Elon Musk has big space ambitions (are there any other kind?), and stoked interest in a pending “really special” announcement by teasing images of proposed bases on Mars and the moon — dubbed “Mars City” and “Moon Base Alpha” — Thursday night.

Musk, chief executive of SpaceX and Tesla Inc. TSLA, -4.69% , among others, is scheduled to speak at the International Astronautical Congress in Australia early Friday morning, Eastern time. Earlier in the evening, Musk tweeted he “Will be announcing something really special at today’s talk” related to “making humanity multi-planetary.”