenon occurs when the rocket and its electrically conductive exhaust pass through a sufficiently strong electrical field. The rocket condenses the field as it passes through by as much as two orders of magnitude (that is lighting is 100 times more likely to occur than naturally). This compression continues until the breakdown electric field of air limit is exceeded and a lightning strike occurs. 

These rules are in place due in part because during the Apollo 12 mission in 1969, the rocket passed through a cumulus cloud and triggered a lighting strike that damaged some nonessential equipment onboard the rocket. The crew was able to complete their mission but things could have gone sideways (and then groundways) dangerously quickly if that bolt had hit a vital component.