Musk has previously hinted that the event might show off the completed prototype ahead of its first suborbital flight. There's also a possibility that SpaceX will narrow down the time frame for that flight, which might happen as soon as October. Other updates may include design tweaks and revisions of the long-term schedule. Many details aren't set in stone, and there's a chance you may see significant changes.

As it stands now, the Starship prototype will have three Raptor engines (six in the finished version) and should reach an altitude of up to 12.5 miles before returning to Earth. The final craft is meant to enter commercial service by 2021, complete a tourist trip around the Moon by 2023 and eventually conduct missions around the Solar System. It's not certain that SpaceX will reach these goals, however. The firm is known for its optimistic timelines (it first expected a Falcon Heavy test flight in 2013), and hasn't even constructed the Super Heavy Booster needed for more ambitious missions. A lot needs to fall into place for SpaceX to meet its goals, and you may have a better sense of its progress after the presentation.

Update 9/28 6PM ET: SpaceX has bumped the update to "no earlier than" 9PM Eastern from the original 8PM due to weather. We've updated the story accordingly.

Update 2: The YouTube stream indicates things should begin around 9:15PM ET.