National Aeronautics and Space Administration
Ames Research Center
Space Science Division and Educational Programs Office
Educational Brief EB-117
Subject: Planetary Science, Astronomy

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Galileo: Probe Into Jupiter

page 5

The Probe Spacecraft: First Entry Into a Giant Planet's Atmosphere

The 340 kilogram (747 lb) Probe spacecraft has its own power supply, communications, command, programming, data processing, data storage systems, and science payload. The Probe will be the first spacecraft to enter the atmosphere of any of the four gas giant planets beyond the orbit of Mars. It will be the first to measure directly Jupiter's atmosphere. The Probe's protective shell is rugged enough to withstand the extreme heat generated by the high-speed plunge into this atmosphere. As the Probe is pulled in by massive Jupiter's strong gravity, an enormous release of energy occurs as the spacecraft is slowed from 170,000 km/hr (106,000 mph) to 160 km/hr (100 mph) in four minutes. The 15,500 deg. C (28,000 deg. F), incandescent plasma envelope generated ahead of the Probe will briefly be brighter than the Sun's surface.

Probe hurtles into Jupiter's atmosphere

The spacecraft consists of a descent module carrying 30 kilograms (66 lb) of seven scientific instruments (see box). A protective shell consists of fore and aft covers to shield the capsule from the heat of entry. When the spacecraft has slowed to 160 km/hr (100 mph), these covers are discarded and the descent module descends on a large parachute, with speeds ranging from 750 m/sec (2500 ft/sec) at high levels of the atmosphere to 27 m/sec (88 ft/sec) deep in the atmosphere. The sequence of events and estimated conditions during the descent are shown in the diagram on the opposite page.

The components of the Galileo Probe

The Probe's nine science experiments will greatly enhance our understanding of Jupiter as a planet. Even before entering the atmosphere, one of the instruments carried by the Probe will measure Jupiter's intense radiation belts within 8050 km (5000 miles) of the cloud tops, far closer than was possible with earlier flyby spacecraft.

As the Probe descends through the atmosphere, its electronic computer will receive information from the science payload, process and encode it, and immediately transmit the coded signal to the Orbiter overhead. There the information will be stored for later transmission to Earth. The Orbiter will be able to receive data from the Probe for up to 75 minutes.

SCIENCE INSTRUMENTS

Atmospheric Structure Instrument

Provides information about temperature, density, pressure, and molecular weight of atmospheric gases.

Neutral Mass Spectrometer

Analyzes the composition of gases by measuring their molecular weights.

Nephelometer

Locates cloud layers in the atmosphere and measures some of the characteristics of the cloud particles.

Lightning and Radio Emission Instrument.

Searches and records radio bursts and optical flashes generated by lightning in Jupiter's atmosphere.

Helium Abundance Detector

Determines the important ratio of hydrogen to helium in Jupiter's atmosphere.

Net Flux Radiometer

Senses the differences between the flux of light and heat radiated downward and upward at various levels in Jupiter's atmosphere.

Energetic Particles

Used before entry to measure fluxes of electrons, protons, alpha particles, and heavy ions as the Probe passes through the inner regions of Jupiter's ionosphere.

Also, variations in the Probe's radio signals to the Orbiter will be used to determine wind speeds and atmospheric absorptions.

Data from the seven instruments and two radio experiments will be the first on-the-spot measurements in the atmosphere of a gas giant planet. By accurately determining the abundances of gaseous atmospheric constituents, such as hydrogen, helium, ammonia, methane and water, experimenters can compare the composition of Jupiter's atmosphere with the composition of the Sun to obtain information about planetary formation. Jupiter's strong gravity has prevented any of the ingredients which formed it 4.5 billion years ago from escaping. As the Probe descends it will search for droplets and solid particles which would indicate the occurrence of clouds in the atmosphere. By combining the measured gaseous composition with measurements of barometric pressure and temperature, scientists can use chemistry and physics to estimate the composition of the cloud layers. Three cloud layers should exist; the top layer of ammonia liquid droplets and ice crystals (believed to be the cloud tops we see), an ammonium hydrosulfide cloud layer, and a deeper water cloud layer. But there are large uncertainties.

Two experiments will help us understand the mysterious multiple jet streams and atmospheric superrotation. Variations in the frequency of the Probe's radio signal received by the Orbiter will reveal the variation of winds with altitude. Measurement of amounts of sunlight and of interior heat at each atmospheric level will give information about power sources driving the winds. Searches for lightning flashes and radio waves emitted from them will provide clues on how internal heat escapes from the planet's interior.

The Orbiter Spacecraft: Monitoring the Atmosphere from Orbit

Data from the Probe will apply to only one latitude, longitude, and time. Observations by the Orbiter will provide a context for the Probe results by intensely studying the entry site from orbit, comparing it with other parts of the planet, and watching its variations over time.

Diagram of the Galileo Orbiter

The Orbiter (see picture on page 2 and diagram alongside) has a unique design allowing the main body of the spacecraft (electronic bays, propulsion systems, power source, main Artist's concept of Probe approaching clouds Path of Probe through Jupiter's atmosphere antenna, and experiments to measure radiation belts and magnetic field) to spin at about three RPM. This stabilizes the spacecraft. Immediately aft of the main body a despun section, driven by an electric motor, counters the main body's rotation. This provides a scan platform with scientific instruments and the radio antenna to receive signals from the Probe.

Path of Probe through Jupiter's atmosphere

Four instruments on the scan platform provide most of the Orbiter's observations of the atmosphere. These instruments (imaging, near infrared mapping spectrometer, photopolarimeter, and ultraviolet spectrometer) can view Jupiter from many angles at high spatial resolution with broad spectral coverage. They can also view the planet's nightside. The data obtained from the Orbiter will be less than planned because its main antenna did not fully open. The antenna problem does not affect the return of all data gathered by the Probe. Communication with Earth will rely on one smaller antenna. Galileo scientists will, nevertheless, maximize scientific returns over the mission by selecting observations that use the uniqueness of the Orbiter.

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