Astronomers turn eyes to New Horizons target beyond Pluto

The New Horizons space probe, which made headlines around the world in 2015 when it beamed back humanity’s best-ever views of Pluto, is currently hurtling through the outer reaches of the solar system on its way to a rendezvous with a lump of ice known as MU69.

New Horizons won’t get to MU69 for another year and a half – the flyby is expected to occur on 31 December 2018 or 1 January 2019 – so the spacecraft is hibernating to preserve its energy.

Meanwhile, scientists on Earth are doing everything they can to find out as much as possible about MU69 before New Horizons gets there. {%recommended 1147%}

MU69 sits in the Kuiper belt, a broad disc of small floating bodies out beyond Neptune at distances between 30 and 50 times as far from the Sun as Earth is. In some ways it mirrors the asteroid belt that occupies a ring between the orbits of Mars and Jupiter, though Kuiper belt objects are on the whole much icier than the rocky asteroids closer to the Sun. 

Very little is known about MU69: it was only discovered in 2014, when astronomers pointed the Hubble Space Telescope at the Kuiper belt to look for something interesting beyond Pluto that New Horizons might be able to manoeuvre itself towards, and because it is so tiny and distant it’s hard to get a good look.

What astronomers have been doing is watching very closely when MU69 passes in front of a background star. In these events, known as stellar occultations, observing the changes to the image of the star can reveal information about the size of MU69 and also whether there is any debris in the space around it that might pose a hazard to New Horizons.

The first such occultation occurred on 3 June, and more than 50 observers set up telescopes in South Africa and Argentina along the path that the tiny, almost imperceptible shadow of MU69 would trace across the Earth’s surface. Though the occultation lasted only 2 seconds, altogether the telescopes capture more than 100,000 images of the area of space around MU69.

While the observers did see the background star dimming, they were unable to make any direct observations of MU69, which suggests that it smaller than earlier estimates that it is perhaps 18–41 kilometres across. It is also possible that MU69 is actually a swarm of smaller objects, rather than a single lump.

Another occultation will occur on 10 July, which NASA’s airborne Stratospheric Observatory for Infrared Astronomy (SOFIA) will watch with its 2.5-metre telescope. A third is due on 17 July, which will be observed by the Hubble Space Telescope and more ground-based observers in southern Argentina. 

When all the data is in and analysed, the New Horizons team should have everything they need to make sure that when New Horizons does wake up on its journey through the void, it will have the best information possible to guide it on its way.