<<An atmospheric probe was released from the Galileo spacecraft in July 1995, entering the planet's atmosphere on December 7. It parachuted through 150 km of the atmosphere, collected data for 57.6 minutes, and was crushed by the pressure to which it was subjected by that time (about 22 times Earth normal, at a temperature of 153 °C)
. It would have melted thereafter, and possibly vaporized. The Galileo orbiter itself experienced a more rapid version of the same fate when it was deliberately steered into the planet on September 21, 2003, at a speed of over 50 km/s, to avoid any possibility of it crashing into and possibly contaminating Europa—a moon which has been hypothesized to have the possibility of harboring life.
The 339-kilogram atmospheric probe, built by Hughes Aircraft Company at its El Segundo, California plant, measured about 1.3 meters across. Inside the heat shield, the scientific instruments were protected from ferocious heat during entry. The probe had to withstand extreme heat and pressure on its high-speed journey into the Jovian atmosphere, traveling at 47.8 km/s.
The probe was released from the main spacecraft in July 1995, five months before reaching Jupiter, and entered Jupiter's atmosphere with no braking beforehand. The probe was slowed from its arrival speed of about 47 kilometers per second to subsonic speed in less than two minutes. This was by far the most difficult atmospheric entry ever attempted; the probe had to withstand 230 g and the probe's 152 kg heat shield, making up almost half of the probe's total mass, lost 80 kg during the entry. NASA built a special laboratory, the Giant Planet Facility, to simulate the heat load, which was similar to that of an ICBM-style straight-down reentry through a thermonuclear fireball.
It then deployed its 2.5-meter parachute, and dropped its heat shield.
As the probe descended through 150 kilometers of the top layers of the Jovian atmosphere, it collected 58 minutes of data on the local weather. It only stopped transmitting when ambient pressure exceeded 23 atmospheres and temperature reached 153 °C. The data was sent to the spacecraft overhead, then transmitted back to Earth. Each of 2 L-band transmitters operated at 128 bits per second and sent nearly identical streams of scientific data to the orbiter. All the probe's electronics were powered by lithium sulfur dioxide (LiSO2
) batteries that provided a nominal power output of about 580 watts with an estimated capacity of about 21 ampere-hours on arrival at Jupiter.
The probe included six [specialized sounding
] instruments for taking data on its plunge into Jupiter:
- an atmospheric structure instrument group measuring temperature, pressure and deceleration,
a neutral mass spectrometer and
a helium-abundance interferometer supporting atmospheric composition studies,
a nephelometer for cloud location and cloud-particle observations,
a net-flux radiometer measuring the difference between upward and downward radiant flux at each altitude, and
a lightning/radio-emission instrument with an energetic-particle detector that measured light and radio emissions associated with lightning and energetic particles in Jupiter's radiation belts.
Total data returned from the probe was about 3.5 megabits. The probe stopped transmitting before the line of sight link with the orbiter was cut. The likely proximal cause of the final probe failure was overheating, which sensors indicated before signal loss.
The atmosphere through which the probe descended was somewhat hotter and more turbulent than expected. The probe was eventually completely destroyed as it continued to descend. The parachute would have melted first, roughly 30 minutes after entry, then the aluminum components after another 40 minutes of free fall. The titanium structure would have lasted 6.5 hours more before disintegrating. Due to the high pressure, the droplets of metals from the probe would finally have vaporized once their critical temperature had been reached, and mixed with Jupiter's liquid metallic hydrogen interior.>>