The most ambitious of the experiments that gradually accreted onto MUSES-B, however, was also its scientific centerpiece, a small penetrator intended to be fired from the spacecraft as it orbited an asteroid and, as the term “penetrator” implies, penetrate into its outer crust. Penetrators had been proposed for use exploring the Moon, Mars, and minor planets since the 1970s, and in theory had many potential advantages compared to conventional landers for exploring the upper subsurface of those bodies. However, for various reasons none had ever been launched, so that these advantages remained unproven. While small, the Japanese penetrator would at least begin to show whether or not penetrators were actually practical tools of inquiry. Even better, the penetrator could be used to demonstrate one of the newest and least-developed forms of asteroid deflection, kinetic bombardment, where a stream of projectiles would be launched to gradually change the orbit of a threatening body. By actually launching a small projectile into an asteroid, MUSES-B could show the effects such a projectile would have on the target body and experimentally demonstrate the velocity change that could be expected from such an object if it were used to deflect a threatening asteroid or comet. The role of the main spacecraft would be to transport the penetrator to the asteroid and serve as a communications relay between the penetrator and Earth, although it would also carry spectrometers to help extend the penetrator’s precise but localized compositional data to the rest of the body, and a camera for navigation and public relations purposes.
After more than five years of research and development, MUSES-B was launched aboard an M-V rocket in late 2012, bound for the asteroid Itokawa, which had been discovered only a few years earlier by one of the automated asteroid searches that had been created since the 1990s and renamed after the “father of Japanese rocketry,” Hideo Itokawa, after its selection as the target of MUSES-B. The spacecraft itself was renamed Yumi, or “bow,” while its accompanying penetrator was named Ya, or “arrow,” after the launch, as with usual Japanese practice. Shortly after injection into interplanetary space, Yumi began firing its ion engines, gradually building up speed as it flew towards Itokawa. It took more than two years for it to rendezvous with the asteroid, but earlier this year it finally reached Itokawa, and is currently settling into its final science orbit. Mission controllers say that they are preparing to fire Ya later this year, and are currently debating site selection using Yumi’s images of Itokawa’s surface.
