Before you break open the dictionary to grapple with terms such as kilohertz, plasma waves or the heliopause, there's a fundamental aspect of space exploration that still can make a 73-year-old astrophysicist put a major breakthrough in terms a child could appreciate.

"Yes, it is cool," Donald Gurnett, a University of Iowa physics and astronomy professor who grew up on a farm near Fairfax, said of a historic discovery in space travel he helped identify.

Clearly, being on the frontier of space research still excites Gurnett, who's been a professor at the UI since 1965 and helped announce the latest milestone in space exploration last week.

Gurnett and the Iowa team detected the Voyager I became the first man-made object to reach interstellar space  a region about 11.3 million miles from the sun, beyond the solar system and the influence of the sun's solar wind.

A 2012 solar flare reached the Voyager I and caused plasma oscillations, which UI research scientist Bill Kurth detected on April 9 while reviewing plasma data from the craft. This allowed Kurth to determine the density of the particles around the craft fell in a range they predicted in a 1993 paper would indicate interstellar space.

Scientists had suspected for about a year that Voyager I crossed the heliopause  the boundary of the solar system  into interstellar space, but they did not want to confirm it until receiving the data from the Iowa-made plasma wave instrument.

According in to NASA, "Plasma is the most important marker that distinguishes whether Voyager 1 is inside the solar bubble, known as the heliosphere, which is inflated by plasma that streams outward from our sun, or in interstellar space and surrounded by material ejected by the explosion of nearby giant stars millions of years ago."

The idea of reaching interstellar space was discussed when the 36-year-old Voyager mission was in its infancy, but it wasn't an expectation of the mission, Gurnett said.

"Reaching the heliopause was discussed in those days, but I don't believe anyone at that time thought it would be 36 years until we got there," Gurnett said. "Most people thought it was much closer, just beyond the orbit of Jupiter."

But, in those days, traveling to any planet, much less Jupiter was a big undertaking, Gurnett said.

After Congress rejected an earlier "grand tour" proposal deeming it too expensive, they approved the Mariner Jupiter/Saturn mission  later named Voyager, Gurnett said.

At the time, the best hopes for the project were if everything went right and funding continued, the craft would pass Saturn, but they didn't think much beyond Uranus and Neptune, Kurth said.

"From my own point of view, I remember putting a 'Uranus or bust' sticker on the case carrying the instrument," Kurth said.

Gurnett was brought on board the mission to study the radiation around Jupiter and Saturn.

In 1973, he and the Iowa team began developing the instrument to measure plasma waves. They built the tool  about the size of a cigar box and weighing about 2.5 pounds  in what now is Van Allen Hall, named for James Van Allen, who discovered the Van Allen radiation belt. This plasma instrument provided the key data scientists used to conclude the Voyager I had reached interstellar space.

Voyager I launched on Sept. 5, 1977, about two weeks after a sister craft, Voyager II, which still is in the solar system.

An alignment of the planets that occurs once every 180 years, allowed a single spacecraft to reach all four planets  Jupiter, Saturn, Uranus and Neptune  and the mission was expanded, Gurnett said. Scientists were able confirm a radiation belts around Saturn, magnetic fields around the outer planets, as well as send back first-of-its-kind images from outer planets, Gurnett said.

After the primary mission was completed in 1989, the mission was renamed Voyager Interstellar with a goal of crossing the heliopause.

But as years passed, scientists began to wonder how far the heliopause was, if they'd run out of plutonium, which fuels the craft and instruments, and if the craft would hold up, before it crossed the edge of the solar system.

"It had been flat as you could imagine for nine years," Kurth said. "Then suddenly our 3 kilohertz channel ... started showing an increase in signal strength. It increased to a stronger signal strength than it had in a large number of years. Then there it was. The big event showed up. The fluctuations in intensity of time was what we expected to see."

Kurth and Gurnett say they hope the mission that has spanned most of their careers can last another seven to 10 years or so  until the fuel runs out.

The craft is in dense plasma. Kurth believes it will take another year or two to pass through, at which point he hopes to begin to gather new information about what is beyond. There's also other questions to answer, such as how long the rotation of a magnetic field they have encountered will last, the influence of the heliosphere, and Gurnett said he's studying the density of dust particles.

"We've only just arrived in this medium," Kurth said. "And we still have a lot of questions about what this medium is like and how the heliosphere is influencing it, if it is, and how things are changing while we are out there."

For space hobbyists, news of the event spread quickly.

Greg Frohner, 66, of Center Point who worked at Rockwell Collins and now is the vice president of the Cedar Amateur Astronomers, said the interesting part of the milestone to him is how it puts the vastness of the universe and what we know about it into context.

"We often talk about other objects in the Milky Way being 50,000 to 100,000 light years away, and to think those satellites have been up there for 36 years and it is still within our galaxy," Frohner said. "You have to go 2.5 million light years to the Andromeda Galaxy. Those are huge numbers, it's only made it something like 17 light hours from the sun, and it's another three light years to the nearest star."

— The Washington Post contributed to this story.