# Hearing Galileo's Whisper Across the Solar System

The larger the collecting area of an antenna, the weaker the radio signal it can collect and focus, and the faster the spacecraft can send down its data without getting "garbled". Very large Earth receiving satellite dishes are needed because Galileo is very far away and the amount of electromagnetic energy decreases with the square of the distance from the source.

• How many Galileos does it take to turn on a light bulb??
• With the existing Earth receiving stations , it would take energy collected from 10 BILLION GALILEOS OVER THE LIFE OF THE SOLAR SYSTEM TO LIGHT ONE 100-WATT LIGHT BULB FOR 1 SECOND!!!!
And yet, we can detect this very weak signal and send back pictures and other information with it.

The antenna "hears" both signal (data) and noise (static) and uses hardware and software to maximize signal and minimize noise. The data rate or number of data bits per second (bps) a spacecraft can transfer to an Earth station depends on the communications capabilities of both systems.

Galileo will record and later transmit all of its data through its low gain antenna (see spacecraft diagram on page 7). The Deep Space Network, with its 70-meter antennae, can receive data from Galileo at Jupiter at the rate of 10 bps (equal to about 1 letter per second). Several new capabilities will be ready by 1996 to increase the rate to 160 bps (about two sentences per second). These include connecting the ground antennas together in arrays (increases received signal), use of an Ultracone (decreases communications noise) and improved receivers (makes more effective use of available received signal).

Compare the "fast" rate of 2 sentences per second to the "slow" rate of 1 letter per second . Use the first 2 lines of a favorite story or poem. It would take one second to recite using the faster rate.... how long using the slower rate?

Q: The Deep Space Network has 70-meter antennae at stations located in Spain, Australia and the U.S. (California). Why do you think these places were chosen?

H: Stars on this map show where they're located.

## Bibliography & Other Sources of Information

Basics of Space Flight Learners' Workbook, 1993, JPL publication D-9774.
Beatty, K. and A. Chaikin, The New Solar System, 1990, Sky Publishing Corporation & Cambridge University Press.
Glass, B.P. Introduction to Planetary Geology, 1982, Cambridge University Press.
Hunt, G. and P. Moore, Jupiter, 1981, Rand McNally Library of Astronomical Atlases for Amateur and Professional Observers.
The Galileo Mission, 1992, Space Science Reviews, Volume 60, Numbers 1/4, Kluwer Academic Publishers. Yeates, C. et al, Galileo: Exploration of Jupiter's System, 1985, NASA publication SP-479.