As the world awaits New Horizons' arrival in the Pluto system, no one knows exactly what to expect. But Suzanne Dodd's experience gives her a better idea than most.
Dodd was responsible for generating and uploading the complex sequence of commands to the Voyager 2 spacecraft ahead of its Neptune flyby in 1989—the last time we explored a planet for the first time at close range. But that will change on July 14, 2015, when the New Horizons spacecraft makes its closest approach to Pluto, the only one of the nine classical planets yet to be explored in detail (you may remember Pluto was controversially reclassified as a dwarf planet in 2006).
New Horizons was launched from Cape Canaveral in January 2006 and has spent the last nine-and-a-half years journeying to the outer edge of the solar system. So far, the mission has gone largely according to plan, but when the probe arrives at Pluto, the predictions are murkier—and not just because Pluto is so far from the sun. Until recently, even our best photos of the tiny rock were heavily pixelated, and New Horizons' photos did not exceed the the resolution of the Hubble telescope's until May 2015.
In other words: there will be surprises.
While there is plenty that can go wrong during a mission, the closest approach to a planet is "the most critical sequence, kind of like the landing sequence for a Mars lander," explained Dodd, now the project manager for the Voyager program at NASA's Jet Propulsion Laboratory (JPL).
Unlike orbiting missions—such as the Cassini probe currently circling Saturn—where there are years to gather necessary data, flyby missions such as Voyager and New Horizons have just one chance to get it right. In the pre-Hubble days, the Voyager team was not even sure of Neptune's exact size, said Dodd, and therefore could not precisely predict the effects of the planet's gravity on the spacecraft until shortly before the encounter.
"We started developing the closest approach sequence about six months beforehand," she continued, "but we continued making adjustments and the final sequence was uploaded approximately 24 hours before the flyby." According to New Horizons principal investigator Alan Stern, writing for Sky & Telescope, the last date to make a course correction was July 1, but the team will continue refining their sequences, as they receive better and better images of Pluto and its moons, to ensure the spacecraft's various instruments are recording at the correct times
Dodd's 24-hour lead time qualifies as last minute in the world of interplanetary missions. When New Horizons reaches Pluto, for example, a radio signal traveling one way to or from the spacecraft will take four hours and 25 minutes. This isn't like a first-person shooter video game or a drone strike in Afghanistan—there is no real-time feedback, nor anybody sitting in mission control flying the probe with a joystick and snapping photos by remote control.
The margins for error are tiny; New Horizons must hit a target circle 300 kilometres in diameter
"Once the final sequence was uploaded, you feel mostly relief," Dodd remembered. "But there is also anticipation and some nervousness, waiting for the first set of data to come back while knowing that everything is out of your hands."
With the whole world watching and waiting—not to mention the investment of hundreds-of-millions of dollars and years of effort—nobody wants to make a mistake. And the margins for error are tiny. In New Horizons' case, according to NASA, the spacecraft must hit a target circle 300 kilometres in diameter during the Pluto flyby in order to accomplish its mission—and that is after its five-billion-kilometre journey from Earth.
Of course, Voyager had lots of redundancies and contingency plans to deal with potential problems, explained Dodd—everything from a potential failure of the onboard computer, to a rain storm in Spain blocking the signal from one of the Deep Space Network sites. The New Horizons team has also had to deal with unexpected challenges, including the discovery of two new Plutonian moons after the spacecraft's launch. Those discoveries led to a worry that the probe could be hit by dust and debris from the moons, although a detailed study ultimately determined that the risk of such an impact was quite low.
In preparation for the Pluto encounter, Dodd said the New Horizons team will have learned plenty from Voyager, as well as NASA's many other interplanetary missions. The two Voyagers were the first missions to benefit from gravity assists by slingshotting past the giant planets, using these speed boosts to escape the solar system sooner. Similarly, New Horizons' Jupiter flyby allowed the spacecraft to cut five years off its own travel time to Pluto.
Asked what she would do if she had her choice of undertaking any new mission, Dodd said she would like to send orbiters to Uranus and Neptune. "We've done it for all the other planets, except Pluto," she said, "and you could theoretically use the same spacecraft and launch vehicle designs for both planets, to save money.
"My first job was going to Uranus and Neptune and, as I get closer to the end of my career, it would be nice to go back."