A multigenerational mission past the sun
LAUREL, Md. — One of the top prizes in the March 1970 Fort Worth Regional Science Fair — a slide rule and a free dinner in Dallas — went to a high school junior named Ralph McNutt, who had written 30 pages on the question, “Interstellar travel: Is it feasible?” and built a cardboard scale model of the spacecraft he said could be the first to visit another sun.
Humans had landed on the moon the previous summer, the 16-year-old noted. Soon, he was sure, we would venture to all the other planets of the solar system. Then it would be time for the next step: “Going to the stars.”
On a sweaty summer afternoon, McNutt, now 65, sits in his office at the Johns Hopkins University Applied Physics Laboratory. On his computer screen is the latest draft of his boyhood dream: a plan for a probe that would travel 1,000 times farther than Earth is from the sun, leaving behind the safety of our solar system to explore the wilds of interstellar space.
From that far-flung vantage point, Interstellar Probe will help humans finally see ourselves for what we truly are, McNutt says: citizens of a galaxy. Our home planet will be just one world among many, and the sun that gives us life just another pinprick of light in the endless dark.
It’s an audacious proposal. The probe would take 50 years to reach its destination, by which time nearly everyone currently involved in the project will be dead.
Nevertheless, McNutt and a cadre of fellow dreamers hope to get an important endorsement in a few years, when the nation’s space scientists release a list of their top research priorities. To get Interstellar Probe on the agenda, its supporters must convince their colleagues that its goal is scientifically valuable, not to mention politically viable, when there are so many questions inside the solar system still unanswered.
What makes McNutt believe it’s possible? The scientist leans back in his chair and crosses his arms. When he answers, it’s in the form of poetry. “I think man’s reach should exceed his grasp,” he said, paraphrasing Robert Browning. “Otherwise, what is a heaven for?”
Our sun sits on a minor arm of the spinning, star-strewn pinwheel of the Milky Way, about 25,000 light-years from the galactic core. Zooming through the cosmos at roughly half a million miles per hour, the solar system is buffeted by gusts of gas and dust and bombarded by energetic particles whose origins are a mystery.
But we on Earth are partly shielded from this chaos by the heliosphere, a balloon-like structure inflated by the solar wind. Charged particles flowing from the sun stream out to the edge of the solar system — past the planets, beyond Pluto, through the frozen halo of the Kuiper belt, to a place called the heliopause.
This is the liminal zone between the river of solar particles and the ocean of interstellar space; the boundary between our celestial neighborhood and the wider universe.
Only two spacecraft have reached that zone and lived to tell the tale: the twin Voyager probes, which launched in 1977 and took more than 35 years to reach the heliopause. (The Pioneer probes left the solar system but were defunct by that time.) Now their radio communications are increasingly feeble, and several instruments have failed.
Voyager 1, the most distant human-built object in the universe, is now 145 astronomical units from Earth (an astronomical unit is equal to the distance between Earth and the sun). At that pace, it would take 283 years to reach 1,000 AU — 93 billion miles from the sun — the place McNutt hopes to reach. “To really explore what’s out there … you want to get out of the solar system as quickly as possible,” he said.
And for that, you need a really big rocket. NASA might soon have one. The ultrapowerful (but long-delayed) Space Launch System, which is capable of nearly twice as much thrust as the biggest rocket in operation, is expected to make its first flight sometime in 2020 or 2021.