Update, 3:45 pm ET: Relativity Space got to within 70 seconds of launching the Terran 1 rocket Wednesday before it halted the countdown due to an "out of bounds" temperature reading for methane in the upper stage. Although the company recycled the countdown for a second attempt during the three-hour launch window, it was called off with time to spare.

"Thanks for playing," launch director Clay Walker told his team.

The company's webcast did not provide a reason for ultimately scrubbing on Wednesday, although it probably was the propellant temperature issue. "The team is working diligently toward our next launch window in the coming days," Relativity tweeted after the webcast ended.

Original post: Today's the day—probably—for Relativity Space to attempt its first launch of the small-lift Terran 1 vehicle.

The three-hour launch window opens at 1 pm ET (18:00 UTC), and weather conditions at the company's Cape Canaveral, Florida, launch site appear to be ideal. The biggest threat to a liftoff today, almost certainly, is some issue during the countdown with the vehicle or ground systems, as commonly occurs with new rockets.

If the rocket does lift off, then nominally, the Terran 1 will reach a 365 km by 373 km orbit at precisely eight minutes. But it's very far from clear that the launch of the Terran 1 rocket, the majority of which was additively manufactured by large 3D printers, will go as planned. In recognition that this is purely a test flight, Relativity has put no customer payloads on the flight. And the mission has a lighthearted name, "Good Luck, Have Fun," that acknowledges there is a bit of a hold-my-beer aspect to this test flight.

You should set your expectations accordingly. No private company has ever launched its first independently developed, liquid-fueled rocket and had it reach orbit on the first try. And Relativity is pushing a lot of boundaries with its methane-fueled booster. Probably the biggest test here is whether the 3D-printed structure of Terran 1 can withstand the dynamic pressure of ascent through the lower atmosphere.

A nominal first-stage performance would therefore be a significant achievement, as it would validate the company's approach to manufacturing. Ground testing can only take you so far; with rockets, the ultimate test comes in flight.

If the Terran 1 beats the odds and reaches orbit, it would become the first methane-fueled rocket to ever do so. Last December, a quasi-private Chinese company, Landspace, failed in its effort to put the methane-fueled Zhuque-2 into orbit. A second-stage mishap doomed the vehicle. Other methane rockets are coming as well, including United Launch Alliance's Vulcan booster and SpaceX's Starship, so this is probably the Terran 1 rocket's only chance to claim the title of "first methane rocket to orbit."

Relativity Space is one of the more intriguing new space launch companies. It was founded seven years ago by Tim Ellis and Jordan Noone with the goal of 3D-printing rockets on Earth and, one day, Mars. Since then, Ellis has proven to be a superstar fundraiser, bringing in more than $1 billion and building a series of successively larger factories in Southern California.

The company has gone through four generations of Stargate metal 3D printers along the way and has hired well-recognized names in the launch industry. Today, Relativity is one of the most ambitious, well-capitalized, and talented launch companies in the world. Within a few years, it aspires to be flying a large, fully reusable rocket named Terran R.

That's all well and good, but launch is the acid test. If you're going to be a real rocket company, you have to build rockets that can reach orbit. Today, Relativity Space will attempt to become a real rocket company.

A webcast for the debut launch of Terran 1 will begin about an hour before the launch window opens.