This compilation photo shows New Horizons’s view of Pluto in its correct position relative to the background stars as seen by the Canada France Hawaii Telescope.Stephen Gwyn
The evening of July 8, 2013, was especially clear and calm on the summit of Mauna Kea when astronomers turned the Canada France Hawaii Telescope toward a star-speckled patch of sky that is about to be at the focal point of world attention.
The razor-sharp image they captured that night was in answer to a special request from the team behind New Horizons, the NASA probe that on Tuesday will become the first to reach Pluto, the most distant object ever explored by a spacecraft.
Just like any family on a holiday road trip, New Horizons is using a map to safely negotiate its way through an unfamiliar place – a map made in Canada.
"I'm ecstatic," said Stephen Gwyn, a data specialist with the Canadian Astronomy Data Centre in Victoria, part of the National Research Council, and the creator of a customized star catalogue that is helping guide New Horizons toward its historic rendezvous.
Perched on the windward side of Mauna Kea at what is considered the best observing site on Earth, the Canada France Hawaii Telescope combines a wide-field camera with the ability to make pinpoint measurements of the positions of stars.
In combination with a method of analysis developed by Dr. Gwyn, the camera is so precise that were one of its pictures blown up to one kilometre in size, the locations of the stars within the picture would be correctly represented to within a few millimetres.
That translates into seeing a lot of stars all at once while knowing precisely where they are – creating the ideal reference field for New Horizons as it speeds toward Pluto.
The spacecraft, which is on course to skim past Pluto around 7:49 a.m. ET on Tuesday, is travelling at nearly 50,000 kilometres an hour. Over the next two days, the biggest risk it faces is the possibility of smacking into a small, undetected moon or bit of debris circling around Pluto.
To mitigate that risk, the mission team has been using the probe's own cameras to scout ahead for potential hazards. But when those hazards look like faint dots against a backdrop of stars, it's not so easy to pick them out.
"You can take a picture, but you need to know where those things really are," Dr. Gwyn said.
In recent weeks the probe's navigation team has used Dr. Gwyn's reference field to look for anything unexpected drifting in the foreground and to make sure the spacecraft is heading precisely where it needs to go.
He recalled how 25 years ago, when he was a summer student working on sailing tours of the Pacific northwest, he would sometimes navigate using a sextant.
"That's sort of what I'm doing now," he said.
New Horizons is currently a staggering 4.7 billion kilometres from Earth. On Sunday, Glen Fountain, the mission's project manager, said the spacecraft is comfortably on track to hit its "delivery box" – a rectangular target in space about 240 km tall by 100 km wide – at the moment when it's at its closest point to Pluto. There has been no further tweaking of the probe's trajectory since a course correction was made about 15 days ago.
"It's best not to take any chances if you don't need to," said Mr. Fountain during a news briefing at the Johns Hopkins University Applied Physics Laboratory, mission control for New Horizons.
The pace of the mission has been picking up dramatically as the encounter approaches and New Horizons begins to work through a carefully planned sequence of science observations to gain as much information as it can about Pluto and its moons, "like a ballet that gets faster and faster as we get closer," Mr. Fountain said.
The spacecraft is scheduled to break off communications with Earth on Monday night at 11:17 p.m. it can position its cameras to get what will be its best images of Pluto's mysterious surface. If all goes well, a full day of silence will be broken around 8:53 p.m. on Tuesday when New Horizons is scheduled to phone home after the encounter.
In the interim, "we try not to think about the things we fear the most," said mission operations manager Alice Bowman.
Dr. Gwyn said that after working with the mission for two years he was looking forward to seeing the results of the encounter, but he'll have to wait a bit longer that the rest of us – he'll be away on a sailing trip this week.
Asked if he'll be navigating with a sextant as he did years ago, Dr. Gwyn laughed and said, "No, I've switched to GPS."
Ivan Semeniuk