Scientists on the New Horizons mission are beginning to plan in earnest how they will evaluate the data that will begin flowing back from Pluto in less than two years, when the nuclear-powered probe begins sending “better than Hubble” imagery of the distant body and its satellites.
The spacecraft's Long-Range Reconnaissance Imager (Lorri) has already resolved Pluto and Charron, its largest satellite, into two distinct objects (see image, page 22). With the resolution improving by the day, the mission team has planned and uploaded its flyby choreography, and has sent out a call to astronomers for parallel observation from Earth and its environs before, during and after the July 14, 2015, encounter.
The team also has completed a rehearsal with the spacecraft, and conducted a detailed scientific workshop at the Johns Hopkins University's Applied Physics Laboratory (APL) here, where New Horizons was built. There are no plans to retarget the probe again.
“The security that we get by knowing that we've practiced all of these observations on the spacecraft outweighs the flexibility that we would want to be able to do any last-minute targeting,” says astronomer Leslie Young of the Southwest Research Institute (SWRI), who led the team that planned the encounter.
On a mission that has been under consideration since beforelaunched its first Voyager mission to the outer planets on Sept. 5. 1977, long before Pluto was downgraded from the ninth planet to a Kuiper Belt object large enough to be termed a dwarf planet, the two years remaining before the flyby are a virtual blink of the eye. Planning for the encounter started in 2000, when the New Horizons team was preparing its mission proposal, and started “in earnest” after the spacecraft hurtled past Jupiter and gave its operators some experience collecting data at a real planet.
“We are really at this moment, in July of 2013, kicking off this encounter,” says SWRI's Alan Stern, the mission's principal investigator. “We've just finished a nine-day close encounter rehearsal on the spacecraft. The spacecraft performed flawlessly. The entire close-approach sequence was put up on the spacecraft, and it ran from July 5 to July 14, and the spacecraft got an A-plus.”
The mission-science conference at APL gave researchers an idea of what they can expect to receive in the year-and-a-half that it will take after the close encounter to transmit all the data over the 5 billion km (3 billion mi.) that will separate New Horizons from Earth. There are seven instruments on New Horizons, carefully selected after a lot of scientific argument to glean as much information about the outer Solar System as possible. The ink on textbooks rewritten to accommodate discoveries from the Hubble Space Telescope is barely dry, but the scientists here expect they will need to be rewritten again once the spacecraft draws close enough to the Pluto system to incorporate an unprecedented flood of information about the unexplored region.
“When I went to graduate school and I learned about astronomy, the Solar System had a very different architecture,” says Stern, who is 55. “What I now call the middle Solar System used to be called the outer Solar System, and the entire geography of the Kuiper Belt and all the small planets there was not even known, with the exception of Pluto.”
The public is likely to be most taken with early data returning from the spacecraft's cameras, which will be imaging Pluto and its moons beginning in January 2015. Five of them are known. The largest of them, Charon, was discovered in 1978. The smallest, named Styx, turned up last year, and no one will be surprised if more are spotted as New Horizons approaches.
“We've planned particular observations to look for small satellites, many of them going deep, deep imaging with the panchromatic camera,” says Young. “We even took one of our color scans of Pluto and Charon and extended the duration of it to cover the entire system. So, even if we discover an object in the data a year and a half after encounter, we can go back and say there ought to have been a moon there.”
It is also possible that the probe will discover planetary rings, and researchers have long been planning how they will study the tenuous atmosphere they expect to find, and how it interacts with the solar wind.
New Horizons carries three imagers. Lorri is a simple 20.8-cm (8.2-in.) telescope that will generate images with a resolution as fine as 100 meters (330 ft.) on the planet's surface at closest approach. Also onboard are visible/infrared and ultraviolet imager/spectrometers dubbed Ralph and Alice, respectively. Ralph will generate color maps and data on composition and temperatures; Alice will study atmospheres at Pluto and perhaps Charon.
The Radio Science Experiment (Rex) is a passive radiometer that will measure the composition and temperatures of atmospheres it encounters at Pluto and perhaps elsewhere. The Solar Wind Around Pluto instrument will measure atmospheric escape at the dwarf planet, and its interaction with the solar wind, while the Pluto Energetic Particle Spectrometer Science Investigation will study plasma leaving the atmosphere.
The Venetia Burney Student Dust Counter, built as an education project by students at the University of Colorado and elsewhere, has been at work since the probe was launched on Jan. 19, 2006, measuring the interplanetary dust along the trajectory.
The science team has just started polling astronomers who use telescopes on and in orbit around Earth to mount an observation campaign that will collect data in parallel with New Horizons' passage through the Pluto system. In addition to more moons, scientists hope to identify at least one Kuiper Belt object beyond Pluto and retarget the probe for a flyby of that object.
The job will be “like looking for somebody sending Morse code signals from a New York City apartment building,” Young says, because the center of the Milky Way galaxy lies behind the target region. But observations from Hawaii have enjoyed “good seeing,” she says, and the team is hopeful the campaign will discover a good Kuiper Belt target.
The encounter with Pluto itself will be broken into phases that begin on Jan. 6, 2015, says Young.
“Approach Phase 1, where we're just beginning our science, mostly doing optical navigation,” she says. “Approach Phase 2 [April 4-June 23, 2015] we begin to pass Hubble resolution and our science picks up. Approach Phase 3 is when we start to do our more exciting science in our last few rotations before closest approach. All of our [priority] required science objectives are taking in the near-encounter phase, just a day before and a day after encounter, and then as we leave on the departure phase, things slow down.”
Stern says the team is “really looking forward to knocking people's socks off” with “several hundred scientific activities” in the final week before closest encounter and “several hundred more” in the closest two days. Surface imagery with resolution comparable to the Voyager images of Triton are likely, and maps of frost on Pluto's far side are possible.
“The data sets that are going to come down from New Horizons are absolutely mouth-watering,” says Stern, who helped overcome opposition from then-Administrator Sean O'Keefe to send the mission on its way. “Because of the diversity and sophistication of the instruments in this payload, there really hasn't been anything like this in the entire history of planetary exploration for a first encounter. . . . We will exceed Hubble resolution not for a week or two, but from April to October. We will image every one of Pluto's satellites and Pluto itself. We will make compositional maps of Pluto and Charon and Nix, and unresolved compositional information on the other three known satellites. We will map certain temperature fields. We'll measure Pluto's [atmospheric] escape rate. We'll determine its temperature and pressure structure as a function of altitude; we'll address interior models and do many, many other things relating to the satellites, the system and the environment.”