Emre Kelly

FLORIDA TODAY

All eyes are on SpaceX today as it attempts to relaunch and land a previously flown – or "flight proven," as the company prefers to call it – Falcon 9 rocket from Kennedy Space Center.

SpaceX and its commercial satellite partner, SES, are targeting 6:27 p.m. for the opening of a launch window at pad 39A that closes at 8:57 p.m.

Here are five things you need to know about the launch:

1. First flight proven attempt for SpaceX

Today marks the first time that California-based SpaceX will try to launch and land a refurbished Falcon 9 first stage.

It's a critical mission for CEO Elon Musk, who envisions a future of reusable rockets that can be flown, landed and reflown at least several times before reaching operational limits.

After liftoff, the flight proven first stage will attempt to again land on the company's "Of Course I Still Love You" drone ship in the Atlantic Ocean, which is usually stationed several hundred miles off the coast of Florida. The ship is expected to return to its home at Port Canaveral with the Falcon 9 a few days after launch.

2. First stage originally flew on mission to ISS

The first stage being reflown originally took flight in April 2016 from Cape Canaveral Air Force Station with supplies for the International Space Station. After flying more than 4,000 mph and dropping about 87 miles, it landed on "Of Course I Still Love You" in the Atlantic, making it the company's first ever successful ship landing.

During a press event with reporters at Port Canaveral on Tuesday, SES Chief Technology Officer Martin Halliwell said SpaceX provided SES with "tremendous transparency" into many aspects of the first stage, ranging from design to avionics to engines. He said the nine Merlin main engines are the original ones flown in 2016.

[How to watch today's launch attempt]

3. On board: SES-10

Atop the 230-foot Falcon 9 will be SES-10, a commercial communications satellite built by Airbus Defence and Space and operated by Luxembourg-based SES. The 11,700-pound spacecraft uses three separate beams, each of which target different regions of Latin America for transmission of video and data.

The satellite will predominantly be used for video, including the expansion of ultra-high definition signals in the region.

Of the three more SES satellite launches scheduled for 2017, Halliwell said the company is considering flying two of them on flight proven first stages – if the SES-10 mission is successful.

"We're fully confident in the work because that may give us a little bit more security in our manifesting position," Halliwell said. "And that's really important to us."

[10 great places to watch a rocket launch]

4. Weather conditions

According to the Air Force, weather is 80 percent "go" for today's launch attempt, but the Eastern Range sees a decline to 40 percent "go" for the backup launch date on Friday. Weather rules involving cumulus and thick clouds are primary concerns.

Sunset is expected at 7:39 p.m. and 7:40 p.m., respectively.

5. Fourth Falcon 9 flight this year

SpaceX kicked off 2017 with a successful return to flight from Vandenberg Air Force Base on Jan. 14 after an explosion in September grounded Falcon 9 rockets for four months. The mission blasted off with 10 Iridium NEXT communications satellites and successfully landed on a drone ship in the Pacific Ocean.

In Florida, SpaceX followed in the footsteps of the Apollo and space shuttle programs when it launched a Falcon 9 from Kennedy Space Center's historic pad 39A. On board was a Dragon spacecraft that delivered more than 5,000 pounds of supplies and science to the International Space Station. The first stage successfully landed at nearby "Landing Zone 1" at Cape Canaveral Air Force Station.

Finally, SpaceX most recently launched on March 16 with the EchoStar 23 communications satellite. The mission, also launched from pad 39A at KSC, successfully deployed the satellite but did not involve a landing attempt due to the Falcon 9 needing all its fuel to deploy the 12,300-pound spacecraft.

Contact Emre Kelly at aekelly@floridatoday.com or 321-242-3715. Follow him on Twitter and Facebook at @EmreKelly.

[Mysterious equipment spotted on SpaceX drone ship at Port Canaveral]

Launch Thursday

Rocket: SpaceX Falcon 9

Mission: SES-10 commercial communications satellite

Launch Time: 6:27 p.m.

Launch Window: to 8:57 p.m.

Launch Complex: 39A at Kennedy Space Center

Weather: 80 percent "go"

Join floridatoday.com starting at 5 p.m. for countdown chat and updates, including streaming of SpaceX's launch webcast starting about 20 minutes before liftoff.