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SYFY WIRE Bad Astronomy

The wink of a star reveals a distant target for New Horizons

By Phil Plait
artwork of New Horizons and MU69

Just over two years ago, the New Horizons spacecraft provided humanity with its first close-up photos of Pluto in history.

These images changed the way we see the icy world forever. What we learned was staggering. It has vast, smooth regions on its surface indicating they’re geologically young; mountains as tall as the Rockies but made entirely of water ice; strong implications of liquid water under its surface despite the bone-shattering cold temperatures on the surface.

The close encounter lasted only a few hours, because you have a choice: Get to Pluto in less than a lifetime, or spend more time there. Pluto is so far away that even New Horizons, barreling across the solar system at 14 kilometers every second, still took nearly a decade to get there. It was traveling so rapidly that the visit was short.

But, despite the rapid flyby, there’s an advantage to moving faster than a speeding bullet: There are other targets out there in the inky depths of the outer solar system, and if you plan things right, you might just get to see them, too.

Even before the Pluto encounter, astronomers started trolling that region of space to look for another suitable target. They found one: 2014 MU69, an icy chunk of debris likely at most 20-40 kilometers across. It orbits far, far past Neptune, 6.5 billion kilometers from the Sun. It’s part of the Kuiper Belt, a ragtag collection of material left over from the formation of the solar system itself. If you don’t count Pluto (and I do), the first Kuiper Belt Object seen was only in 1992, and we now know of thousands.

But they’re so far away and so small that it’s hard to know what they’re like in detail. And that’s why MU69 is so important. New Horizons will show it to us up close for the first time.

The plan is for the spacecraft to fly within 10,000 km of MU69 on January 1, 2019. Maybe closer. But, to do that, we need to know more about it. How big is it? What shape is it? Is there anything else around it that could interfere with the flyby, like moons, rings, or debris?

These things are difficult to determine, but astronomers got a big clue this week due to geometry. In this case, the stars literally aligned.

Well, the Earth, MU69, and a star aligned. On July 17, 2017, from certain points on Earth, MU69 appeared to pass directly in front of a faint star. Astronomers call this kind of event an occultation, and when it happens, the star’s light is blocked, and it seems to momentarily disappear! In a sense, in this case, we’re in the shadow of MU69.

The occultation provides critical information: Because we know how fast MU69 is moving across the sky, the length of time the star blinks out tells us the width of MU69.

But there’s more. If you observe the occultation from different locations, you see different parts of MU69 passing in front of the star. If it’s a perfect sphere, then some locations will see a shorter occultation because the star cuts a chord behind it, not the full diameter. In fact, the shape itself can be determined by how long the occultation lasts at different positions on Earth.

map of occultation

So New Horizons scientists dispatched telescopes to South America, where the shadow of MU69 was determined to fall across the Earth. In all, a couple of dozen small (40 cm) ‘scopes were deployed, equipped with cameras to record the event.

And … they caught it! At least five telescopes saw the star blink out. That, too, is very useful: If a ‘scope didn’t see it, then that provides an upper limit to the size of MU69 as well. The entire occultation lasted less than two seconds, too, so timing and location were everything here.

animation of occultation

The data are still being processed, and we should have some numbers soon. I’ll note that there were two predicted occultations of two different stars before July 17, but nothing was seen. That means MU69 is probably smaller than previously thought, which, in turn, means it might be more reflective — if we know the distance and how bright it is, then its size depends on how shiny it is. A darker object would have to be bigger to look brighter, so even this non-detection tells us more about it.

My friend and super-solar-system-science communicator Emily Lakdawalla has more about the efforts to record this event. She also wrote a nice piece on what we knew about MU69 from a couple of years back, too.

I can’t stress enough just how difficult this sort of event is to plan! MU69 was only discovered in 2014 using Hubble images. It has a visual magnitude of 27 — that means the faintest star you can see with your unaided eye is 250 million times brighter! Then, using those images, the team had to calculate an orbit for it, and do so with such precision that they could extrapolate where it would be over the next year or two and see if it would pass in front of any stars. Then they had to plan the logistics of all that travel, coordinating the mission and making sure the data were recorded. Yet, as difficult as all that was, they were able to do it so well and with such accurate timing that several of the telescopes did in fact see the star blink out.

Mind you, MU69 is far, far too faint to even see with the telescopes used. So the astronomers had to keep taking data and hope.

And it paid off. Now, armed with more data, they’ll be able to plan the upcoming encounter with a little more confidence. As for what we’ll actually see when New Horizons gets to MU69, well, no one really knows.

If we did, it wouldn’t be exploration now, would it? But in less than 17 months, we’ll find out.