NASA sends probe to the edge of the solar system

March 23, 2006
The New Horizons spacecraft will collect a storehouse of data on Pluto, but it will take about a decade before we see any of it.

Senior Editor

Smoke and steam filled the launch pad as NASA's New Horizons Pluto probe roared into the sky aboard an Atlas V rocket last January from Complex 41 on Cape Canaveral Air Force Station in Florida. NASA

The Navy's new C-band, 3-MW radar at Kennedy Space Center sports a 50-ft dish that makes it the largest of its kind. It tracked New Horizons as it left the Earth and headed for Pluto.

In the Payload Hazardous Servicing Facility at the Kennedy Space Center, workers in clean-room suits lift the New Horizons spacecraft from its work stand. The probe is clad in insulating gold-colored thermal blankets that will retain heat from operating electronics so the interior of the spacecraft remains between 50 and 85°F during its entire journey. The craft will carry a Florida quarter engraved with the Gateway to Discovery design. The coin will serve a practical purpose as a spin-balance weight. The probe will also carry a CD containing 435,000 names of individuals who signed up to send their names to Pluto and beyond.

Last month, NASA launched the New Horizons space probe on a 9.5 year, 4.92 billion mile voyage to explore Pluto, its moon, Charon, and the Kuiper Belt (a ring of more than 100,000 small planetoids circling the sun at roughly the same distance as Pluto). Launching in January let NASA take advantage of a close flyby of Jupiter to slingshot the craft toward its destination and shave three to five years off the travel time. So expect to hear, and see, a lot more about Pluto in early summer of 2015.

The piano-sized New Horizons probe weighs about 1,054 lb, which includes 170 lb of propellant for its thrusters and 66 lb of scientific instruments. It went into space atop a Lockheed Martin Atlas V-551 rocket with five solid-rocket boosters, a Centaur second stage, and a Boeing Star-48B solid-rocket third stage. The Atlas alone, NASA's brawniest launcher, puts out almost 2.5 million pounds of thrust. Altogether, the combined thrust pushed the satellite to 36,000 mph, making it the fastest spacecraft ever. It passed the Moon's orbit in 9 hr, a trip that took Apollo missions three days. The craft will pass Jupiter in 13 months, a journey that took the two most recent Jovian probes, Galileo and Cassini, six and four years, respectively. And after it accelerates around Jupiter (next year), it will be traveling at 47,000 mph (13 miles/sec).

The gravity assist from Jupiter will also give NASA scientist a chance to get more detailed data on Jupiter and Io, one of its moons, while flight-testing the instruments and apparatus for pointing the various instruments. The probe will zoom to within 1.4 million miles of Jupiter, one-third the distance of the Cassini spacecraft's closest approach in 2000 when it was on its way to Saturn.

After passing Jupiter, it will go into hibernation, spinning at 5 rpm with its antenna pointed toward Earth and most of its electronics shut down. Once a week it will send a beacon signal to Earth, either green for "all is well," or one of several red codes if problems are detected. And once a year, it will get a checkup lasting 50 days to make sure the antennae are aimed at Earth, the probe is on the right track, and that the electronics are working and calibrated. This is the first NASA mission to hibernate during travel and the first to communicate in "beacon" mode.

Five months before its closest approach to Pluto, the craft will activate itself and fire thrusters to spin down to a stable attitude. Then it will start taking pictures of the planet and its moon as it gets to within 6,200 miles of the planet.

Even after it has flown past Pluto, the probe will still be gathering data. For example, it will receive commands from Earth and record UV sunlight, both of which will pass though Pluto's atmosphere. The difference between signals that go through Pluto's atmosphere and those that don't will let scientist plot temperature and density profiles of the tiny planet's atmosphere. Back lighting will also make it easier to spot possible rings around Pluto.

After the probe has gotten all the information it can and beamed it back to Earth, NASA will decide, based on funding, whether to continue the mission. Up to this point, the mission, including researchers' time, building and designing the satellite, and launching and tracking it, should total $700 million. If more funds are found, the probe will go on to explore-one or two planetoids in the Kuiper Belt, a two to three-year extension.

The probe was designed and assembled at the Johns Hopkins University Applied Physics Lab. Control signals and data are handled by a radiation-hardened 12-GHz processor. The craft carries two, low-powered solid-state recorders that each hold 4-Gbites. One is a dedicated backup unit.

The probe also sports 16 hydrazine thrusters, but none provide speed on the trip to Pluto. Four of them, each with 1 lb of thrust, are for course corrections. A dozen smaller ones, each with 3 oz of thrust, point the craft and control spinning.

New Horizons will use startracking cameras, an inertial navigation system, and solar sensors to determine its location and attitude. The system has a 3,000-star map, and the cameras snap wideangle images 10 times/sec. Images are compared to the map to calculate the probe's attitude. The inertial system keeps the vehicle on track. And the sun sensors provide backup and can point the New Horizons antennae toward Earth in emergencies.

The spacecraft operates on a radioisotope thermoelectric generator that puts out 240 W (at 30 Vdc) at the beginning of the mission, which will degrade to about 200 W by the time it gets to Pluto. The generator is fueled with 24 lb of plutonium dioxide. The probe carries seven scientific instruments — Alice, Lorri, Pepssi, Ralph, Rex, Swap, and the Student Dust counter — and each use between 2 and 10 W.

Alice is a UV (500 to 1,800 A) imaging spectrometer that will probe the composition, density, structure and temperature of Pluto's atmosphere. It will also search for an ionosphere around Pluto and Charon.

Ralph, the eyes of the New Horizons, will map Pluto, its moons, and other Kuiper Belt objects. It has three black-and-white and four color CCD imagers, as well as an IR mapping spectrometer built for Pluto's sunlight levels which are 1,000 times fainter than daylight on Earth. All these imagers are fed by a telescope with 10 times the resolution of a human eye. The telescope will let Ralph map Pluto with a resolution of 820-ft/pixel, take stereo images that will highlight surface topography, and find clouds and hazes in Pluto's atmosphere. It will work with Alice. The two are named for Jackie Gleason's Honeymooners.

Rex, or radio science experiment, consists of a pair of circuit boards (one is a backup) crammed with sophisticated signal processors. It will detect the temperature and composition of the atmosphere on Pluto (and Charon, if it has one), and finetune the values for the mass of Pluto and Charon.

Lorri, long-range reconnaissance imager, is a digital camera with a telephoto lens. It uses CCD technology and has an 8.2-in. aperture. It will start photographing Pluto 200 days before the closest approach. And at 90 days before closest approach, its images will have better clarity and detail than those taken by the Hubble telescope. At closest approach, Lorri will pick out any objects larger than 150 ft in diameter. The camera uses siliconcarbide construction to keep it and its mirrors focused despite extreme temperature dips.

Pepssi, Pluto energetic particle spectrometer science investigation is said to be the most compact-particle spectrometer ever flown on a space mission. It will detect neutral atoms escaping Pluto and interacting with the solar wind. This will tell scientists what Pluto's atmosphere consists of and how quickly it is escaping.

Swap, solar wind at Pluto works alongside Pepssi, measuring perturbations in the solar wind caused by material — such as molecular nitrogen, carbon monoxide, and methane — escaping Pluto's atmosphere.

SDC, Student dust counter, built by University of Colorado students, will count and measure the size of dust particles that hit the spacecraft during its voyage. It will also search for dust near Pluto. This will be the first dust collector flown past Uranus.

New Horizons and its seven scientific instruments were designed and integrated by researchers at the Johns Hopkins Applied Physics Lab, except for the dust counter which was built by students at the University of Colorado.

Keeping track of it all

When New Horizons passes Jupiter, it will be sending data home at 38 kbps, a little slower than most modems. When it gets to Pluto and begins gathering data, the satellite will only be able to send it back at between 300 and 600 bps. It will take almost 4.5 hr for those signals to reach Earth. At that rate, it would take 12 hr to download a single image from New Horizons' long-range imager, and nearly 40 days to send the entire 10 Gbits of data it will collect during the entire mission. And that's with sole access to NASA's Deep Space Network. But the Network, a triad of steerable, high-gain parabolic antenna sites located about 120° of longitude apart to ensure constant coverage of all areas of the sky, has other tasks. For example, it is receiving data from Martian rovers Spirit and Opportunity, getting information from the Cassini probe as it orbits Saturn and the Ulysses probe as it orbits the Sun, and still communicating with Voyager I and II.

To save time and resources, NASA scientists will make do with compressed (20:1) data sent to Earth starting 10 days after New Horizons' closest approach to Pluto. Some of the original data will be lost to compression, but if researchers determine they need more detail, they will command the probe to "losslessly compress" those files and retransmit them to Earth. This could take nine months.

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