On December 7, 1995, a crowd gathered in an auditorium at the Jet Propulsion Laboratory, in Pasadena, California, waiting for an event that had been years in the making. At 8:04 pm PST, a stout, 4-foot (1.25 meter)-wide, wok-shaped robotic probe, which had been released from the unmanned Galileo spacecraft five months before, finally reached its destination: the atmosphere of the giant planet Jupiter.


As planned by NASA engineers, the small probe, with a mass of 750 pounds (339 kilograms) and packed with seven scientific instruments, plunged into the planet's atmosphere at the mind-boggling speed of about 106,000 miles (170,000 kilometers) per hour. "From down inside Jupiter's atmosphere," a NASA report lyrically imagined, the probe would resemble "a spectacular fireball, streaking east out of the setting sun and towards the gathering night."



Before the probe melted in the intense heat and pressure of the Jovian atmosphere, scientists hoped it would take measurements temperature, winds, and more—including measurements that would help provide more evidence supporting the Big Bang theory of the formation of the universe. In Dark Universe, the new Hayden Planetarium Space Show open at the Museum, viewers will have a you-are-there view of the probe's breathtaking drop—one of the more dramatic episodes in the history of unmanned missions and an exhilarating story about the new age of discovery that is revealing more about our surprising universe.


The descent lasted 57 minutes. Quickly slowed by the thick layer of gases in Jupiter's atmosphere, the probe released one of its parachutes, dropped remnants of its heat shield, and, swaying beneath its main chute, established a radio link with the Galileo Orbiter. In less than an hour, the little probe managed to send home data about the planet's sunlight, temperature, winds, and more—including measurements that would help provide more evidence supporting the Big Bang theory of the formation of the universe.


Named for the famed Florentine astronomer, the spacecraft Galileo was launched aboard Space Shuttle Atlantis in 1989. By then, a mission to Jupiter had been in the works for decades.

The most massive planet in our solar system, the gas giant Jupiter has no solid surface and boasts dozens of moons with a variety of fascinating features, from active volcanoes on Io to icy surfaces on Europa. Flybys by four other spacecraft in the 1970s suggested there was much more to learn from Jupiter, not just about the planet itself but also about the formation of the universe.

"At that point, we had tantalizing hints about the structure and composition of the Jovian atmosphere, but had no solid evidence from direct measurements," explains Mordecai-Mark Mac Low, curator in the Museum's Department of Astrophysics who curated the new Space Show. "The Galileo probe was designed to dive into the atmosphere to make those measurements."

But the Galileo mission—which included flybys of Venus, Earth, and even an asteroid en route to Jupiter—faced a big setback a year and a half into its six-year journey. The spacecraft's 15-foot (4.6 meter)-wide, umbrella-like communications antenna—called a high-gain antenna and designed to transmit data back to Earth—did not open. Without that antenna, the ability of the spacecraft to send home information about what it learned at Jupiter was in jeopardy.

For several weeks after the failure, a special team of more than 100 technical experts from the Jet Propulsion Lab and elsewhere worked to diagnose the problem. Was the issue something to do with the orbital path of the spacecraft? Or had the problem existed at the time the craft left Earth? (Ultimately, the long delay for launch after the Challenger space shuttle explosion in 1986 was determined to have played a role: lubricant had drained out of the umbrella mechanism.) The antenna remained useless, forcing a different approach.

Finally, the team developed new software so that data could trickle back via the functional low-gain transmission antennas instead—saving the mission, but at a cost. Instead of a high-speed data link, the spacecraft was communicating at the speed of an ancient 300-baud modem.

Still, the solution kept Galileo in service—and on track to collect important data well through 2003, when the spacecraft finally ran out of power. (That year, NASA crashed the craft into Jupiter, after it had covered more than 4 billion miles.) The mission's discoveries about Jupiter were many: that meteorite impacts had created the planet's dust-grain rings, that its moon Europa seems to have a salty ocean beneath its frozen surface, and that Jupiter's atmosphere has thunderstorms, with lightning strikes up to 1,000 times more powerful than lightning on Earth.

And then there was the data transmitted back by the probe. A mass spectrometer was on board to measure the chemical composition of Jupiter's atmosphere. At the time, NASA engineers voiced excitement about collecting information about the formation of the solar system, such as measuring the concentration of noble gases (which never freeze or liquefy) in Jupiter's atmosphere, to see whether their concentrations matched those known to be present during the formation of the solar system, about 4.5 billion years ago. But a different measurement—one that wasn't touted much in the run-up to Galileo's arrival at Jupiter—ended up gathering evidence in support of the Big Bang theory as well.

To understand how the Galileo probe could find information about the formation of the vast universe on relatively nearby Jupiter, it helps to review the Big Bang theory.


Some 13.8 billion years ago, the universe was everywhere extremely hot and dense. It expanded explosively fast, cooling as it did so, and its energy condensed into particles of matter—the protons, neutrons, and electrons that make up the atoms we know. The Big Bang theory posits that the universe remained hot enough for a short time for heavier atoms to fuse together from the protons, normal hydrogen nuclei, and neutrons, including helium, lithium, and a small amount of hydrogen with an extra neutron tacked on, called deuterium. Once the temperature had fallen too low to sustain further nuclear fusion everywhere, heavy atoms could only be produced by fusion in the centers of stars. But stars cannot form deuterium, only destroy it. If the Big Bang theory holds, there should be evidence in the universe that the deuterium abundance is lower than it was predicted to be right after the Big Bang.

"If you can get a measurement of deuterium someplace where it's been reasonably well preserved in the eons since the Big Bang," says Mac Low, "then you can check the prediction of the abundance formed during the initial expansion of the universe."

But there aren't that many places that retain the primordial proportions of these elements. The abundance of deuterium in stars falls over time as it is destroyed. Our own planet Earth is so small that most of its hydrogen has escaped, so there is actually more heavy deuterium relative to hydrogen here than you might expect. But since Jupiter is so massive—it comprises some two-thirds of all the mass of the planets—its gravity keeps hydrogen and deuterium from escaping. As Mac Low explains, "Jupiter functions as a cold storage locker for deuterium."

When Galileo's probe parachuted into Jupiter's atmosphere, its mass spectrometer measured information about the hydrogendeuterium ratio—data that researchers back on Earth were able to analyze. "It turns out," Mac Low says, "to agree extremely well with the predictions of the Big Bang theory."

Other measurements from even more ancient sites provided additional confirmation. During the same period, researchers managed to measure light from bright, distant quasars that passes through intervening, almost pristine gas clouds in space that had never formed stars and so were a relatively clean sample of the atoms produced in the Big Bang. By measuring which light-wavelengths are absorbed by these clouds, astronomers have been able to infer that they, like the much younger Jupiter, contain the ratio of deuterium to hydrogen predicted by the Big Bang theory.

While the probe's mission to Jupiter's atmosphere didn't provide the definitive proof of the Big Bang theory by itself, it is, like many scenes in the new Space Show, an example of how researchers work to make the most of each and every foray into space, and how discoveries build on one another to confirm astonishing ideas–such as that the entire universe was once hotter than the center of a star. Each new finding adds to our knowledge of the universe beyond–and raises intriguing new questions as well.

Learn more about the new Space Show, Dark Universe.