Color image of rocket engines on the surface of the Moon.

Apollo 17's attitude control thrusters on the Moon. Cassini used the same design on its mission to explore Saturn.

Giving and Receiving: A Mission Tradition

By Jay R. Thompson

Feature | November 17, 2021

At Thanksgiving gatherings, food is shared, leftovers are coveted, and different people have different specialties.

Interplanetary missions are similar, but with spaceship parts instead of potatoes or pie.

Like Thanksgiving, spaceflight is about heritage, people’s specialties, the right ingredients, and leftovers. And if Grandma’s pies are legendary, that’s what she’ll be asked to bring.

Color image of men working on spacecraft
A spare Radioisotope Thermoelectric Generator (RTG) from Cassini helped power the New Horizons mission to explore Pluto and beyond. The RTG is the black cylinder on the left.

“You always go back to who does it best,” said Julie Webster, who served as manager of spacecraft operations for NASA's Cassini spacecraft at the Jet Propulsion Laboratory (JPL). For example, the same California company that provided several previous missions with propulsion system heaters was invited to make Cassini’s heaters, and the Italian company that built star trackers for NASA's Galileo mission to Jupiter was asked to provide Cassini’s.

Responsibilities on the Cassini mission were also sometimes shared and sometimes not, just like in a kitchen. Cassini’s magnetometer instrument was built in the U.S. but it was operated by scientists in the United Kingdom, and Cassini’s cosmic dust analyzer was built in Germany and it was operated by Germans.

Mission developers choose their ingredients based on what has and hasn’t worked in the past.

“Much of the Cassini spacecraft was based on Voyager and Galileo heritage,” Webster said. From those missions, engineers learned what makes an interplanetary mission succeed or fail. So they designed Cassini with complete backups of several systems including its computer, main engine, thruster system, radio, and others.

"Much of the Cassini spacecraft was based on Voyager and Galileo heritage."
- Julie Webster, Cassini Chief Engineer

Cassini also used hardware with a proven record. The spacecraft’s attitude control thrusters followed the same design as those on the Voyager spacecraft, and Cassini’s main engine was the same design as the Apollo lunar module’s attitude control thrusters. That meant the Cassini engine wouldn’t need to be designed from scratch and that it was already known to be reliable. “That’s why we didn’t have to do much testing on it,” Webster said.

Julie Webster in a cleanroom with Cassini spacecraft.
Julie Webster with the Cassini-Huygens spacecraft before it was sent to environmental testing in 1996.

Spaceflight also frequently makes use of leftovers, and some of Cassini’s hardware was left over from previous missions. For example, one of Cassini’s power supply units, called an RTG (for radioisotope thermoelectric generator), was a leftover from the batch of RTGs originally made for Galileo and Ulysses spacecraft. Another spare RTG that wasn't used for Cassini ended up being used to provide power for New Horizons the first spacecraft to explore Pluto and the Kuiper Belt.

Leftover hardware results from the same reasoning that Thanksgiving leftovers do: Sometimes it’s better to have too much than not enough.

When an interplanetary space probe requires something – a spectrometer, magnetometer, or a power source – the manufacturers don’t build just one. “Typically, anytime you have flight hardware, you will also build at least an engineering model and a qualification unit,” Webster said.

Flight hardware is the component that actually gets attached to the spacecraft and launched into space. The qualification unit is a functioning replica of the flight hardware, and it is used for testing before launch. The engineering model is a functioning replica as well, and engineers use it for testing and troubleshooting even after the spacecraft has left.

Every piece of hardware on the spacecraft needs to be tested before it's used, like tasting a dish before serving it to guests. Flight hardware and flight spares alike are temperature tested, for example. Engineers cycle the hardware’s temperature up to as hot as they think it could experience in flight, and then down to as cold as it could experience. They do this multiple times until they’re confident it’ll survive whatever space has to offer. Engineers do the same for the engineering model even though it’s not going into space.

The qualification unit, however, isn’t treated so kindly. It’s baked and frozen repeatedly in much greater temperature extremes than the spacecraft will actually experience, to see what the hardware can really handle.

But sometimes three copies of a component aren’t enough.

"The most dangerous thing that a spacecraft goes through is humans touching it before launch."
- Julie Webster, Cassini Chief Engineer

“When you absolutely need something for the mission, you might make five,” Webster said. That is, some components on the spacecraft are more critical than others. For example, if one of Cassini’s dozen science instruments had been damaged beyond repair close enough to launch day that it couldn’t be replaced, the spacecraft could have launched without it. The spacecraft could have launched without that one instrument and still achieved most of its science goals.

Some components, however, could have ended the mission if they broke. Cassini absolutely needed to carry two radios – one primary and one backup – or it might have faced a situation where it couldn’t have shared its awesome discoveries with Earth. “We were not going to launch Cassini without two radios,” Webster said.

But the team wasn't going to build only two radios and hope nothing happened to them before launch. “The most dangerous thing that a spacecraft goes through is humans touching it before launch,” Webster said.

Flight hardware can be dropped, crushed, or otherwise damaged, just like a Thanksgiving pie. So engineers build more copies of some components than they actually plan to use, and then engineers and scientists test them to determine which ones are the most precise, or the strongest, or the most effective of the bunch. The best devices are selected for use on the spacecraft, and the second-best become “flight spares,” which can replace the instrument on the spacecraft if that hardware gets damaged before launch. If all goes well, however, the instrument on the spacecraft launches undamaged, and the spare unit stays home.

But extra hardware isn’t simply tossed in the trash. It’s protected and stored because another mission might use it. Building new spaceflight hardware can be time-consuming and expensive. Even though a backup camera or dust analyzer isn’t quite as good as the best one, the difference in quality between the two can be so tiny as to be almost meaningless. For example, when Cassini's design needed to change several years before launch on Oct. 15, 1997, the mission was able to reduce costs by using spare optical components from the Voyager mission to construct the spacecraft's wide-angle imaging camera.

Cassini not only accepted leftovers from other missions, but it shared some leftovers of its own. One of Cassini’s radios was used by the NEAR-Shoemaker spacecraft, which studied (and later landed on) the near-Earth asteroid Eros. The Chandra X-ray Observatory, a space telescope, used Cassini’s spare data recorder.

Unfortunately, said Webster, “No one took our solid-state power switch, which is sad.” It seems that, like with Thanksgiving, some dishes are no one’s favorite.

Man at computer console
During its 20 years in space, the Cassini spacecraft was monitored from the Spaceflight Operations Facility at NASA's Jet Propulsion Laboratory (JPL).

In 2016, for the mission's final Thanksgiving, some members of the Cassini team spent part of the day at JPL performing an orbit trim maneuver. It’s not unusual for spaceflight engineers to spend holidays at work. “We have a track record of hitting major holidays with maneuvers and other mission events,” Webster said. And someone had to keep an eye on Cassini that holiday.

The spacecraft would keep zooming through space at thousands of miles per hour no matter what day of the year it was, Webster said. “The spacecraft didn't care that it was thanksgiving.”

Julie Webster, the spacecraft operations team manager for the Cassini mission, rips up the final contingency plan for the mission on Sept. 15, 2017. On that date, Cassini ended its remarkable journey of exploration. | Full image and caption

After 20 years in space – 13 of those years exploring Saturn – Cassini exhausted its fuel supply. And so, to protect moons of Saturn that could have conditions suitable for life, Cassini was sent on a daring final mission that would seal its fate. After a series of nearly two dozen nail-biting dives between the planet and its icy rings, Cassini plunged into Saturn’s atmosphere on Sept. 15, 2017, returning science data to the very end.

And for that, we are all very grateful.

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