SpaceX successfully launched its Falcon 9 rocket today from California — over four months after one of the company’s vehicles exploded on a launchpad in Cape Canaveral, Florida. The rocket took off just before 1PM ET from Vandenberg Air Force Base, putting 10 satellites into orbit for communications company Iridium. After the initial takeoff, SpaceX was also able to land the Falcon 9’s first stage on a drone ship in the Pacific Ocean. It was the first successful launch of the Falcon 9 since August, as the company was forced to ground all of its vehicles following the September explosion.

It’s an incredibly short turnaround for a rocket company following an accident

Today’s launch marks an incredibly short turnaround for a rocket company following an accident. It took SpaceX six months to recover from its previous rocket failure in June 2015, when one of the company’s Falcon 9s disintegrated en route to the International Space Station. Other companies have taken years to recover from the loss of a vehicle. Orbital ATK, for instance, spent two years updating the engines on its Antares rocket after a previous version of the vehicle exploded during take off in 2014. "It typically takes nine to 12 months for people to return to flight. That's what the history is," Tory Bruno, CEO of United Launch Alliance, told Reuters.

Beginning of today's successful mission to launch 10 communications satellites to low Earth orbit. A video posted by SpaceX (@spacex) on Jan 14, 2017 at 11:15am PST

SpaceX struggled to understand what caused the launchpad failure in September, with Elon Musk calling the accident “the most difficult and complex failure we have ever had in 14 years.” The explosion occurred as the Falcon 9 was being loaded with propellant in preparation for a static fire test — a routine procedure SpaceX does to test the rocket’s engines before flight. During propellant loading, the rocket ignited in a fireball that lead to the vehicle’s destruction and the loss of the Amos 6 satellite it was supposed to carry into space a few days later.

SpaceX grounded all of its rockets and spent its hiatus from spaceflight investigating the accident with help from NASA, the Federal Aviation Administration (FAA), the US Air Force, and the National Transportation Safety Board. Two weeks ago, the company came to the conclusion that the problem started in the Falcon 9’s upper liquid oxygen tank, which stores super chilled liquid oxygen propellant. That oxygen seems to have had a bad reaction with vessels housed inside the tank known as composite overwrapped pressure vessels, or COPVs.

These vessels, which house helium needed to pressurize the tank, consist of an inner aluminum liner surrounded by carbon fibers. SpaceX was able to find some debris of the COPVs after the accident and found that the aluminum liner had buckled, or was scrunched up. The idea is that some of the liquid oxygen from the propellant tank got between the carbon fibers and the aluminum, where it pooled into the recesses of these buckles. Meanwhile, the helium from the COPV was so cold that it may have caused this pooled oxygen to turn into a solid, making it extra vulnerable to ignition. Friction from the carbon overwrap, or even breaking carbon fibers, may have provided the spark needed to create the fireball.

The FAA accepted SpaceX’s explanation for the accident

The FAA accepted SpaceX’s explanation for the accident, granting the company a launch license last week for today’s mission. Today’s success signals that SpaceX is ready to get back to launching its rockets again. That’s important given the company’s busy year ahead. SpaceX planned to launch 20 rockets last year, and 27 this year, according to financial documents released to The Wall Street Journal. The September explosion meant the company only launched eight rockets in 2016, so there’s definitely a backlog of customers waiting to fly this year.

Today’s launch was the first part of the Iridium NEXT mission, an effort by Iridium to create a large constellation of 66 satellites in lower Earth orbit. The company also plans to have 15 spare satellites — six in orbit and nine on the ground. Altogether, these satellites will deliver mobile voice and data coverage all over the planet, including over the ocean and at the poles, according to Iridium. SpaceX has a contract with Iridium to launch 70 of these satellites over seven launches over this year and next.