King of the planets will propel probe to Pluto
BY STEPHEN CLARK
Posted: February 27, 2007
NASA's $700 million New Horizons spacecraft was launched last January, and the piano-sized voyager sped past the Moon's orbit just nine hours after blastoff. Thirteen months later, the probe has arrived at the solar system's largest planet.
"We've checked out our spacecraft, checked out our payload, and flown almost half of a billion miles across our solar system," said Alan Stern, New Horizons principal investigator from the Southwest Research Institute.
New Horizons will slingshot around Jupiter at about 0543 GMT (12:43 a.m. EST) Wednesday, passing about 1.4 million miles from the planet's center. The path takes the probe just outside the orbit of Callisto, the outermost of Jupiter's four largest moons.
"We have a very narrow window in space that we have to hit," Stern said. "It's about 500 miles across, and we have to hit it from 500 million miles away."
New Horizons should avoid being exposed to the harsh radiation environment around Jupiter because it is flying far enough away from the planet, said Glen Fountain, the mission's project manager.
"By flying very close, we expect to get a kick and have it fling us outward towards Pluto at an even greater speed," said James Green, acting director of NASA's planetary science division.
Jupiter's gravity will send the 1,000-pound probe on an arcing path toward the outer solar system, where it will reach Pluto in July 2015. New Horizons should receive a speed boost of about 9,000 miles per hour from the flyby.
Pluto is a long, long, long way away, so we've asked a planetary friend - Jupiter - to help us out," Green said.
Using Jupiter's gravity shaves three years off the journey to Pluto and gives ground teams an opportunity to put New Horizons through a realistic test to prepare for the one-shot pass through the solar system's most enigmatic region.
"We designed this particular flyby to be a stress test on our spacecraft," Stern said.
Stern, who will take office as NASA's next associate administrator for science in April, said New Horizons will conduct more than 700 science observations at Jupiter using its payload of seven instruments.
More observations are on tap during the Jupiter encounter than are currently planned for the Pluto flyby, allowing controllers to gain confidence in the craft before the make-or-break swing past the icy dwarf planet.
"We specifically want to exercise the spacecraft hard enough that hopefully we turn up any small vulnerabilities or weaknesses," Stern said.
Earth-orbiting observatories including the Hubble Space Telescope and the
Far Ultraviolet Spectroscopic Explorer will devote time to Jupiter
observations during the time of closest approach. Ground telescopes will
also be looking at the planet, Stern said.
The spacecraft will be the first to fully infiltrate Jupiter's magnetotail, a long tail-like river of plasma blown back by the solar wind. By coincidence, the trajectory of the probe will take it through at least one-fourth of the magnetotail, which is believed to stretch as far back as the orbit of Saturn.
"No spacecraft has ever been there," Spencer said. "We don't know what happens there."
Plasma trapped inside Jupiter's expansive magnetic field, or magnetosphere, escapes the Jovian system through the magnetotail, Spencer said.
Plasma and space environment monitoring instruments will investigate the composition of the magnetotail as the probe flies away from Jupiter. New Horizons is projected to exit the magnetotail by June, officially ending the Jupiter encounter phase of the mission.
Infrared and ultraviolet imaging spectrometers will turn their sensors toward Jupiter to observe the gas giant's turbulent cloud formations and massive storms, including the Great Red Spot, which is more than twice the size of Earth.
The imagers should be able to gaze several layers into Jupiter's thick atmosphere to create a three-dimensional view. Instruments will also watch for auroral storms over Jupiter's poles, Spencer said.
"We'll take what will probably be the most detailed pictures in infrared light that have ever been taken of Jupiter's aurora," Spencer said.
New Horizons will also study Jupiter's four largest moons in the hours surrounding closest approach. The moons will be located on the opposite side of the planet during the flyby, but the craft's instruments should still be able to make meaningful measurements.
"We won't get very close to these moons, but there's quite a lot that we can do even from long range given the capabilities of our spacecraft," Spencer said.
Specific targets for observations include volcanic activity on Io, the innermost of Jupiter's large moons. New Horizons will also probe the ice shelf on the surface of Europa, which is believed to harbor a liquid water ocean.
Surface and atmospheric measurements of Ganymede and Callisto are also planned.
"For all the moons, we'll be getting very detailed views of their atmospheres, in some ways more detailed than any previous observations, using our ultraviolet instrument," Spencer said.
Jupiter's tenuous ring system will also be closely observed by the craft's high resolution visual camera. Scientists will analyze downlinked imagery to look for evidence of tiny new moons to add to the growing list of Jovian satellites, which now stands at 63.
"This should be the most detailed investigation of the ring system that's ever been done," Spencer said.
New Horizons is the eighth spacecraft to visit Jupiter, and the first since NASA's Galileo orbiter was deliberately crashed into the planet's atmosphere at the end of its mission in 2003.
Wednesday's flyby presents one of the best opportunities for science operations at Jupiter in history, Spencer said.
The probe carries a unique and powerful set of instruments outclassing the payloads of most earlier visitors to Jupiter, and it will fly four times closer to the planet than Cassini did in December 2000 as it was en route to Saturn, Stern said.
"We're really looking forward to following up on questions that Galileo and Cassini put before us," Stern said.
Scientists will be searching for changes in the planet and its moons since the last detailed images of the Jovian system were gathered in 2003. New Horizons will be conducting other observations for the first time.
"We're not just repeating what's already been done," Spencer said. "We're carefully tailoring our observations so we learn new stuff about the system."
Science data gathered by New Horizons is not expected to stream back into the mission control room in Laurel, Md., until after the most intense observations are completed on March 4.
"We take data first and we spool it back later," Stern said.
Data will be stored on solid state recorders, and science results will begin to trickle back to Earth after closest approach, he said.
New Horizons is the only spacecraft planned to visit Jupiter during a 13-year gap between permanent missions to study the planet. The Galileo orbiter ended its mission in 2003, and NASA's Juno probe is scheduled to arrive at Jupiter in 2016. Juno's launch is slated for 2011.
"This is the only train going this way, and with these high technology instruments, we are going to produce some stunning data sets, and we can't wait to get them on the ground," Stern said.
Most of the electronic equipment aboard New Horizons will be turned off this summer to begin a hibernation phase designed to reduce mission operation costs. The nuclear-powered probe's flight computer will routinely monitor the health of the spacecraft's systems and broadcast a weekly beacon tone informing controllers of the craft's status.
Ground teams will perform annual checkouts of the spacecraft lasting about 50 days to activate the probe's systems and instruments for tests. Course correction thruster firings will also be planned for the checkout periods.
Formal observations of Pluto and its moons will begin about five months prior to the spacecraft's flyby in July 2015. After passing Pluto, scientists will select targets for an extended mission to study objects in the Kuiper Belt, a collection of icy dwarf planets orbiting the Sun beyond Neptune.