(Every day until December 25, I’m posting a science-related image or video and description.)

Day 15

While you go about your business, robots are exploring the Solar System in your name. The most recent addition to these ranks arrived on the Moon yesterday (December 14): the Chang’E 3 probe landed and deployed the Yutu (Jade Rabbit) rover. This marks the first soft landing of a mission on the Moon since the Soviet Luna 26 mission in 1976. To put it another way: this is only the second Moon landing in my lifetime. Emily Lakdawalla assembled several videos from the official broadcast, including the one above; you can see all of them at the Planetary Society blog.[1]

A significant number of other probes have orbited, so it’s not like we’ve ignored the Moon. However, some things are best done at ground level, such as chemical analyses of the regolith, the Moon’s “soil”. Yutu is smaller and less sophisticated than the Curiosity or Opportunity rovers currently active on Mars, but then again it doesn’t need to be. Depending on the relative position of Mars and Earth, a radio signal takes somewhere between a little over 3 minutes (when both planets are on the same side of the Sun) or as much as 22 minutes (when they’re on opposite sides) to travel from controllers on Earth to a rover on Mars. And that’s a one-way signal! To know if a command was executed properly or other responses, you have to double the amount of time.

Thus, it’s impractical to completely control a Mars rover remotely: it needs to act autonomously for the most part, with occasional input from humans. The Moon, by contrast, is about 1 light-second away, so a rover on the near side could be controlled pretty much in real time by operators on Earth. (A rover on the far side would be a completely different matter, likely requiring a dedicated relay station for communications.) A lag of two seconds is annoying in video games, but pretty reasonable for controlling a robot on the Moon — and unlike a video game, there’s no fireball-hurling demons to react to.

Yutu has six wheels and two large solar panels. Both the rover and the stationary platform are equipped with a variety of scientific instruments for studying the lunar surface, and a small ultraviolet telescope for astronomy work. (The Moon is a great place for astronomy, since it’s beyond Earth’s atmosphere while still providing a solid surface for an observatory.) Chang’E landed near the Sinus Iridum, or Bay of Rainbows, a relatively crater-free region of the Moon that flooded with lava in the distant past.

Yutu is named for the mythological pet of the moon goddess in Chinese folklore. The “rabbit in the Moon” is a bit of pareidolia from the patterns of lava-filled lowlands and cratered highlands, equivalent to the “man in the Moon” in European traditions. The lunar rabbit is also the inspiration for a very weird short film.

Note

1. See also Sean Carroll’s excellent post on the landing, which explains why I’m not making a big deal about the fact that this is a probe from China.