Somewhere in the considerable reading I’ve done on the Apollo program, a comment from William Anders stuck with me. Anders was the lunar module pilot on the historic Apollo 8 mission in 1968. It marked the first time a manned spacecraft had departed earth orbit to journey to another body in the solar system.
Anders would have known all about the trajectory calculations and the burns necessary to complete the trip, but still, he said, when Apollo 8 approached the moon in shadow, all he could see was a giant, moon-shaped hole in the star field. That gave him cause to worry about someone misplacing a decimal point, turning what was supposed to be a close flyby into a direct hit.
Didn’t happen, of course. Apollo 8 grazed by 70 miles above the lunar surface, injected itself into orbit and made history. I thought about this story when looking over the coverage of NASA’s latest triumphal mission, the Juno Jupiter exploration project. There are some interesting parallels. Apollo 8 was the first manned spacecraft to go to and orbit the moon; Juno is the most sophisticated spacecraft of any kind to orbit Jupiter, although it’s not the first. Jupiter is, in fact, the most visited of the outer planets, with eight other missions aimed at the gas giant, the first all the way back in 1973, with Pioneer 10, which whizzed within 82,000 miles of Jupiter. (Pioneer 10 plugged away until 2003, when it finally ran out of power. At that point, it was more than 7 billion miles from earth.)
Juno’s orbital insertion was more of the Apollo 8 variety. It approached to about 3000 miles of the surface, a mere 3 percent of the planet’s diameter. Apollo 8 similarly flew about 3 percent of the moon’s diameter above the lunar surface. But Apollo’s trajectory calculations were quite a bit simpler. It had a 70-hour trip of 240,000 miles. Juno’s trajectory design was shaped by the available launch energy and consisted of a heliocentric orbit, followed by an earth gravity assist flyby: five years and 1.7 billion miles.
The calcs on both missions were obviously pretty impressive. Apollo 8 required one mid-course corrective burn, Juno needed two, despite flying nearly 7000 times farther. It hit its insertion window within one second and a handful of kilometers. Although Juno got a little help from the ground, it may be the most autonomous—there’s that word again—spacecraft ever simply because it has to be. Jupiter’s distance from earth varies widely from a minimum of 365 million miles to more than 600 million miles. Currently, it’s about 550 million miles, meaning round-trip radio signals take about 90 minutes; far too sluggish for meaningful ground command. Apollo, of course, had guys onboard to monitor the relatively crude computer overseeing the orbital insertion.
And speaking of computers, you’d think that 45 years after Apollo Juno would have one of supreme sophistication in order to fly through deep space autonomously for so many years and miles. But, not exactly. Or at least sophisticated in a different way. The spacecraft’s main computer isa flight-proven system called a RAD750, built by BAE Systems. It’s a single-board affair with a whopping 256 megabytes of flash memory and 128 megabytes of DRAM. If that sounds like a fraction of the power of the computer you’re reading this on, it is. The design trades sophistication and power for something more important: reliability. The system is capable of enduring radiation a million times higher than what a human could survive. We’ve been getting spacecraft out to Jupiter for nearly a half century in one form or another on computing power far less impressive than Juno’s.
Juno’s instrument package didn’t require any new technology, but it’s got a lot of sensors aboard, including a radiometer for atmospheric sounding, instruments for magnetic field studies and a UV imager/spectrometer system. Interestingly, it has a camera, but it was added for PR purposes, simply to show the paying public some nice shots of Jupiter for its $1.1 billion price tag. That’s less than the Mars rover missions cost, but more than simple orbiter projects to other planets.
Is it worth it? I’m the wrong guy to ask, frankly. My kneejerk answer is of course it’s worth it. This is basic exploration of the sort man has been doing since he figured out how to bang rocks together to make tools. It’s nothing more or nothing less than pure inquiry for the sake of inquiry. Run the clock back to the turn of the last century and two brothers from Dayton, Ohio, were doing the same thing on a beach in North Carolina, albeit with their own money. I’m sure they couldn’t have imagined we would be flying to Jupiter barely 70 years later.
