On Thursday, a SpaceX Falcon 9 rocket blew up on the launch pad, shaking nearby buildings and spewing smoke into the sky, two days before its planned liftoff with an Israeli satellite.

According to SpaceX CEO Elon Musk , the Cape Canaveral mishap was not an explosion but a "fast fire."

In his Thursday tweet, Musk did not call the incident an explosion.

@elonmusk: Loss of Falcon vehicle today during propellant fill operation. Originated around upper stage oxygen tank. Cause still unknown. More soon.

Generally, the difference between what might be a "fast fire" and an explosion is the speed of combustion, said Daniel M. Nosenchuck, an associate professor in Princeton University's Mechanical & Aerospace Engineering Department.

While he couldn't comment directly on the SpaceX incident, as he is out of the country and has not followed it closely, he told CNBC Friday that one observable difference between a fire and an explosion is the sound that witnesses hear. The sound of a deflagration, Elon's "fast fire," is different from that of a detonation wave.

@John_Gardi: @njcygni Actually, Challenger's destruction wasn't an explosion for the same reason, the shock wave was less than supersonic; a deflagration

A deflagration wave is subsonic, or slower than the speed of sound, whereas a detonation wave is supersonic, faster than the speed of sound.

"So, a 'fast fire' might be a relatively slowly moving deflagration wave, which might produce a 'whooshing' sound. An explosion, which produces the detonation wave, would be characterized by an extremely loud bang as the supersonic combustion front moved past an observer," explained Nosenchuck, who has taught classes on heat transfer.

He also noted that "the pressure rise that occurs behind an explosive detonation wave is nearly instantaneous and can be very large and cause much more structural damage than a deflagration event."

In its latest update on the "anomaly," SpaceX pledged to carefully and thoroughly investigate the event.

Late Friday, the company provided an update on Thursday's events:





Yesterday, at SpaceX's Launch Complex 40 at Cape Canaveral Air Force Station, an anomaly took place about eight minutes in advance of a scheduled test firing of a Falcon 9 rocket.





The anomaly on the pad resulted in the loss of the vehicle.





This was part of a standard pre-launch static fire to demonstrate the health of the vehicle prior to an eventual launch.





At the time of the loss, the launch vehicle was vertical and in the process of being fueled for the test. At this time, the data indicates the anomaly originated around the upper stage liquid oxygen tank. Per standard operating procedure, all personnel were clear of the pad. There were no injuries. Overview of the incident:

SpaceX emphasized that its "number one priority is to safely and reliably return to flight for [its] customers, as well as to take all the necessary steps to ensure the highest possible levels of safety for future crewed missions with the Falcon 9."

Read the full updated statement from SpaceX on the its website.

— CNBC's Robert Ferris contributed to this report.