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Japan’s Kaguya spacecraft has released its second mini-probe into orbit around the Moon. The two ‘baby’ probes will work in concert with their mothership to reveal the Moon’s internal structure by making sensitive measurements of its gravity field.

Formerly known as SELENE, for SELenological and ENgineering Explorer, Kaguya is named after a character from a Japanese legend who travels to the Moon. It blasted into space on 14 September and entered lunar orbit on 4 October.

The two mini-spacecraft are each just 1 metre long with a mass of about 50 kilograms, while the main spacecraft, which weighs in at nearly 3 tonnes and measures about 5 metres long.

They clung to the main spacecraft during launch, but detached on 9 and 12 October. They are called Okina and Ouna, after the foster parents of princess Kaguya in the legend.

Guardian angels

“[The two] satellites are flying in a higher orbit than the main satellite, Kaguya, as if they are watching over the Kaguya as its guardians,” a statement from the Japan Aerospace Exploration Agency (JAXA) says. “Therefore, we selected Okina and Ouna as their nicknames.”

Okina will travel in an elongated orbit that is 100 kilometres from the Moon at one end and 2400 kilometres at the other, while Ouna’s path will take it from 100 to 800 kilometres from the Moon.

Using just 70 watts of power each, about what is needed to power a light bulb, the two probes, together with the main spacecraft, will measure the Moon’s gravity field to probe the distribution of mass in its interior.

Far side

The motions of the three spacecraft, which are sensitive to variations in the gravity field, will be measured from Earth by tracking their radio signals. Okina and Ouna will beam radio signals directly to Earth, and Okina will also relay signals to Earth from Kaguya while the main spacecraft is out of contact on the far side of the Moon.

The Moon has a much “lumpier” gravity field than Earth because its mass is not as smoothly distributed inside. These variations cause spacecraft orbits to change over time, a problem better gravity maps may help solve, says Paul Spudis of Johns Hopkins University in Laurel, Maryland, US.

“If people go, or even [for] future unmanned spacecraft, the better you know the gravity field, the better you can find stable orbits,” he told New ¾«¶«´«Ã½ before the launch.

High-definition videos

It may also help scientists figure out the events that made the Moon’s crust uneven in thickness, with the thickest part on the far side, he says, something that would have been set early in the Moon’s early history.

The mission will start collecting science data in a couple of months, says Kaguya project manager Yoshisada Takizawa. “The checkout of the mission instruments will be finished in December,” he told New ¾«¶«´«Ã½. “After the mission instruments’ checkout, the planned observation of the Moon will be started.”

Between now and then, the main spacecraft also needs to finish adjusting its orbit from the current elliptical one into a circular orbit 100 kilometres above the lunar surface.

Since reaching space, Kaguya has recorded some high-definition videos of Earth. JAXA has released a still frame from the footage, showing Earth from 110,000 kilometres away (see image at right).