Date: October 19, 2009

Title: When Galileo Fell into Jupiter


Podcaster: Adam Fuller from Columbia Astronomy

Organization: Columbia University Astronomy –

Description: In December 1995, NASA’s unmanned spacecraft, Galileo, dropped a probe into Jupiter’s atmosphere and provided us with the first in situ observations of an outer planet’s atmosphere. Getting the probe into an atmosphere with six times Earth’s gravity and twenty times Earth’s atmospheric pressure was extremely difficult. Having the probe enter in a scientifically interesting location even more so. In this podcast we’ll discuss how NASA dealt with these issues as well as what we learned from the Galileo probe and orbiter.

Bio: Adam Fuller is currently a graduate student at the University of Unknown. He graduated from Columbia University with a B.S. in Astrophysics in 2009. He also has a B.A. in Journalism from North Carolina (Go Heels!) His research interests include planetary science, meteorology, and astrobiology. Outside of school, he is an avid marathoner, a failed comedian, and a dedicated uncle to three proto-astrophysicists.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the American Association of Variable Star Observers, the world’s leader in variable star data and information, bringing professional and amateur astronomers together to observe and analyze variable stars, and promoting research and education using variable star data. Visit the AAVSO on the web at


Hello everyone, and welcome to Columbia Mondays! My name is Adam Fuller, and I’m a recent graduate of the undergrad astrophysics program at Columbia University in the City of New York. I’m currently a first year grad student in the Planetary Sciences department at Johns Hopkins University.

In today’s 365 Days of Astronomy podcast we’re talking about the Galileo Mission to Jupiter, the unmanned spacecraft NASA sent to Jupiter over 14 years ago. It was an incredibly successful mission that watched Jupiter swallow a comet, discovered an asteroid with a moon, and detected life on Earth. More importantly, it provided us with the first in situ observations of an outer planet’s atmosphere. To do that it had to survive “history’s most difficult atmospheric entry” by enduring two and a half times Earth’s gravity, atmospheric pressures over twenty times greater than Earth’s, and temperatures that dropped 1,400 degrees Fahrenheit in less than three minutes! Today we’ll talk about the design of the Galileo spacecraft and how it survived these extreme conditions. I recommend following along with this podcast’s transcript so you don’t miss any of the amazing details of this journey.

The Galileo spacecraft consisted of a planetary orbiter with an attached atmospheric entry probe. The orbiter weighed about 5,250 lbs. (2,380 kg) on Earth and stood over 17 feet tall (5.3 meters.) The probe weighed roughly 750 lbs. (338.93 kg) and had a diameter of about 4.3 feet (1.3 meters.) The spacecraft had a 1.6 MHz main processor that was fabricated on sapphire to protect it from radiation. It had the computing power of an Apple II desktop computer or an Atari 2600 — appropriate at the time when the spacecraft was constructed in the late 1970s. Galileo was launched by the Space Shuttle Atlantis on October 18, 1989. It was originally planned to launch in January 1982, but various delays, including the Challenger disaster in January 1986, postponed its launch for seven years.

Galileo had three primary scientific objectives: (1) study the geological composition and structure of Galilean moons Io, Europa, Ganymede, and Callisto, (2) characterize the plasma physics at work in the Jovian magnetosphere, and, most importantly, (3) deploy the entry probe into Jupiter’s atmosphere to study its composition, structure, and dynamics. The probe piggy-backed on the orbiter during liftoff and most of the spacecraft’s trip to Jupiter. On July 13, 1995, the Galileo spacecraft deployed the atmospheric entry probe while both were still roughly 50,000,000 miles (80,000,000 km) and five months from Jupiter. They traveled together the remainder of the way and arrived simultaneously at Jupiter on December 7, 1995.

Jupiter is the largest planet in the Solar System. It’s about 89,000 miles (142,984 km) across, slightly wider than 11 Earths lined up in a row. It has the mass of 318 Earths. It has 2.58 times Earth’s gravity which makes its escape velocity nearly 37.3 miles per second (60 km/s), a potentially fatal entry speed for a probe. (Earth’s escape velocity is 6.95 miles per second [11.186 km/s].) So how do we get the probe into Jupiter without it getting ripped apart by gravity or burned up upon entry into the atmosphere? To “ease” the Galileo probe into Jupiter, its arrival path was along Jupiter’s equator, and it traveled in the same direction the planet rotates. This subtracts about 7.46 miles per second (12 km/s) from its incoming velocity. Its flight entry angle also needed to be a tiny 8.6 degrees above Jupiter with only ±1.5 degrees for error. This is equivalent to shooting a softball from Venus to the Earth during inferior conjunction and getting it to enter our atmosphere through a region over the equator roughly the size of New Mexico at a speed of 5.6 miles per second (8.94 km/s).

Jupiter is a gas giant planet made up of 85% hydrogen, 13% helium, and 2% everything else. There is no solid surface like Earth’s, and, in fact, we don’t even know if it has a solid core like every other planet in our Solar System. Instead, descend into Jupiter one twentieth of its diameter (4,440 miles, 7150 km), and you’ll encounter atomic hydrogen compressed under such enormous pressures and temperatures that it behaves more like a gigantic metallic ocean, nothing like anything we’ve ever experienced here on Earth.

With no solid surface from which to measure altitude, we instead use the area of Jupiter’s atmosphere where the atmospheric pressure equals 1 bar. On Earth this is at sea level. To get an idea of how atmospheric pressure scales between Earth and Jupiter, we compare the location where the pressure drops to only 0.2 bars. On Earth this marks the boundary between the troposphere–under which weather occurs, planes fly, and humans live–and the stratosphere. It’s roughly 7.5 miles (12 km) above the ground. On Jupiter, the atmospheric pressure drops to 0.2 bars around 21.7 miles (35 km) above the 1 bar level. This is because of the differences in the two planets’ atmospheric compositions. While Jupiter is 85% hydrogen, Earth’s atmosphere is 78% nitrogen. Jupiter’s mean molecular weight is between 2.0 and 2.3 g/mol while Earth’s is 28.97 g/mol, almost 13 times greater than Jupiter’s; Earth’s atmosphere is much heavier than Jupiter’s.

The Galileo probe entered Jupiter’s atmosphere 640 miles (1,030 km) above the 1 bar level. Even though the atmosphere is very tenuous here, its temperature is 1,160 degrees F (900 K). The probe traveled at 29.3 miles per second (47.13 km/s). Its peak velocity was 29.5 miles per second (47.468 km/s) at an altitude of 134 miles (215 km). Here the temperature of the atmosphere had fallen to -185 degrees F (152.8 K). The probe traveled a radial distance of 506 miles (815 km) and experienced a 1,345 degrees drop in temperature in just 110 seconds.

The probe then experienced an extreme deceleration, slowing from 29.4 miles per second (47.3 km/s) to 1.72 miles per second (2.767 km/s) over the next 50 seconds. 45% of the probe’s total mass (335 lbs (152 kg) of 747.21 lbs (338.93 kg)) was the heat shield that protected the probe during entry into Jupiter’s atmosphere. During the extreme deceleration, 64% of the heat shield burned away (215.2 lbs (97.6 kg)), accounting for 29% of the probe’s total mass before entry. Again, this mass was burned off in 50 seconds. The atmospheric temperature at the beginning of this phase of the descent was about -177 degrees F (157.2 K) and -239 degrees F (122.6 K) at the end. The temperature *decreased* during deceleration, but the friction with the atmosphere was high enough to burn away the heat shield. Even with the remaining 120 lbs (54.4 kg) of heat shield, it still almost failed. It’s been calculated that the peak heating for the Galileo probe during this period of the descent exceeded 4.6 times the radiant energy flux above the Sun’s photosphere. The softball we launched from Venus had just fallen “straight down [into] a thermonuclear fireball” while still in Earth’s upper stratosphere.

160 seconds after entry, the probe had descended to an altitude of 38.8 miles (62.5 km), moving at 1.72 miles per second (2.767 km/s). The atmosphere’s temperature is roughly -239 degrees F (122.6 K). The speed of sound in this region of Jupiter’s atmosphere is around 0.65 miles per second (1,050 m/s), so the probe was moving at 2.6 times the speed of sound on Jupiter. If it were moving this fast in Earth’s atmosphere at sea level, it would be traveling at 8.3 times the speed of sound.

Between 32.3-34 miles (52-55 km) above 1 bar, the probe finally slowed to subsonic speed (in Jupiter’s atmosphere). This was between 15-20 seconds after the heat shield lost the bulk of its mass, 75-80 seconds after the probe’s peak velocity of 29.5 miles per second (47.468 km/s), and 175-180 seconds after the probe entered the atmosphere. The atmospheric temperature here hadn’t changed much and hovered between -255 degrees and -239 degrees F (113.2 to 122.6 K).

The probe continued its free fall for another 10-15 seconds before the parachute deployed at an altitude of roughly 30 miles (48.323 km). Here the probe’s velocity was now 1,600 mph (0.712 km/s) and had fallen 608 miles (979 km) in 190 seconds. It fell for another 50 seconds before the orbiter picked up the probe’s signal when the probe was at an altitude of 15.5 miles (25 km). So 4 minutes after the probe entered the atmosphere, it was fluttering downward at a pedestrian 980 mph (0.438 km/s). It continued to record and transmit measurements back to the orbiter for another 58 minutes and 40 seconds (for a total of 62 minutes and 40 seconds). During that time it descended another 82.3 miles (132.4 km) to a pressure of 22 bars and surrounding atmospheric temperature of 310 degrees F (427.71 K) before it finally failed.

According to Wikipedia, as the temperature of the surrounding atmosphere continued to climb higher, the parachute should have melted about 30 minutes later, the aluminum components after another 40 minutes of free fall, and finally the titanium structure six and a half hours later. The entire probe was probably completely vaporized less than 9 hours after it entered Jupiter’s atmosphere. The probe had just survived “history’s most difficult atmospheric entry” only to be swallowed up whole by Jupiter’s crushing atmosphere.

The orbiter remained in orbit around Jupiter, visiting the Galilean moons Io, Europa, Ganymede, and Callisto and making more measurements of Jupiter’s atmosphere and magnetosphere, until September 21, 2003. By then several onboard instruments had been damaged beyond repair by radiation, so the orbiter was deorbited and sent to join the remains of the probe in Jupiter’s atmosphere. Originally a two year mission, the Galileo spacecraft was in service for 8 years and spent a total of 14 years in space. Over the entire course of the mission, Galileo traveled almost 3 billion miles (4,631,778,000 km) and completed 34 orbits of Jupiter.

Well, I hope you’ve enjoyed this latest Columbia Monday podcast about when Galileo fell into Jupiter. For more information about the public events at Columbia Astronomy visit Our next Columbia Monday podcast will be “Radio Astronomy: An Important Tool” by Nitza Santiago on November 8th. I’m Adam Fuller. Have a great day and keep listening.

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