Stamatios “Tom” Krimigis with the charged particle detector that is flying on Voyager 1 and 2.

Stamatios “Tom” Krimigis with the charged particle detector that is flying on Voyager 1 and 2.

JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY

Pluto caps one scientist's 50-year exploration of the solar system

Ermis, Aphrodite, Aris, Días, Kronos, Ouranos, Poseidon, Pluton. Growing up near the beach on the Greek island of Chios, Stamatios “Tom” Krimigis would look for wandering planets in the night sky. “How could I anticipate at the time that I would be one of humanity’s representatives [to those bodies]?” asks Krimigis, the former head of the space department at Johns Hopkins University’s Applied Physics Laboratory (APL) in Laurel, Maryland. On 14 July, when the New Horizons spacecraft swoops past Pluto, he will become the only person to have visited them all, instrumentally speaking. “It’s the stuff dreams are made of.”

The Pluto flyby will cap a remarkable half-century of planetary reconnaissance, 50 years to the day after Mariner 4 flew past Mars and returned the first image from another planet. Krimigis, an expert in planetary magnetospheres, had a hand in both missions, as well as in visits to all the other worlds in the solar system. “Timing is certainly critical,” says Ed Stone, the project scientist for Voyager, the grand tour of the outer planets that accounted for half of Krimigis’s tally. “He’s obviously had the right ideas to propose to NASA, time after time.”

Born in 1938 in Greece, Krimigis came to the United States at the age of 18 to enroll at the University of Minnesota. The launch of Sputnik in 1957 galvanized him, and many other 5 of his generation, to study physics and space science. After college, he went to the University of Iowa to work for James Van Allen, who had been principal investigator on Explorer 1—the United States’ first satellite. It detected the first traces of the belts of protons and electrons, trapped by Earth’s magnetic field, that bear Van Allen’s name. In 1963, Krimigis recalls, Van Allen asked him a question that was to shape his career: “He said, ‘How would you like to be a coinvestigator on the first mission to Mars?’ ”

Scientists wondered whether radiation belts like Earth’s existed around other planets. Van Allen was principal investigator for Mariner 4’s trapped-radiation detector, designed to answer that question for Mars. The detectors were essentially Geiger counters, which detect the tiny jolts of ionizing radiation but cannot distinguish between xrays, protons, and electrons. Krimigis’s job was to add a solid-state detector that could tell them apart by measuring the energy of incoming particles. “I finally got the thing working about a month before we launched it,” he says.

When Mariner 4 flew past Mars on 14 July 1965, Krimigis and Van Allen watched for jitters on a stripchart recorder needle as their data came down at 8.33 bits per second. The needle barely moved: There were no radiation belts at Mars. The first picture was disappointing, too, with none of the hoped-for canals of lore. “It looked like a crater on the moon!” Krimigis remembers. A few years later, Van Allen and Krimigis sent a particle detector to Venus on Mariner 5; no radiation belts turned up there, either.

By the time of the payload selection for the twin Voyager spacecraft in the early 1970s, Krimigis, still focused on charged particles, was ready to compete against Van Allen with an instrument proposal of his own. “The prodigal son was going up against his mentor, and he had a whole team of us young guys,” says Louis Lanzerotti, a space physicist at the New Jersey Institute of Technology in Newark, who joined Krimigis on his winning Low Energy Charged Particle (LECP) experiment, designed to detect nuclei of elements heavier than hydrogen or helium.

In 1979, when Voyager 1 and Voyager 2 passed by Jupiter, Krimigis’s detectors picked up sulfur and oxygen ions whipped up by the giant planet’s powerful magnetic fields. Later, scientists discovered that these elements originated in the volcanoes on Jupiter’s moon Io. The two probes went on to Saturn, where they found oxygen ions, fed by the watery geysers of the moon Enceladus. Then, Voyager 2 swung by Uranus and Neptune, and discovered simple belts of protons and electrons.

Krimigis’s instruments also explored deep space. The LECP has a scanning motor that sweeps through a near circle and measures particles from different directions. The motor—still working today after 6.5 million pivots—helped Voyager 1 scientists improvise measurements of the solar wind when a dedicated instrument failed. 

It took a while for Krimigis to notch Mercury onto his planetary belt. He missed out on the payload for Mariner 10, which flew past Mercury twice in 1974. Scientists on that mission claimed to find protons—perhaps a small radiation belt trapped by Mercury’s magnetic field. But in 2011, APL’s MESSENGER mission arrived at Mercury with an innovative particle detector Krimigis had helped design, able to determine the charge and speed of incoming ions with unprecedented precision. It found no protons, only short-lived belts of electrons that the nearby sun’s powerful magnetic field disrupts almost as soon as they form.

Now, finally, comes Pluto’s visit by New Horizons, a mission that Krimigis helped usher into existence while space department head at APL. He recruited its principal investigator, Alan Stern (Science26 June, p. 1414), and lobbied Congress to keep the mission afloat in the early 2000s. The charged-particle detector on New Horizons, PEPSSI, is a descendant of the one flown on MESSENGER. Pluto has a thin, wispy atmosphere and probably lacks a magnetic field, so Krimigis isn’t expecting to find much more than a few stray protons and electrons. But he’s prepared to be surprised: “It’s not what you think you’re going to find, but the things you don’t expect,” he says. The New Horizons encounter will return 5000 times as much data as Mariner 4 did at Mars, Stern says.

Norman Ness, a veteran of 21 NASA missions and an emeritus researcher at the University of Delaware in Newark, has nearly as many trophies as Krimigis. In addition to leading a magnetic experiment on Voyager (which netted him the giant planets), he was a principal investigator on Mariner 10, which flew past Venus on its way to Mercury, and a co-investigator on the Mars Global Surveyor. But Ness says he doesn’t put much stock in the tallies. “I have never looked upon that as a metric to worry about,” he says.

In any case, Krimigis isn’t done yet. He’s waiting for the telltale signs that Voyager 2 has left the solar system, as Voyager 1 did in 2012, and is preparing for his next project: a turn inward, toward the sun. He is a co-investigator for an ion-measuring instrument on Solar Probe Plus, scheduled to launch in 2018 and fly within the sun’s wispy, scorching atmosphere, the corona. Krimigis has been involved in planning the mission since the late 1970s. “My lifelong dream project had always been the solar probe,” he says. “It’s the first mission to a star.”

*See Science’full coverage of Pluto, including regular updates on the New Horizons flyby.