Japan's space agency says it's communicating with its un-crewed craft "Moon Sniper" after it landed on the lunar surface.
But scientists say the craft's solar generators aren't working - it's relying entirely on batteries.
Japan's space agency is now rushing to fix the solar generators before the batteries run out.
The landing means Japan becomes only the fifth country to land on the Moon after the US, the former Soviet Union, China and India.
Last year, India made history when it successfully landed a spacecraft near the south pole of the Moon.
The Japanese spacecraft aimed to land within 100m (328ft) of a location near the Shioli crater, on the near side of the Moon - scientists are still confirming if they've managed this.
The lander may have just a few hours of life ahead of it.
Officials are currently prioritising activities. They've turned off heaters and are now pulling down pictures from the craft and data that will tell them how well the landing software worked.
The agency won't immediately give up on Slim if it does fall silent. It's always possible the solar cells have somehow become oriented in a way that prevents them from seeing the Sun.
As the light changes angle, it's always possible Slim could come back to life. But the temperatures on the Moon in the dark get very low - often so low that they can break electronics.
