For the first time, we’re about to get a close look at Pluto and its cold, outer region of the solar system. The Post's Joel Achenbach explains NASA's New Horizons mission. (Tom LeGro/The Washington Post)

Barring a freak collision or some other cosmic disaster, the indefatigable NASA New Horizons spacecraft will fly within shouting distance of Pluto early Tuesday morning. The robotic probe has already captured dramatic images of the dwarf planet, which has mysterious dark spots, possible cliffs and craters, and an unusually large moon with a gash across its face the size of the Grand Canyon.

The speedy spacecraft is on track to pass about 7,750 miles from Pluto at 7:50 a.m. Eastern time Tuesday.

“People talk a lot about how surreal it is that we’re really here,” principal investigator Alan Stern said Monday. “It feels like you’ve been walking on an escalator for almost a decade, and then you step upon a supersonic transport.”

The New Horizons team, based at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., won’t know whether the spacecraft successfully passed through the Pluto system until 8:53 p.m. Tuesday. That’s when New Horizons is supposed to check in with a short burst of data to report that it’s healthy.

This 9 1/2 -year journey to a tiny keyhole in space some 3 billion miles from Earth is the equivalent of a golfer on the East Coast hitting a ball across the continent and making a hole-in-one in Los Angeles, said the project manager, Glen Fountain.

“Pluto is perfectly spherical, like every respectable planet,” Stern said at a public lecture Monday at APL, in a dig at the astronomers who in 2006 demoted Pluto from “planet” to “dwarf planet” status. It has also turned out to be slightly larger than previously estimated.

“I didn’t know what to expect, because [Pluto] is a point of light when seen from Earth. It’s fascinating. It’s complex. You’ve got these dark regions and this bright, heart-shaped region,” said planetary scientist Cathy Olkin, another member of the New Horizons team. “I’m thrilled by how it looks.”

This is not an encounter without risks. The Pluto system may have a fair amount of debris, and even a small rock could disable a spacecraft going upward of 31,000 mph. Stern said computer models show that the risk of a collision is 1 in 10,000.

Because Pluto is about 3 billion miles from Earth, New Horizons can’t be joysticked from mission operations at APL. The nine-hour round-trip communication time means the spacecraft has to function autonomously. Olkin said the probe must make about 600 maneuvers over a nine-day period that started last Tuesday; that will enable 433 separate observations.

The designers of this $720 million mission have built in some redundancy, with multiple observations and slight wiggle room on the timing. New Horizons, launched in 2006, has star-trackers and other guidance capabilities to orient itself correctly and turn toward Earth to transmit data. What it doesn’t have, primarily for cost reasons, according to Fountain, is the kind of robust sensing and computing power to lock onto a target — say, one of Pluto’s moons — if that target is not precisely where it had been anticipated to be.

One inevitable feature of exploring the outer solar system is that the technology is relatively ancient by the time the spacecraft gets there.

9 years and 3,000,000,000 miles to Pluto

“In some ways, these are technologies from the ’90s,” Fountain told reporters Sunday.

Critical to the intense period of observations during the flyby is knowing exactly where Pluto and its moons are at any given moment relative to the spacecraft. To that end, the New Horizons team obtained a number of old-fashioned photographic plates of Pluto from the 1930s at the Lowell Observatory in Arizona, where the planet was discovered in 1930 by astronomer Clyde Tombaugh. The plates were digitized and added to other data to create a better model of Pluto’s orbit.

New Horizons had a trial run of its observation sequence when it passed through the Jupiter system.

“We did all this work at Jupiter,” Fountain said. “We had a couple of miscues. We knew exactly what went wrong, so we modified it.”

This is a one-shot deal, with time so precious that for nearly 22 hours, from late Monday to late Tuesday, New Horizons won’t communicate with Earth. The team will receive its last bulletin from the spacecraft at 11:17 p.m. EDT Monday. Then everyone will wait and hope the probe is executing its commands properly.

If New Horizons doesn’t check in as expected Tuesday night, that wouldn’t necessarily mean something had gone disastrously wrong, mission operations manager Alice Bowman said. There are benign reasons the radio link across 3 billion miles of space might not work, she said.

“We’re out there on the frontier, you know?” Bowman said. “Things can happen. And things do happen. And we have to be prepared for that. If we don’t get the signal, then eight hours later we should have another opportunity to get that signal.”

The team already endured a July 4 weekend scare when the spacecraft’s workhorse computer became overloaded and forced the craft to hunker down for three days in a safe mode, with the backup computer in charge. New Horizons went into a controlled spin initially, a configuration that would have made the main scientific objectives impossible.

The computer problem meant the team lost three days of planned scientific observations. But Tuesday, the team managed to get the spaceship prepared anew for the most important part of the mission: the close encounter.

The computer drama left the team in a somewhat conservative mood, Fountain acknowledged.

“If you don’t need to change anything, you don’t. And that’s where we’re at,” he said.

He wants his team to enjoy the ride, tense though it may be.

“It’s a wonderful time to be alive and to be aware of what’s going on,” Fountain said.

Stern, who began pushing for a Pluto mission in 1988, looked buoyant Monday afternoon.

“It’s a rock-solid spacecraft,” Stern said. “We’ve flown this bird for 3,500 days and change. I’m very confident about tomorrow.”

Robert Gebelhoff and Rachel Feltman contributed to this report.