Peeking at Pluto and Beyond

Two members share their experiences of working on NASA’s New Horizons spacecraft

6 November 2015

Imagine working on a really expensive, high-profile spacecraft and not knowing if it will work for almost a decade after it’s finished. That’s the dilemma faced by IEEE Members Chris Hersman, Ron Schulze, and the team of engineers at Johns Hopkins Applied Physics Laboratory, in Laurel, Md., who designed, built, and continues to operate the New Horizons spacecraft launched in January 2006.

APL, along with New Horizons principal investigator Alan Stern and the Southwest Research Institute, manages the NASA mission to explore Pluto and its system of moons and other objects in the Kuiper Belt, a ring of icy miniworlds thought to be left over from the solar system’s creation. The spacecraft, which spent about two-thirds of its nearly 10-year flight asleep so as to reduce wear and tear on its electronics, woke up in late 2014 and then, like E.T., “phoned home” on 14 July to tell APL and the rest of the world that it was carrying out the first-ever close approach to the Pluto system.

While images of the planet’s unusual heart-shaped surface patterns, water ice–rich crust, and atmospheric blue haze have captured the public’s attention, it’s the engineers who made the flyby possible. Hersman is a mission systems engineer and Schulze a lead antenna engineer.

“You may only have one opportunity in your entire career to do something like this,” says Schulze. “We gave it our all. That’s what we needed if the mission were to succeed.”

Adds Hersman, “Even when you get the telemetry along the way and everything looks like it’s going well, there’s so much that still has to happen and go right. Yet the flyby could not have gone more smoothly.”

NASA began work on the spacecraft in 2001. Schulze joined APL’s space department in 1997, and Hersman started out as the spacecraft’s systems engineer in 2001.

PLUTO’S IMPORTANCE

For the U.S. National Academy of Sciences, learning about Pluto and the Kuiper Belt has the highest priority in solar system exploration. Pluto was our solar system’s smallest planet, the farthest one from the sun, and so small as to have been recently designated a “dwarf planet.” It lies anywhere from 4.28 billion to 7.5 billion kilometers from Earth, depending on where the two are in their orbits around the sun. The New Horizons mission seeks to understand where Pluto and its moons fit in with the other objects in the solar system, such as the inner rocky planets (Earth, Mars, Venus, and Mercury) and the outer gas giants (Jupiter, Saturn, Uranus, and Neptune). The program will cost about US $700 million over 15 years (from 2001 to 2016).

Pluto and its largest moon, Charon, belong to a third planetary category known as ice dwarfs. They have solid surfaces, but unlike the terrestrial planets a significant portion of their mass consists of icy material. Pluto sits at minus 190 to 204 °C. The close-up look from New Horizons could help scientists understand the Pluto system’s origins.

COMMUNICATING WITH EARTH

About the size of a baby grand piano, New Horizons weighed 478 kilograms at launch. Its power source is a radioisotope thermoelectric generator that produced about 200 watts during the flyby. The propulsion system relies on hydrazine, which undergoes a chemical reaction to produce thrust from a superheated gas. The communication system—the spacecraft’s link to Earth—beams back scientific data, including images, and exchanges commands and status information with mission control at APL. It also allows for precise radiometric tracking using the antenna stations of NASA’s Deep Space Network. It has seven scientific instruments.

Schulze helped build the spacecraft’s antenna systems, critical for the science missions and communications. Two broad-beam, low-gain antennas were mostly for near-Earth communications. Farther from Earth, a 30-centimeter-diameter, medium-gain dish antenna and a larger 2.1-meter-diameter, high-gain dish antenna take over. The spacecraft navigates using onboard gyros, star trackers, and sun sensors, which automatically determine its position in space.

According to Schulze, New Horizons operated mostly in a spin-stabilized mode on its way to Pluto. But it can also operate in a three-axis “pointing” mode that lets instruments point or scan during calibrations and planetary encounters, such as the one with Pluto and a flyby of Jupiter in 2007.

“The spacecraft was naturally stabilized because it was spinning, allowing it to go into long hibernation periods when we wouldn’t need to send any commands to check on its orientation,” Schulze says.

Schulze wrote about his work in “The New Horizons High-Gain Antenna: Reflector Design for a Spin-Stabilized Bus at Cryogenic Temperatures,” published in 2004 and available in the IEEE Xplore Digital Library. Since the launch, Schulze and a small team of APL’s radio-frequency experts have been monitoring the health of the communication system.

As a missions systems engineer, Hersman is in charge of the mission’s technical aspects, including reviewing the commands given to the spacecraft’s key systems and its science instruments. About once a year over the last decade, Hersman has led reviews of the trending data from the spacecraft’s subsystems and onboard components to make sure they’re still working. During 18 separate hibernation periods, ranging in duration from 36 to 202 days, the New Horizons team has checked the spacecraft’s weekly beacon signal. To do this while the craft was spinning, they used the medium-gain antenna at the point when the high-gain antenna was oriented to where the Earth was going to be when New Horizons exited hibernation, “so we would know on wake-up day we could communicate with the spacecraft.”

Both engineers worked on other projects when they weren’t checking up on New Horizons.

INCREDIBLE INSTRUMENTS

The instruments—each uses between 2 and 10 watts—send data to two onboard, solid-state memory banks before transmitting it to mission control. The seven instruments include Alice, an ultraviolet imaging spectrometer, which analyzed the composition and structure of Pluto’s atmosphere and looked for atmosphere around Charon. It consists of a telescope, a spectrograph, and a sensitive electronic detector with 1,024 channels at each of 32 separate spatial locations in its long, rectangular field of view.

Ralph, the main eyes of the spacecraft, created the photolike maps of Pluto and its moons that captured the public’s imagination. It does this with three panchromatic (black-and-white) and four color imagers in its multispectral visible imaging camera, as well as an infrared compositional mapping spectrometer.

(Alice was named on a previous space mission, so when this instrument was added it was named for Ralph, Alice’s husband in the U.S. television series “The Honeymooners.”)

Then there’s the Radio Science Experiment. REX relied on an occultation technique to probe Pluto’s atmosphere and search for an atmosphere around Charon.

The “eagle eye” of New Horizons is the Long Range Reconnaissance Imager—essentially a digital camera with a large telephoto telescope. LORRI is responsible for most of the visual images beamed back to Earth. It imaged sections of Pluto's sunlit surface at football-field-size resolution, resolving features at about 50 meters across. A panchromatic high-magnification imager, it consists of a telescope with a 20.8-centimeter aperture that focuses visible light onto a charge-coupled device.

The appropriately named Solar Wind Around Pluto instrument measured interactions of Pluto with the stream of fast, charged particles flowing from the sun. SWAP is the largest-aperture instrument ever used to measure solar wind. Using SWAP measurements of how the solar wind is perturbed by its interaction with Pluto’s escaping atmosphere, scientists will determine the escape rate of atmospheric material from Pluto.

The Pluto Energetic Particle Spectrometer Science Investigation— PEPSSI, for short—is the smallest, lowest-power directional energetic particle spectrometer ever flown on a space mission. It searched for neutral particles such as nitrogen, carbon monoxide, and methane that escape Pluto’s atmosphere and become charged by their interaction with the solar wind.

Finally, there’s the SDC. Built and operated by students at the University of Colorado at Boulder, the Venetia Burney Student Dust Counter measures the space dust peppering New Horizons as it moves across the solar system.

“For us, the engineers, the problem is to get the data down from all the instruments,” Hersman explains. “It’s for the scientists to piece together the images and analyze them.” The data rate is only 1 to 2 kilobits per second, so it can take more than an hour to send even one high-resolution image back to Earth. It will take about a year for New Horizons to send home the entire set of Pluto flyby data, according to Hersman.

THE SECOND ACT

Hersman and Schulze are now gearing up for a flyby of the spacecraft’s next target: a small Kuiper Belt object named 2014 MU69. The KBO is roughly 1.6 billion kilometers from Pluto. At the edge of the solar system, it is 20 times wider than the asteroid belt between Mars and Jupiter. MU69 is estimated to be 45 kilometers in diameter. Astronomers think it could hold chunks of celestial objects left over from our solar system’s formation. New Horizons won’t reach it for another three years.

The first of four maneuvers to reach 2014 MU69 was conducted on 22 October to fire New Horizons’ engines and redirect the spacecraft. According to Hersman’s calculations, New Horizons has enough power to continue to fly well into the mid-2030s before its propellant freezes in the icy cold.

In terms of public recognition, the attention surrounding the spacecraft’s arrival at Pluto has breathed new life into the U.S. space program. Taxpayer dollars made the mission possible, Hersman points out. He wants to “thank the public for the opportunity to explore these amazing worlds.”

It is no surprise that both men see the mission as the highlight of their careers.

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