You know Juno? Yes, thanks to ALMA!

You know Juno? Yes, thanks to ALMA!

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On 19 October 2014, ALMA has made ten images of asteroid Juno. The images were made over a period of four hours. During that time, Juno rotated around its axis, so ALMA has seen different parts of the space rock. It’s as if someone took photos of your head while you were slowly turning around – the photos would show both your face and the back of your head.

Juno is one of the largest asteroids – rocky bodies that orbit the sun between the orbits of the planets Mars and Jupiter. It was the third asteroid that was discovered, back in 1804. It was named after the Roman goddess who was the daughter of Saturn, the wife of Jupiter and the mother of Mars.


But although Juno is some 250 kilometres across, it is hard to see in much detail, because of its large distance. During the ALMA observations, the asteroid was 295 million kilometres away. Still, ALMA was able to see the slightly elongated shape of Juno, thanks to the large separation between the individual ALMA antennas during the Long Baseline Campaign: up to 15 kilometres.

Asteroids like Juno are cold and dark. We can only see them because they reflect some sunlight. Still, they also emit their own – invisible – radiation. The surface of an asteroid is warmed by the sun. As a result, it glows at millimetre wavelengths – precisely the type of radiation that ALMA is sensitive to.

Astronomers had already discovered that the sunlight reflected by Juno varies every 7.2 hours, because of the rotation of the asteroid. From the brightness variations, it was deduced that Juno is a little bit egg-shaped. The new ALMA observations clearly show this slightly elongated shape.

By studying the millimetre radiation of an asteroid like Juno, astronomers hope to learn more about its composition (what types of rock is Juno made of) and its surface properties (is it smooth like a slab of stone, or porous like sand). In November 2018, Juno will again be relatively close to the Earth, so ALMA will be able to observe the asteroid again.

Juno (officially known as (3) Juno because it was the third asteroid to be discovered) was found by German astronomer Karl Ludwig Harding on 1 September 1804. Back then, it was listed as a new planet. Only later did astronomers realize there were so many small objects between the orbits of Mars and Jupiter that you couldn’t possibly call them all planets. These rocky bodies are now known as asteroids (or minor planets). Juno completes one orbit around the sun every 4.36 years.

The ALMA observations of Juno were carried out by a large international team of astronomers, led by Todd Hunter of the National Radio Astronomy Observatory in Charlottesville, Virginia. Todd was the calibration and imaging expert during the ALMA Long Baseline Campaign in the fall of 2014. The new results will be published in Astrophysical Journal Letters.