Montage of our solar system

NASA Administrator James M. Beggs has approved the addition of an asteroid flyby option to the Galileo mission. The option will permit post-launch decision to fly by the asteroid 29 Amphitrite in December 1986.

The decision to launch Galileo on an Amphitrite- Jupiter trajectory or on Jupiter only trajectory will be made between now and launch -- as late as the launch day itself. decision to fly past Amphitrite does not, however, guarantee that scientific investigation of the asteroid will actually be carried out by the spacecraft; the decision to conduct such investigations would be made no earlier than two months after launch, and would be made based on an assessment at that time of mission operations and the state of the spacecraft.

Asteroid mission planning and science investi- gations sequencing would occur after if the asteroid trajectroy is chosen and if decision is made to conduct an asteroid flyby mission.

Approval follows two years of study by numerous scientific groups, mission designers and program officials to devise means to include the option. If the option to encounter the asteroid is selected -- decision will be made after launch -- it would add significant scientific "first" to the Galileo mission.

The Galileo mission to Jupiter, designed to orbit the planet and send an instrumented probe into its atmosphere, is scheduled for May 1986 launch by the Space Shuttle and Centaur upper stage.

The National Academy of Sciences, as well as NASA's Solar System Exploration Committee (SSEC), has identified the investigation of the asteroids by spacecraft as an essential element of balanced planetary exploration program. The flyby has the endorsement of the Galileo Project Science Group and the Small Bodies Working Group of the SSEC.

The asteroid flyby would be treated as an add-on, not primary mission objective, and will not be permitted to compromise the basic mission objectives or add any risk to the already-ambitious planetary mission.

Amphitrite is in near-circular solar orbit in the middle of the asteroid belt at 2.5 astronomical units (AU) from the Sun. (Earth's average distance is one AU or about 150 million kilometers). Amphitrite, about 200 km in diameter, is one of the larger asteroids.

The flyby distance will be determined by spacecraft safety considerations. specially convened hazards workshop has concluded that with 10,000- to 20,000-km flyby distance, the hazard to the Galileo spacecraft is no greater than merely flying through the asteroid belt, as has been done by two Pioneer and two Voyager spacecraft. Significant scientific objectives can be realized by analyzing measurements and Doppler tracking data obtained from that distance.

From Earth, Amphitrite appears as the twelfth- brightest asteroid and has fluctuations suggesting rotation period of 5.39 hours. At that rotation rate, most of the surface can be photographed and scanned by Galileo's mapping spectrometer. Analysis of these data will reveal the asteroid's size, shape, mass and density, its exact rotation rate and pole orientation and its detailed surface morphology and mineral composition.

Together, these properties will allow scientists to determine whether Amphitrite is primitive accumulation of solar nebulae condensates or whether it is an evolved body that is fragment, or perhaps core, of broken-up minor planet.

Information obtained will make it possible to confirm or refute hypothesis that Amphitrite and other asteroids are sources of many of the meteorites which fall on Earth.

This first encounter with an asteroid would also supply ground truth for interpreting many previous and forthcoming ground-based observations of other asteroids.

Amphitrite, the 29th asteroid discovered, was detected by Albert Marth, in London, England, at the William Bishop Observatory on March 1, 1854. The asteroid was named for one of the mythical god Neptune's wives.

A new optional trajectory, containing both Amphitrite and Jupiter and constrained by the launch vehicle energy and existing launch window, has been developed. If taken, the optional trajectory would result in delay in the Jupiter arrival date from Aug. 29, 1988, to Dec. 10, 1988, regardless of whether an asteroid encounter were carried out.

No mission operations work or added software capability related to the flyby will be accomplished prior to launch other than that required to plan the primary mission based on the optional asteroid flyby trajectory.

The effect of the change in trajectory or additional encounter on the Jupiter mission would be minor. Since the asteroid flyby would require added expenditure of propellant in the early mission phase, the number of tour orbits of Jupiter would be decreased from 11 to 10. Consequently, the length of the planned tour has been extended from 20 months to 22 months to permit the achievement of all the major objectives previously encompassed by the 11-orbit tour.

There will be no near-term cost impact due to the incorporation of the flyby option. Major added costs, estimated at $20 million to $25 million, are attributable to five-month mission extension due to the delayed arrival date and increased tour time.

The Galileo project is managed by the Jet Propulsion Laboratory, Pasadena, Calif., under the direction of the Office of Space Science and Applications, NASA Headquarters, Washington, D.C.

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