2017-08-08 11:02:02
Chasing Shadows for a Glimpse of a Tiny World Beyond Pluto

11:02, August 08 43 0

This summer, scientists crisscrossed two oceans, braved wind and cold and deployed two dozen telescopes — all for five blinks of starlight that lasted a second or less.

For the team working with NASA’s New Horizons spacecraft, which made a spectacular flyby of Pluto two years ago, those smidgens of data provide intriguing hints about the spacecraft’s next destination, a distant frozen world that is believed to be a pristine, undisturbed fragment from the earliest days of the solar system.

New Horizons will fly past it on Jan. 1, 2019.

But the object is so far away — a billion miles beyond Pluto — and so small — no more than 20 miles wide — that almost nothing was known about it.

From the five blinks, obtained with exhausting effort, scientists now know that it has an odd shape.

Instead of round like a ball it appears to be more like a long, skinny potato — or maybe two objects in close orbit around each other, possibly even touching.

“It’s like, wow, this is going to be really cool,” said Marc W. Buie, an astronomer at the Southwest Research Institute in Boulder, Colo., who led the observations. “We don’t know what we’re going to find.”

While Pluto is the biggest object in the ring of icy debris beyond Neptune known as the Kuiper belt, this object with the designation 2014 MU69 is among the smallest. It orbits more than four billion miles from the sun, and it is like a time capsule, promising clues about how the planets formed.

Astronomers using the Hubble Space Telescope first spied it three years ago as they searched for somewhere for New Horizons to visit after Pluto.

All Hubble could see was a slowly moving speck of light — enough to calculate an orbit and determine that New Horizons could reach it. But almost everything else about MU69 was a mystery or a guess. Not even the largest, most powerful telescopes on Earth can see it at all.

The New Horizons scientists could, however, learn more about it during a few chance moments when a star in the night sky momentarily vanished because MU69 passed in front of it.

From the distance to MU69, its speed and how long the star winks out, astronomers can calculate the width of the object.

It turned out that a bonanza of three such events, which are known as occultations, were expected to occur within a two-month period this year, on June 3, July 10 and July 17, as MU69 passed in front of three different stars.

“That’s just crazy,” Dr. Buie said. “That’s just how the dice rolled on this one.”

But they did not have the data — sufficiently precise star locations — to know exactly where to set up telescopes to observe the events.

“We say it’s hard, and it is,” Dr. Buie said. “If you’re not in the shadow, you don’t see it. Even a year ago, this would have been categorically impossible.”

But they were lucky again because a European Space Agency mission called Gaia was methodically mapping more than a billion stars in the Milky Way galaxy. New Horizons scientists got an early look at the catalog in late April.

Dr. Buie had been scrambling to gather equipment and observers, and on June 3, 50 team members turned 24 telescopes on two continents, in Argentina and South Africa, to the sky.

The telescopes took more than 100,000 images of the star, waiting for MU69 to pass in front of it.

But the star never vanished. The scientists had completely missed the shadow.

“I was physically and emotionally exhausted, psychically damaged,” Dr. Buie said.

In late June and early July, Hubble made additional observations of MU69 that refined the orbit.

The July 10 occultation track mostly passed over the southern Pacific. This time, the New Horizons scientists took off from Christchurch, New Zealand, in Sofia, a NASA 747 equipped with an 8.2-foot-diameter telescope. They headed north, toward Fiji, to intercept the shadow and returned 10 hours later.

Again, the star never disappeared. They had missed the opportunity again.

They immediately headed back to Argentina for one more try.

S. Alan Stern, the principal investigator of New Horizons, said he was confident, with the additional Hubble measurements, that they would capture the vanishing of a star this time. But mission managers also always worry about the so-called unknown unknowns. Perhaps something unanticipated in the Hubble data was deceiving them about MU69’s position.

The July 17 shadow was predicted to pass over Comodoro Rivadavia, a city along the Atlantic coast of South America. Comodoro’s nickname is “the capital of wind.” In the middle of winter, the weather was also cold.

“It was a pretty intense event,” Dr. Stern said. “Your telescopes were shaking.”

At several observing sites, tractor-trailer trucks served as windbreaks, as did contraptions made of poles and canvas.

A highway was shut for a couple of hours so that the headlights of cars and trucks would not spoil the observations.

The skies were clear, and the time of the shadow, 12:50 a.m., passed.

A few hours later, Amanda M. Zangari, one of the Southwest Research Institute scientists on the New Horizons team, was staring blearily at her laptop analyzing the data from one of the telescopes.

In her exhaustion, the data was not making sense to her. Then it hit her. “I realized it didn’t make any sense, because the occultation star was missing,” she said.

This is what she saw:

Five telescopes, it turned out, had detected the star’s vanishing for up to about a second.

The success also confirmed that the Sofia observations a week earlier had barely missed the occultation and close enough to the shadow to be useful. Preliminary analysis found no signs of dimming, indicating there are no clouds of debris in the neighborhood of MU69 that could imperil New Horizons.

The five blinks established the odd shape. If it is one skinny potato, MU69 is no more than 20 miles long. If it is two spheres circling each other, each is about nine to 12 miles wide.

Many Kuiper belt objects in this region are binaries, although most are considerably larger than MU69. If one this small can be a binary, that may change the understanding of how Kuiper belt objects formed.

Mission managers can still tweak the flyby time of the New Horizons spacecraft by a couple of hours. Ideally, they want to view the broad side of MU69 and optimize geometry of tracking stations on Earth during the flyby.

“We’re working through all those mathematical issues,” Dr. Stern said.

Another occultation is possible in August next year. The scientists have not decided whether they will chase the shadow one more time.