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Lessons from a failed Jupiter probe
Lessons from a failed Jupiter probe

Stories of tech

Lessons from a failed Jupiter probe

A 1991 Jupiter mission that went awry shows us that what makes a moonshot a success isn’t always landing on the moon.

Written by David W. Brown

Illustrations by Ryan Nguyen

Things were bad, and were about to get worse. John* could see the wear of recent weeks, the disappointment in the faces of his team. The men and women in the room with him at Jet Propulsion Laboratory (JPL), NASA’s research and development center, had spent much of 1991 trying to save the Galileo spacecraft. Nothing was working.

NASA launched the probe two years earlier, with a goal of studying Jupiter and its mysterious moons of fire and ice. It had spent those two years soaring across the solar system, its enormous, umbrella-like high gain antenna in a stowed configuration to protect it from the Sun while the spacecraft swung by Venus. (In space, nothing separates you from solar radiation, and to be that close to a star is a rough business for a little robot.)

Until now, things seemed to have been going well. The Venus slingshot maneuver — in which the planet’s gravity hurled the spacecraft around the Sun — went off without a hitch. After a subsequent Earth gravity assist to build up more speed, Galileo was finally in a safe spot in space, 37M miles away from Earth. But when JPL engineers ordered the spacecraft to open the sixteen-foot antenna, the spacecraft reported that it could not.

And that was that.

Everyone on John’s team, gathered now in one of the lab’s myriad conference rooms, was well aware of the magnitude of the problem. The large antenna was an integral part of this mission. NASA spacecrafts are flying laboratories that carry precision scientific instruments. When building them, scientists spend years working out precisely what is and is not known about the destination, and what experiments might solve those mysteries.

Those experiments collect vast quantities of data, which must be transmitted back to Earth. To do so, it would need to use its giant high-gain antenna. The spacecraft had a second antenna about the size of a coffee can, of course, but it was only good for sending back very simple telemetry data (such as: “No, I cannot open the high gain antenna”).

Maybe there was water in those moons, too. And conceivably a lot of it — entire oceans. And where there is water, there might be life.

They had come so far, sacrificed so much to build this billion dollar spacecraft. There was no flying it back to Earth to repair. John needed his team to find their mojo, and fast, because NASA, the science community, indeed, the world, were all counting on them.

John stood, looking around the room at his team. Engineers young and old stared back. It would take one hell of a speech to get things back on track.


The story of Galileo began much, much earlier than that tense moment at JPL. When NASA’s two Voyager probes flew by Jupiter in 1979, they were supposed to find a few chunks of icy rock circling a swirling ball of hydrogen. At five times the distance of the Earth from the Sun, the Jovian system was too cold for anything all that interesting, aside from Jupiter itself.

What the Voyagers found, however, defied imagination.

Io, the closest large moon to Jupiter, was literally spilling its molten guts into space. Jupiter’s gravity and its interplay with other moons made Io the most volcanic moon in the solar system. Europa, the next moon out, was crystalline and colorful — the jewel of the Jovian system. And Ganymede and Callisto were bronze and speckled, like animal skins or hammered sheets of metal. The ice was there, yes, and the rock, as expected, but with that much gravity and heat, maybe there was water in those moons, too. And conceivably a lot of it — entire oceans. And where there is water, there might be life.

In 1981, after the two Voyager discoveries of a wonderland circling Jupiter, JPL engineers settled on a spacecraft design dedicated to the Jupiter system, studying the planet and its moons in exquisite detail. They would later name it Galileo.

Though Galileo could launch on just about any big rocket, the U.S. at the time had gone all-in on the Space Shuttle, and tied all its high priority missions to the “space truck” as a way of preserving its funding. So it would carry Galileo. JPL in Pasadena, California finished building the Jupiter probe in December 1985, and shipped it by truck to Cape Canaveral, Florida, for launch. The idea was to launch Galileo directly from Earth to Jupiter.

When the shuttle Challenger blew up the following month, however, the fleet was grounded, and NASA drove Galileo back to California, now to launch in 1989. JPL did a few upgrades to the spacecraft in the interim, and Galileo spent the rest of the time in storage waiting for its turn. Then, for the third time, technicians trucked it across the country. This time, it did leave Earth. But because the positions of the planets had changed in that time, Galileo could no longer launch directly to Jupiter. Instead, NASA would fly it to Venus to get a “gravity assist”; the planet would slingshot the spacecraft back to Earth for two further speed-building slingshots.

When engineers send a command to a spacecraft, they get back data on the nature of every part of the spacecraft. In the case of the high-gain antenna, it’s not that a blinking red light said, “STILL CLOSED!” Rather, they received telemetry stating that a screw mechanism that opened the parabolic antenna had not traveled the entire distance necessary to indicate an open antenna.

“The first question was: What does that mean?” John told me in an interview. Was the antenna open or not? There were no little cameras you could point at the dish. To determine how far it had opened, they tilted the spacecraft this way and that to see when it blocked an onboard sun detector, the same way a beachgoer might adjust an umbrella to block daylight. By doing this, with minimal math they could determine precisely how “open” the antenna was.

“As we made the turn, instead of just saying, ‘Okay, it's covered,’ the telemetry began saying, ‘It's covered, not covered, covered, not covered, covered, not covered.’ Corresponding to the rotation, speed, and rate of the spacecraft, it told us that the antenna was not symmetrically out,” he said.

In other words, one or more of the umbrella’s ribs were stuck. John speculated that pins got jammed during the multiple cross-country drives to and from Cape Canaveral. It could also have been that lubricant on the antenna ribs dried. The good news is that they could simply reverse the process, closing the antenna and reopening it, the same way closing an umbrella and reopening it will help it open properly. The bad news?

“We didn't have a reverse control on it,” said John. They had removed it, using its circuitry instead to install another instrument on the spacecraft. “They decided that the risk of the high gain antenna needing to be reversed was not very likely to happen.”

Until it did.


In the conference room that day, John’s team was despondent. Their job was to write commands to send to the spacecraft to try. So far, they had tried tilting the spacecraft repeatedly into the Sun’s full light, heating up the antenna ribs and hopefully loosening things up. But metallurgy from tens of millions of miles didn’t work. Then they tried doing the opposite: tilting the antenna away from the Sun, hoping that by freezing the antenna ribs, it might loosen things up. That, too, didn’t work. They considered spinning the spacecraft like crazy, hoping that centrifugal forces might open the antenna. No good. The team was ready to throw in the towel.

So John addressed them.

“You’ve got to believe we can do this,” he said. “JPL is really good at these sorts of things. We will solve it. So you can either be sad, depressed, and quit, or you can be part of something that could recover a mission. And you'll look back on this all your life, no matter which choice you make.”

It was true JPL was good at that sort of thing. No two missions NASA flew were alike. Every spacecraft was essentially built from nothing, engineered for the specifics of the body about to be explored. A spacecraft at Saturn, where the Sun was a mere dot in space, would have different needs than a spacecraft at Mercury, where the Sun would be enormous. And JPL had by then explored nearly every planet in the solar system. They had orbited Venus and landed spacecraft on Mars. The first American satellite in space? JPL. Solving hard problems was their entire purpose. The lab’s motto said it all: “Dare mighty things.”

John knew it, and John’s team knew it. And his team got to work.

Like many moonshots, their big dreams didn’t work out. In the end, the high gain antenna never opened. Instead, the engineers came up with a different strategy. What could they do with that other coffee can antenna, the little one for the telemetry? An array of three powerful antennas on Earth talked to spacecraft millions of miles away. Was there some way to crank up its sensitivity? There was. First, rather than talk to any single antenna of three, as usual, what if they pointed two antennas at Galileo? They could combine and clean up the data received and pull more signal from the noise. They also upgraded the dish hardware for better reception.

Meanwhile, they rewrote the software on Galileo — literally upgraded its entire computer core from the other side of the solar system — to use super high compression algorithms. And they cranked up the coffee can antenna’s output. They found a way to transmit 1,000 bits per second — six times faster than its previous best. They would lose a third of the mission’s science, but a kilobit was enough: the mission would be saved.

Because of their efforts, Galileo discovered that Ganymede has its own internal magnetic field — the only moon in the solar system to do so. They discovered the nature of the powerful, 330 mile per hour permanent winds that swept across Jupiter’s atmosphere. (For comparison, these winds, unchanged for centuries, are more powerful that the most powerful tornado to ever strike on Earth).

Most critically, Galileo discovered that beneath Europa’s ice shell is, indeed, an ocean bearing three times more liquid saltwater than can be found on the planet Earth. In that ocean are all the ingredients necessary for life. Not just microbes, but complex life. There might be fish down there. There might be sea monsters. In 2024, NASA will launch its next spacecraft to the Jupiter system: Europa Clipper. And it will answer whether, indeed, Europa is habitable.

If one day scientists find life, nothing — not biology, not astronomy, not philosophy or religion — will ever be the same again. And even though Galileo didn’t achieve its mission as intended, it did, indeed achieve something incredibly important.

The astronauts of Apollo 13 never landed on the moon, but the capsule’s amazing rescue transformed NASA engineering culture evermore. Ernest Shackleton never reached the South Pole, but the fact that after losing his ship and all hope of rescue from Antarctica in 1915, he somehow led his men on a 19-month escape from the ice — without losing a single member of his team. Sometimes in the course of exploration, hindsight reveals that the mission objective is far less important than the fact of the mission itself.

And with Galileo, NASA engineers were reminded that as long as they can talk to a spacecraft, they can solve any problem, no matter how seemingly-insurmountable. What makes a moonshot a success isn’t always landing on the moon. Sometimes, it is the marshaling and measurement of tenacity and ingenuity. It’s not always about what happened when the plans worked; it’s about what happened when everything went wrong.

*A pseudonym; because of the sensitivity of his current position at the laboratory, John requested not to be identified.

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