Just after midnight Wednesday, a Maryland-built spacecraft bound for distant Pluto will soar past a key milepost in its nine-year voyage -- giant Jupiter and its turbulent system of moons and rings.
It's not the first visit to Jupiter by a robotic mission from Earth. Six other spacecraft have passed by, and one, Galileo, orbited there for eight years.
But scientists say their $700 million New Horizons craft will give them a new perspective on the Jovian system and on secrets that were inaccessible to prior missions.
"With these high-tech instruments, we are going to produce stunning data sets, and we can't wait to get them on the ground," said Alan Stern of the Southwest Research Institute in Boulder, Colo., principal investigator on the New Horizons mission.
NASA's New Horizons was designed and built, and is managed and controlled, by scientists and engineers at the Johns Hopkins University's Applied Physics Laboratory near Laurel.
This morning, it is about 505 million miles from Earth, and 5.6 million miles from Jupiter, closing on the planet at a cool 44,200 mph.
Controllers at APL sent their final package of 45,000 commands to the spacecraft this week -- the instructions to guide New Horizons automatically through days of observations as it flies by Jupiter and its moons.
"It's a one-shot deal, and everything has to work," said APL's Hal Weaver, project scientist on the mission. "If we do get a hiccup, there is no chance for us to recover. ... We just have to miss out on some science we were hoping to get."
The most crucial aspect of Wednesday's flyby is putting New Horizons through a 500-mile-wide "keyhole" in space. That's where it must be to get the course change and 9,000 mph speed boost from Jupiter's gravity needed to cut its risk of failure by knocking three years off its flight to Pluto.
"It's the equivalent of skeet shooting from downtown Washington and hitting that skeet ... in Baltimore. That's a pretty tough shot, and we are right on course," Stern said.
New Horizons' primary mission is to visit Pluto and its two moons in July 2015, then to sail to the icy Kuiper Belt beyond. It will be mankind's first venture into what Stern called "a whole new zone of the solar system."
"It's really quite the frontier, beyond where the giant planets lie," Stern said. "We're opening up both a window into the deep, outer solar system and a window back in time, 4 1/2 billion years to the birth of the planets. We think it's very exciting."
Next week's Jupiter flyby is a kind of "stress test" for the mission after 18 years of planning and building. "Hopefully, we will turn up any small vulnerabilities and weaknesses, either in the ground-planning system or with the bird up in flight," Stern said.
"Everybody is feeling anxious," Weaver said. "We won't know until Wednesday afternoon if everything was successful and we really are still on course."
But Stern expressed confidence. "I don't feel very much anxiety," he said at a recent news briefing. "If I have any ... it's that somebody will wake me up and it's still three days from launch."
New Horizons was launched from Cape Canaveral on Jan. 19 last year. The fastest spacecraft ever rocketed from Earth orbit, it passed the moon's orbit in just nine hours, and Mars' orbit in three months.
On Wednesday, the spacecraft will pass within 1.4 million miles of Jupiter's center, just outside the orbits of its four largest moons -- Io, Ganymede, Europa and Callisto. That's four times closer than NASA's Cassini spacecraft flew in 2000 en route to Saturn.
New Horizons began sending back pictures of Jupiter early last month, while still 50 million miles from the planet. The pace of observations of the planet, its moons, rings and space environment will accelerate tomorrow and continue throughout its flyby.
"We have carefully tailored our observations so we will not be repeating things that have been done on the ... other missions that have already been to Jupiter," said John Spencer, lead scientist with the mission's Jupiter encounter team.
Only five images will be sent back during the next few days, and only to reassure scientists and to give the public a glimpse of what New Horizons is seeing.
That's because the spacecraft has no moving parts. It cannot turn its cameras and instruments toward Jupiter and its moons, and aim its radio antenna back toward Earth at the same time.
"We take data first and spool it back [to Earth] later," Stern said. It may be months before all the Jupiter data gets back to Earth.
Among the scientists' key targets:
The Jovian moon Io --Contorted by Jupiter's gravity, Io is hot. Its volcanos spew ionized dust and gas into space, which streams into Jupiter's atmosphere, creating polar aurorae physicists want to study with ultraviolet and infrared sensors.
Jupiter's atmosphere --Northwest of the planet's famous Red Spot -- a storm that's been raging for at least 100 years -- there are younger storms and areas of surprisingly clear air. Scientists will probe them with "Ralph," the craft's infrared imager and sensor.
Jupiter's rings --Discovered in 1976, the dusty Jovian ring system is very faint. But New Horizons' telephoto camera will study its structure and search it for new moons.
Europa, Callisto and Ganymede --New Horizons won't get very close to these other moons of Jupiter. But scientists will use high-resolution telescopic imagery to study their surface structure and composition, and get detailed views of their atmospheres.
Jupiter's magnetotail --This is the stream of ionized sulfur and oxygen molecules, lofted from Io and blown by the solar wind for hundreds of millions of miles, perhaps as far as the orbit of Saturn.
New Horizons will fly straight down the magnetotail after leaving Jupiter, gathering data all the way. "No spacecraft has ever been there," Stern said. "It is ... a unique opportunity to study that type of physics. ... We think we're going to have interesting discoveries."
Jupiter is already visible, for free, to anyone who goes outside before dawn on a clear morning and looks for the brightest "star" in the southeastern sky.
"I haven't tried that yet," Weaver confessed. "I'll probably step out on the next couple of mornings to look."