Today is the day. At 4:33 pm ET, SpaceX is on track to become the first private space company to deliver NASA astronauts to the International Space Station (ISS).

The final launch preparations for the Demo-2 mission are currently taking place at Cape Canaveral, Florida, and we're here to bring you a live blog of the final countdown and the launch as it happens - weather pending.

The two astronauts already strapped in and aboard the Crew Dragon will be Bob Behnken and Doug Hurley, both veteran spacefarers with several missions under their belts. This will mark the first time since the 2011 end of the Space Shuttle program that American spacecraft will carry NASA astronauts to the ISS.

You can watch the SpaceX livestream here:

LIVE BLOG (all timestamps ET/refresh to see the latest):

3:30 pm: While we wait, here's a brief overview of SpaceX's history - from early failures to successfully docking the Dragon capsule at the ISS in 2012, and landing the Falcon 9 re-usable rocket in 2015.

3:31 pm: While excitement is mounting, we're still waiting to hear for conditions to clear for #LaunchAmerica: a weather briefing is expected shortly. The instantaneous launch window is at 4:33 pm, and that will be the only chance today.

3:33 pm: T minus one hour!

3:36 pm: We've now heard one "go" on the comms, but here's what we are expecting next: first up, we need a "go/no go", which we're expecting at T minus 45 minutes… If that happens, the crew access arm will retract at T minus 42 minutes. As we've already heard, the launch window is tight today, so there's not much wiggle room in the countdown.

3:44 pm: While we wait for that crucial final announcement, we've been hearing more about the two astronauts already strapped in the Crew Dragon capsule. You can read more about them and their preparations here!

3:46 pm: The weather is still in the red…

3:48 pm: We're at "go" for propellant load! The astronauts are currently hearing a safety briefing reminder for evacuation procedures over the comms; the crew access arm is retracting!

3:50 pm: The weather is trending in the right direction! But this whole thing could still be postponed, so we can't get too excited just yet. Absolutely nail-biting…

3:57 pm: We're about a minute away from propellant load starting. Goosebumps!

3:59 pm: Propellant load has started! Liquid oxygen is being pumped into the first stage of Falcon 9, and RP-1 kerosene is flowing into both stages. Those two propellants won't ignite on their own - the rocket uses the addition of a third fluid known as TEA-TEB (triethylaluminium-triethylborane) to ignite the engines. That third fluid ignites spontaneously when it comes into contact with oxygen, this is known as a pyrophoric substance.

4:06 pm: WEATHER UPDATE! The launch team is currently assessing the situation with lightning, and we'll know in about six minutes if we're launching or not. Oh my god.

4:10 pm: Apparently, 86 percent of people queried in a survey earlier said they would love to go to space if they had the chance. If that was amongst the live-stream viewers, we have a bit of a self-selected audience there… While we still wait for that final weather ruling, here's a story by our senior writer Michelle Starr on how living in space might shape human evolution in the future.

4:13 pm: Stage two propellant load complete. There are clouds of liquid oxygen steaming away, "this is perfectly normal", we are assured.

4:14 pm: Uh oh, it's not looking good regarding weather.

4:16 pm: OH NOOOOOO.

4:17 pm: The launch has been aborted at T minus 16 minutes. We're heartbroken. The launch is now going into a normal scrubbing sequence the team practices every single launch. The hardware was looking great, and all propellant save the liquid oxygen in the second stage was loaded, but the weather just did not clear on time. Ten minutes after the instantaneous launch window it would have potentially been okay, but it was not to be.

4:19 pm: It's not all lost! The next launch window is on Saturday 30 May, a little earlier in the day, at 3:22 pm ET.

4:21 pm: Now what? Well, the team has to pump out all that propellant… this will take about 40 minutes. Once that has taken place, the crew access arm will move back into position, and Bob Behnken and Doug Hurley will climb out of the capsule.

4:25 pm: The livestream will stay up until the astronauts leave the vehicle. The two astronauts just have to sit tight.

4:26 pm: What's really good news is that, from a hardware perspective, everything went flawlessly. The team can treat this as an additional rehearsal, and it was looking really, really good. But with a tropical storm forming off the coast of South Carolina, today was just not the day.

4:28 pm: Why wasn't the launch scrubbed earlier, if weather was looking iffy? Well, Florida weather is highly variable, and sometimes it's possible to get clear conditions right in the launch window, even if things are looking weird a few hours out. That's why launches such as this one continue checks all the way up to the last possible moment.

4:32 pm: It would have been T minus 1 minute right now… Sigh.

4:33 pm: Well, that's all for today, folks. We'll be wrapping this live blog early for, uh, an obvious reason. As we noted, the livestream will continue as the scrub procedures continue. In the meantime, let's hope for clear skies on Saturday!