"In contrast to optical telescopes, which capture the reflected light from the Sun, the new ALMA images show the actual millimeter-wavelength light emitted by the asteroid," said Todd Hunter, an astronomer with the National Radio Astronomy Observatory (NRAO) in Charlottesville, Va., according to a press release. The paper detailing the findings has been accepted for publication in the Astrophysical Journal, Letters.

"By using ALMA to image the thermal glow from asteroids like Juno and other bodies in our solar system, astronomers will be able to study their shape, composition, and surface properties," Hunter said, according to the press release.

The complete set of images documents about 60 percent of one asteroid rotation. The 10 images took four hours to capture on Oct. 19, 2014 when Juno was about 295 million kilometers from Earth. The axis of rotation was tilted away from the Earth, so the southern hemisphere is predominant.

ALMA achieved a resolution of 40 milliarcseconds, "meaning that each 'pixel' in the images is about 60 kilometers across, covering approximately one fourth of the surface of Juno," according to the press release. "This resolution is a vast improvement over earlier observations made at similar wavelengths and is enough to clearly resolve the shape of the asteroid and potentially tease out prominent surface features."

Juno's next close approach to Earth will be in November 2018. Juno will actually be much closer than it was in 2014, so ALMA will be able to double its resolution.

"This new observation clearly demonstrates that ALMA will be a very powerful tool for studying asteroids," said NRAO astronomer Arielle Moullet, according to the press release. "At its highest resolution, ALMA is powerful enough to resolve the surface of many asteroids."