Galileo (1989-2003)

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Overview

The Galileo craft was sent to study Jupiter in great detail. It consisted of an orbiter and an atmospheric probe. The probe was launched into Jupiter's atmosphere upon arrival (see below), and the orbiter studied Jupiter for about 14 years. The orbiter chiefly studied Jupiter's atmosphere, four largest moons, and its magnetosphere.

There were eight main objectives for the orbiter:

1. investigate the circulation and dynamics of the Jovian atmosphere
2. investigate the upper Jovian atmosphere and ionosphere
3. characterize the morphology, geology, and physical state of the Galilean satellites
4. investigate the composition and distribution of surface minerals on the Galilean satellites
5. determine the gravitational and magnetic fields and dynamic properties of the Galilean satellites
6. study the atmospheres, ionospheres, and extended gas clouds of the Galilean satellites
7. study the interaction of the Jovian magnetosphere with the Galilean satellites
8. characterize the vector magnetic field and the energy spectra, composition, and angular distribution of energetic particles and plasma to a distance of 150 Jupiter radii

The atmospheric probe had five main science goals:

1. determine the chemical composition of the Jovian atmosphere
2. characterize the structure of the atmosphere to a depth of at least 10 bars
3. investigate the nature of cloud particles and the location and structure of cloud layers
4. examine the Jovian radiative heat balance
5. study the nature of Jovian lightning activity
6. measure the flux of energetic charged particles down to the top of the atmosphere

Mission

The Galileo craft was supposed to be launched in 1986, and take a more direct two-year journey to Jupiter. Unfortunately, the space shuttle Challenger's explosion in that year delayed the craft until 1989. Galileo was launched on October 18, 1989 at 22:23:00 UTC.

The orbiter released the probe 147 days prior to the probe's entry. The atmospheric probe provided the first direct evidence as to the make-up and dynamics of Jupiter's atmosphere. On December 9, 1995, at about 17:04 (EDT), the 346 kg (760 lbs) probe opened its 2.5 m (8 ft) parachute and fell through Jupiter's atmosphere at speeds exceeding 160,000 kph (100,000 mph). Two minutes later it dropped its heat shield so that it could collect data on atmospheric structure, temperature, cloud and chemical composition, while also detecting lightning within 12,000 km (8,000 miles) of the probe's entry point (the probe found that lightning occurs about one tenth as often as on Earth, but that individual events were over 10 times stronger). A detailed timeline of the probe's entry is at the end of this section.

Some of the probe's findings indicated that there was less water than previously expected, but further study showed that the probe had merely encountered a dry spot. The probe also encountered winds of up to 530 kph (330 mph) with intense turbulence, which suggested that Jupiter's winds are driven by heat escaping from the planet's interior. The probe found less helium, neon, carbon, oxygen, and sulfur than expected. As expected, the probe encountered no solid objects or surfaces during its entire 600 km (373 mile) voyage. After 57 minutes, the extreme temperature and pressure of Jupiter's atmosphere destroyed the probe.

There was a single tape recorder on board the spacecraft; it was a four-track digital model manufactured by Odetics Corporation that could store up to 914,489,344 bits of data (that's about 109 Mb, or about 300,000 pages of text). Unfortunately, in 1995, Galileo's tape recorder became stuck in the "rewind" position for 15 hours, which permanently damaged a section of the tape.

The Galileo orbiter could only complete approximately 70% of its original science objectives. This is because in 1991 its 4.8 m (16 ft) diameter High Gain Antenna became stuck, and was unable to open completely. This forced NASA to only utilize the much slower Low Gain Antenna. The Low Gain Antenna could only transmit information at about 10 bits per second - about 100 times slower than the High Gain Antenna (if you are viewing this site with a 56k modem, then it is about 5,500 times faster than the Low Gain Antenna). The graphics-sensitive weather monitoring suffered the most.

The orbiter was expected to make over twenty total passes of Jupiter before its extended mission ends in December 1999 (its planned mission ended in December of 1997, two years after arrival). The mission was extended again to 2002.

The Galileo orbiter was terminated in a controlled entry of Jupiter's atmosphere on September 21, 2003. This entry destroyed the craft and any possibility for the craft to contaminate the moons, especially Europa, which scientists believe is one of the best candidates to find extraterrestrial life in the solar system.

One of the most important discoveries made is that the moon Europa might contain a liquid ocean of water underneath its thin ice crust. Galileo has also shown that the three large moons Ganymede, Europa, and Io have fairly strong magnetic fields, which means that the moons probably have cores of liquid metal. Molten metal cores provide heat that could make the moons hospitable to some forms of life. Galileo has imaged the four Galilean Satellites, named after Galileo Galilei.

Timeline of Probe's Entry

All times are given as EST:

• 11:04 - coast timer initiated probe operation
• 12:46 - orbiter flyby of Io (~1000 km), but no imaging nor spectral data were collected
• 14:04 - Energetic Particles Investigation (EPI) began to measure trapped radiation in a region previously unexplored
• 16:54 - Galileo orbiter reached closest point to Jupiter
• 17:04 - probe entry and data relay
• 17:05:52 - pilot parachute deployed
• 17:05:54 - main parachute deployed
• 17:06:02 - deceleration module jettisoned
• 17:06:06 - direct scientific measurements began
• 17:06:15 - radio transmission to orbiter began
• 17:08 - visible cloud tops of Jupiter reached
• 17:12 - atmospheric pressure the same as Earth's sea-level pressure
• 17:17 - second major cloud deck was encountered (uncertain)
• 17:28 - water clouds encountered (uncertain)
• 17:34 - atmospheric temperature equal to room temperature on Earth
• 17:46 - probe entered twilight
• 18:04 - end of baseline mission; probe may cease to operate due to lack of battery power, attenuation of signal due to atmosphere, or crushing due to atmospheric pressure
• 18:19 - orbiter ceased to receive probe data (if still transmitting)
• 19:27 - ignition of Galileo main engine to insert into Jupiter orbit

It is assumed that after 03:00 EST on December 8, 1995, that the probe had completely vaporized in Jupiter's atmosphere.

Important Firsts

• first mission to make a close flyby of an asteroid (Gaspra)
• first mission to discover a satellite of an asteroid (Ida's satellite Dactyl)
• first multispectral study of the moon
• first atmospheric probe to enter Jupiter's atmosphere
• first spacecraft to go into orbit around Jupiter
• first direct observations of a comet impacting a planet (Shoemaker-Levy 9)

Craft

Galileo Orbiter

The structure of the orbiter was divided into two sections. The main body of the spacecraft, comprised of the electronics bays, propellant system, RTG and science booms, and high-gain antenna, rotated at rates of 3.25 or 10.5 rpm. The despun section, aft of the main body, used an electric motor to drive it counter to the rotation of the main section. This dual spin attitude control system accommodated instruments which required stable, accurate pointing (the imaging instruments) and those which benefit from repetitive, broad-angular coverage (the various particles and fields instruments). The length of the spacecraft was 9 m and, with the high-gain antenna (HGA) deployed, was 4.6 m in diameter.

Included on the spinning section were instruments which detected low-energy charged particles, high-energy and potentially dangerous charged particles, and cosmic and Jovian dust. Other instruments included on this section studied waves generated in planetary magnetospheres and by lightning discharges. Galileo's magnetometer sensors, designed to measure planetary magnetic fields, were mounted on a boom 11 m (36 ft) long; the boom was long as to escape interference from the spacecraft.

The second part of the orbiter was stationary, and contained the instruments which required stability. They consisted of a high-resolution camera system, a near-infrared mapping spectrometer, an ultraviolet spectrometer to help analyze the chemistry of Jupiter's atmosphere, a photo-polarimeter radiometer to measure radiant and reflected energy, and a dish antenna which was used to track the above mentioned probe as it entered Jupiter's atmosphere while the orbiter relayed the data to Earth.

Power was provided to the spacecraft through the use of two radioisotope thermal generators (RTGs), each of which was located at the end of a short boom. These converted the natural radioactive decay of plutonium 238 dioxide into electricity.

Huygens Atmospheric Probe

There were two modules that comprised the probe. The deceleration module consisted of the fore and aft heat shields and their accompanying support structure, as well as the thermal control hardware for the phases of the mission through entry into the atmosphere.

The descent module contained the science instruments and the subsystems required to support them. This was the actual package that went through the atmosphere. The descent module differed from the Pioneer Venus Large Probe design, which included a sealed pressure vessel, in that the mass was minimized by venting the module and protecting individual units as necessary with hermetically sealed housings. These housings were designed to survive to pressures of 20 bars and were tested to 16 bars.

Craft Data