The problem with space is that it’s big. Like, really big! It takes light almost an hour to get to the Earth from Jupiter, and that’s the closest of the outer planets. In terms of space missions, this means that visiting the outer planets either requires very long transit times or very large launch vehicles. Long transits could mean decades, likely resulting in the original science team retiring before the spacecraft arrives. Large launch vehicles are risky and have a high price tag, severely limiting how many missions we can afford to launch. What we needed was a game changer, something that would let us use smaller rockets and still get there quickly; that game changer was the gravitational assist.
In the early years of the space race, mathematicians set to work coming up with a method to get to the outer planets more rapidly. The first breakthrough was in 1961 when then-graduate student Michael Minovich presented a paper describing a gravity assist. Here’s how he did it.
His first task was to calculate a change in orbit when a spacecraft encountered a planet — the so-called two-body problem. (This method is similar to how the game Kerbal Space Program calculates orbits.) After arriving at a solution, he tried a much more difficult task — the three-body problem. The three-body problem calculates a two-body encounter in the context of both bodies starting in orbit of the Sun. This is a much harder calculation, but the result was spectacular: Minovich demonstrated how a spacecraft could change its orbit using the encounter. This can be used to effectively fling the spacecraft into a higher orbit by stealing a tiny amount of the planet’s rotation.
Minovich’s work gained prominence when a scientist named Max Hunter used gravity assists to design a Grand Tour mission to the outer planets in 1964; the proposed mission only required five kilograms of propellent rather than the 145 metric tons required without using gravity assists. In 1965 Gary Flandro, a NASA engineer, identified a once-in-176-year opportunity to visit all four outer planets in a single mission, an opportunity that would happen twelve years later in 1977.
The gravity assist concept was demonstrated on the Pioneer 11 mission, which used it to do a massive handbrake turn in space to go from Jupiter to Saturn when the mission was only planned to go to Jupiter.
Initially, the Voyager program was only funded to visit Jupiter and Saturn and was called Mariner Jupiter-Saturn. But JPL was sneaky and designed the spacecraft to last long enough to do the full Grand Tour.
The next thing to determine was which trajectories to use. NASA engineers looked at ten thousand different trajectories. Very important to the science goals of the mission was a close encounter with Saturn’s moon, Titan. Titan was the only moon known to have an atmosphere at the time. Flying by Titan, however, would prevent the spacecraft from visiting Uranus and Neptune because the trajectory to get close to Titan would send it out of the ecliptic, the plane in which all of the planets orbit the Sun. One spacecraft would go to Jupiter, Saturn, and Titan and out of the ecliptic. The other would go to Jupiter, Saturn, Uranus, and Neptune. If the first spacecraft failed to get to Titan, the other would go to Titan and forgo the rest of the Grand Tour.
One major engineering consideration for spacecraft design involved the distance they would travel. They couldn’t use solar panels billions of kilometers from the Sun, so they would need nuclear generators known as Radioisotope Thermoelectric Generators or RTGs. These produce tiny amounts of electricity from radioactive decay over long periods of time. The heat is turned into electricity with a series of devices known as thermocouples which generate electricity based on their temperature.
That’s the trajectory and the spacecraft itself sorted, what about the science?
NASA put out a formal Request for Proposals in April 1972 in the following investigation areas: imaging, radio science, infrared and ultraviolet spectroscopy, magnetometry, charged particles, cosmic rays, photo-polarimetry, planetary radio astronomy, plasma, and particulate matter. They received over 200 responses from which ninety scientists were invited. These scientists were further organized into twelve teams, each in charge of a different instrument. You can read more about the different instruments in the Patreon bonus content for this week’s episode.
The instruments for the two spacecraft were chosen years in advance based on initial observations by Pioneer 10 and 11 at Jupiter and Saturn. For example, a planned ultraviolet polarimeter for Voyager was removed in favor of a plasma experiment after Pioneer 10 revealed a thousand times more intense radiation environment around Jupiter than expected.
One of the more famous things on Voyager 2 wasn’t an instrument and didn’t have a Principal Investigator — the “Sounds of Earth” record, more commonly called the Voyager Golden Record. Created by a committee led by Carl Sagan, it was included on the off chance that, if an intelligent species encountered the spacecraft millennia in the future, it would tell them something about humanity. It was a 32-centimeter wide gold-plated copper disk that had two hours of content recorded onto it, including nature sounds, music, and greetings in sixty languages.
On the outside of the disk were diagrams depicting human beings and different fundamental concepts in math and science as well as instructions on how to play it. In forty thousand years, Voyager 2 will pass within two light-years of the star Ross 248 in the constellation Andromeda.
Voyager 2 was launched on August 20, 1977, to go on the full Grand Tour trajectory. Its Titan IIIE rocket’s maximum performance was barely able to put it on the planned trajectory to eventually meet up with Uranus. To find out what happened with the rest of the mission and with Voyager 1, stay tuned to Rocket Roundup for future episodes of This Week in Rocket History.
Slingshot Magic (Gravity Assist)
Interstellar Mission (NASA)