The Falcon Heavy rocket will launch at the end of this month, SpaceX founder Elon Musk said Thursday night.

“Hold-down test fire next week. Launch end of the month,” Musk writes in a post on his Instagram.

The official launch date for the new rocket system had not been announced, but it was believed it might happen around early to mid-January, based on previous comments from SpaceX president Gwynne Shotwell.

When it launches, the Falcon Heavy will be the “most powerful rocket in the world,” say SpaceX officials. Its first stage is made up of a combined three Falcon 9 engine cores and 27 Merlin engines.

“With more than 5 million pounds of thrust at liftoff — equal to approximately eighteen 747 aircraft at full power — Falcon Heavy will be the most powerful operational rocket in the world by a factor of two,” the company said on its Instagram account earlier this month.

When the rocket launches from the Kennedy Space Center, it will send Musk’s own Tesla Roadster, looping David Bowie’s “Space Oddity” into a billion-year orbit around Mars.

A Falcon 9 rocket booster lands on the SpaceX droneship Of Course I Still Love You in this file photo.

If all goes as planned, and the rocket doesn’t blow up on the launch pad or during ascent, the three boosters will all land back on Earth. The center core will land on the drone ship Of Course I Still Love You off the coast of Florida, and the two side boosters will land on pads at Cape Canaveral, Florida, near the launch site.

The new rocket has been a long time coming. Musk first announced the Falcon Heavy in April 2011 with a 2013 launch date. This date slipped after it became apparent that the company couldn’t strap three Falcon 9 rockets together and call it a day, with the Falcon Heavy’s development shifting into a far more complex affair.

Falcon Heavy Demo Mission
 The Falcon Heavy on the launch pad at Kennedy Space Center in Cape Canaveral, Florida.

In July, Musk teased what we can expect for the inaugural launch of the Falcon Heavy rocket, at the 2017 ISS R&D Conference in Washington, D.C. He had this wry line to sum up his feelings on the entire project: “Major pucker factor.”

“There’s, like, a lot that can go wrong there,” Musk said at the time. He encouraged people to come down to Florida to watch the launch, though. “It’s guaranteed to be exciting,” he said.

Musk reiterated those cautious — this-thing-might-explode — comments Thursday in the same Instagram update: “Excitement on launch day guaranteed, one way or another.”