Robotic Exploration of the Solar System
Part 2: Hiatus and Renewal 1983±1996


Paolo Ulivi with David M. Harland

Robotic Exploration of
the Solar System
Part 2: Hiatus and Renewal 1983±1996

Published in association with

Praxis Publishing
Chichester, UK


Dr Paolo Ulivi
Cernusco Sul Naviglio
Italy

Dr David M. Harland
Space Historian
Kelvinbridge
Glasgow
UK

SPRINGER±PRAXIS BOOKS IN SPACE EXPLORATION
SUBJECT ADVISORY EDITOR: John Mason B.Sc., M.Sc., Ph.D.
ISBN 978-0-387-78904-0 Springer Berlin Heidelberg New York
Springer is a part of Springer Science + Business Media (springer.com)

Library of Congress Control Number: 2007927751

Front cover image: Copyright David A. Hardy/www.astroart.org/STFC
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# Copyright, 2009 Praxis Publishing Ltd.
The use of general descriptive names, registered names, trademarks, etc. in this publication does
not imply, even in the absence of a specific statement, that such names are exempt from the
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Cover design: Jim Wilkie
Copy editing: David M. Harland
Typesetting: BookEns Ltd, Royston, Herts., UK
Printed in Germany on acid-free paper


Contents
In Part 1
1.
2.
3.

Introduction
The beginning
Of landers and orbiters
The grandest tour


Now in Part 2
Illustrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
Author's preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi
4.

The decade of Halley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
The crisis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
The face of Venus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
The mission of a lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Balloons to Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Two lives, one spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
``But now Giotto has the shout''. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
Extended missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Low-cost missions: Take one . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Comet frenzy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
The rise of the vermin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
An arrow to the Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
Into the infinite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
Europe tries harder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135


vi

Contents

5.


The era of flagships. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The final Soviet debacle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mapping Hell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The reluctant flagship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Asteroids into minor planets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galileo becomes a satellite of Jupiter . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Returning to Europa and Io . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Beyond the Pillars of Hercules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The darkest hour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Overdue and too expensive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

145
145
167
196
217
237
278
311
327
335

6.

Faster, cheaper, better . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The return of sails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A new hope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
In love with Eros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Completing the census . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Low-cost masterpiece . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sinking the heritage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wheels on Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Martians worldwide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Meanwhile in America . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

347
347
349
359
373
379
423
442
461
464

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter references. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Previous volumes in this series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

468
477
483
521
523
525



Illustrations
Front cover: The Ulysses spacecraft passing through the tail of Comet Hyakutake
Rear cover:

The Galileo spacecraft on its IUS stage in Earth parking orbit

Chapter 4
A Seasat synthetic-aperture radar image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
The antenna of JPL's prototype planetary radar . . . . . . . . . . . . . . . . . . . . . . . . . 5
Impressions of the Venus Orbiting Imaging Radar . . . . . . . . . . . . . . . . . . . . . . . 7
VOIR aerobraking, and in its mapping configuration . . . . . . . . . . . . . . . . . . . . . 8
A Venera radar-mapping orbiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
One of the first Venera radar images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Radar imaging and altimetry running across Cleopatra Patera . . . . . . . . . . . . . 13
Volcanic structures on Venus called `arachnoids' . . . . . . . . . . . . . . . . . . . . . . . . 14
The lava flow of Sedna Planitia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
The elongated orbit of Halley's comet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Rendezvousing with Halley's comet using a Jupiter gravity-assist . . . . . . . . . . . 18
An electric-propulsion Halley rendezvous mission . . . . . . . . . . . . . . . . . . . . . . . 19
Ballistic orbits for a Halley flyby. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Dr Tsung-Chi Tsu of the Westinghouse Research Laboratory . . . . . . . . . . . . . . 23
NASA's proposed solar sail for a Halley mission . . . . . . . . . . . . . . . . . . . . . . . 24
NASA's proposed electric-propulsion flyby of Halley . . . . . . . . . . . . . . . . . . . . 25
The Planet-A (Suisei) and MS-T5 (Sakigake) spacecraft . . . . . . . . . . . . . . . . . . 28
The orbits of the Suisei and Sakigake missions . . . . . . . . . . . . . . . . . . . . . . . . . 30
The Giotto spacecraft in tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Giotto was Europe's first deep-space mission . . . . . . . . . . . . . . . . . . . . . . . . . . 33
The trajectory of the Giotto flyby of Halley's comet . . . . . . . . . . . . . . . . . . . . . 35

The Halley Multicolor Camera of the Giotto spacecraft . . . . . . . . . . . . . . . . . . 36
The trajectory flown by the twin-spacecraft Vega mission . . . . . . . . . . . . . . . . . 38
A mockup of the Vega spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Another view of the Vega spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
A cutaway of the Vega lander sphere for Venus . . . . . . . . . . . . . . . . . . . . . . . . 45


viii

Illustrations

The Vega balloon probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
The descent profile of the Vega lander and balloon . . . . . . . . . . . . . . . . . . . . . . 49
NASA's fast-flyby spacecraft for the Halley Intercept Mission . . . . . . . . . . . . . 51
Observing Halley's comet using instruments on a Shuttle . . . . . . . . . . . . . . . . . 52
A Proton rocket launches Vega 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
The tracks of the Vega balloons in the upper atmosphere of Venus . . . . . . . . . . 57
International Sun±Earth Explorer 3 in preparation . . . . . . . . . . . . . . . . . . . . . . 59
ISEE 3 was initially placed into a `halo' orbit . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Comet Giacobini±Zinner during its 1972 return. . . . . . . . . . . . . . . . . . . . . . . . . 63
The magnetic field as reported by ICE while passing Giacobini±Zinner . . . . . . . 65
The orbits of Giacobini±Zinner, Halley and related spacecraft. . . . . . . . . . . . . . 66
A Mu-3SII rocket launches a Japanese Halley spacecraft. . . . . . . . . . . . . . . . . . 67
An Ariane launches Giotto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Giotto viewed Earth from a range of 20 million km . . . . . . . . . . . . . . . . . . . . . 70
A telescopic image of Halley's comet on 8 March 1986 . . . . . . . . . . . . . . . . . . . 72
The best pictures of the nucleus of Halley's comet taken by Vega 1. . . . . . . . . . 74
Ultraviolet views of Halley's comet taken by Suisei . . . . . . . . . . . . . . . . . . . . . . 75
Vega 2's best image of Halley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Steering Giotto close to Halley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

How a comet interacts with the solar wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Giotto's view of the nucleus of Halley's comet . . . . . . . . . . . . . . . . . . . . . . . . . 86
A distant view of Halley's comet in September 2003 . . . . . . . . . . . . . . . . . . . . . 90
Sakigake's return to Earth in 1992 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Giotto's encounters with Halley and Grigg±Skjellerup . . . . . . . . . . . . . . . . . . . . 94
An early timetable of the Solar System Exploration Committee. . . . . . . . . . . . . 97
The Mars Geoscience/Climatology Orbiter Planetary Observer . . . . . . . . . . . . . 98
An early-1980s Mariner Mark II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Four Mariner Mark II interplanetary bus configurations. . . . . . . . . . . . . . . . . 101
The initial concept of CRAF exploiting Voyager technology . . . . . . . . . . . . . . 107
CRAF as envisaged prior to the Challenger disaster . . . . . . . . . . . . . . . . . . . . 108
The Mariner Mark II CRAF to be launched by a Titan IV. . . . . . . . . . . . . . . 109
The final version of the CRAF closely resembled the Cassini spacecraft . . . . . 110
CRAF was to use a Venus gravity-assist to reach Tempel 2 . . . . . . . . . . . . . . 112
Rosetta: a joint ESA±NASA Comet Nucleus Sample Return. . . . . . . . . . . . . . 116
The ion-propelled European AGORA asteroid spacecraft . . . . . . . . . . . . . . . . 119
Italy's Piazzi spacecraft approaching an asteroid . . . . . . . . . . . . . . . . . . . . . . . 121
Vesta considered as a joint Soviet±European mission. . . . . . . . . . . . . . . . . . . . 122
The trajectory of the Russian Mars±Aster mission. . . . . . . . . . . . . . . . . . . . . . 123
The surface penetrator module for the Mars±Aster mission . . . . . . . . . . . . . . . 124
The trajectory of ESA's close-perihelion Solar Probe. . . . . . . . . . . . . . . . . . . . 127
The configuration of ESA's Solar Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
Two configurations of JPL's Starprobe close-perihelion spacecraft . . . . . . . . . 129
The Soviet YuS spacecraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
JPL's Interstellar Precursor spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
ESA's Kepler Mars orbiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137


Illustrations


ix

The Mercury Orbiter proposed by ESA in 1992 . . . . . . . . . . . . . . . . . . . . . . . 139
Chapter 5
The Soviet Fobos mission used the UMVL bus . . . . . . . . . . . . . . . . . . . . . . . .
The PrOP-F hopper for the Martian moon Phobos . . . . . . . . . . . . . . . . . . . . .
The DAS long-duration lander for Phobos . . . . . . . . . . . . . . . . . . . . . . . . . . .
The integrated CCD camera and spectrometer for Fobos . . . . . . . . . . . . . . . .
The Martian magnetospheric boundaries as observed by Fobos 2 . . . . . . . . . .
A Fobos 2 view of Phobos hovering over Mars. . . . . . . . . . . . . . . . . . . . . . . .
Another image of Phobos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Termoskan image obtained by Fobos 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
An image of Phobos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A section of the final Termoskan by Fobos 2 . . . . . . . . . . . . . . . . . . . . . . . . .
A Fobos mission press meeting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The observing geometry of the Magellan synthetic-aperture radar . . . . . . . . . .
Magellan's eccentric orbit of Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Magellan spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Magellan/IUS stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Magellan after deploying its solar panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Magellan radar image of the impact crater Golubkina. . . . . . . . . . . . . . . . .
The complex structure of Maxwell Montes . . . . . . . . . . . . . . . . . . . . . . . . . . .
A hemispherical view of Venus as revealed by Magellan . . . . . . . . . . . . . . . . .
A field of small volcanoes on Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Magellan's view of the Venera 8 landing site . . . . . . . . . . . . . . . . . . . . . . . . . .
Pancake volcanoes and an impact crater in the Eistla region . . . . . . . . . . . . . .
A portion of Baltis Vallis on Venus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coronae in the Fortuna region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dark features in the Lakshmi region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A series of wrinkle ridges and a small volcano on Venus . . . . . . . . . . . . . . . . .

On Venus gravity anomalies closely correlate with topography . . . . . . . . . . . .
An early-1980s rendition of the Galileo Jupiter orbiter and probe . . . . . . . . . .
The Centaur G-prime hydrogen-oxygen upper stage . . . . . . . . . . . . . . . . . . . .
The `General-Purpose Heat Source' of an RTG. . . . . . . . . . . . . . . . . . . . . . . .
Testing Galileo's high-gain antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Galileo spacecraft as revised after the Challenger accident . . . . . . . . . . . .
The capsule for the Galileo atmospheric probe . . . . . . . . . . . . . . . . . . . . . . . .
Testing Galileo's atmospheric probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The parachute of Galileo's atmospheric probe . . . . . . . . . . . . . . . . . . . . . . . . .
The main components of the Galileo probe . . . . . . . . . . . . . . . . . . . . . . . . . . .
The solid-state imager for the Galileo mission . . . . . . . . . . . . . . . . . . . . . . . . .
The many configurations of the Galileo spacecraft . . . . . . . . . . . . . . . . . . . . .
The circuitous journey taken by Galileo to Jupiter . . . . . . . . . . . . . . . . . . . . .
Galileo is prepared for mating with its IUS stage . . . . . . . . . . . . . . . . . . . . . .
Galileo took a `self picture' in flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ultraviolet pictures of Venus by Galileo . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

148
150
152
154
159
161
162
163
164
165
166
169
170

171
173
175
178
180
181
183
184
185
186
188
190
192
194
198
199
201
202
203
205
206
207
209
211
214
215
216
219
221



x

Illustrations

The temperature field of the middle atmosphere of Venus . . . . . . . . . . . . . . . .
A mosaic of Galileo images of the Ross Ice Shelf in Antarctica. . . . . . . . . . . .
Galileo's fouled high-gain antenna. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galileo's best view of Gaspra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Laser beams fired at Galileo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galileo's view of Ida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dactyl orbiting Ida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Galileo view of Shoemaker±Levy 9 fragment K striking Jupiter. . . . . . . . . .
Galileo documents the impact of Shoemaker±Levy 9 fragment W . . . . . . . . . .
Galileo's arrival in the Jovian system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
An impression of Galileo's atmospheric probe on its parachute . . . . . . . . . . . .
Uruk Sulcus on Ganymede viewed by Galileo . . . . . . . . . . . . . . . . . . . . . . . . .
Galileo Regio on Ganymede . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Jupiter's Great Red Spot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Early views of Io by Galileo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The border of Marius Regio of Ganymede . . . . . . . . . . . . . . . . . . . . . . . . . . .
A chain of craters in northern Valhalla on Callisto . . . . . . . . . . . . . . . . . . . . .
A section of the outermost ring of Valhalla . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galileo views Io from a range of 244,000 km . . . . . . . . . . . . . . . . . . . . . . . . .
Surface `hot spots' and sky glows around Io . . . . . . . . . . . . . . . . . . . . . . . . . .
Galileo views Jupiter's ring forward-scattering sunlight . . . . . . . . . . . . . . . . . .
An early close-up view of Europa by Galileo. . . . . . . . . . . . . . . . . . . . . . . . . .
An `ice rink' on Europa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A jumble of ice `rafts' in the Conamara region of Europa . . . . . . . . . . . . . . . .
An ice peak in western Conamara . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A double ridge in northern Conamara. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Views of Io in eclipse by Galileo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`White ovals' in Jupiter's atmosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Craters near the north pole of Ganymede . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Zonal circulation in the northern hemisphere of Jupiter. . . . . . . . . . . . . . . . . .
The intersection between Erech and Sippar Sulci on Ganymede . . . . . . . . . . .
An image of Io in eclipse showing `hot spots' . . . . . . . . . . . . . . . . . . . . . . . . .
The impact crater Har on Callisto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A variety of terrains on Europa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Jupiter's small inner moons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galileo's trajectory during its primary mission. . . . . . . . . . . . . . . . . . . . . . . . .
A close up of an icy `raft' in Conamara . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Tyre multi-ringed basin on Europa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Astypalaea Linea in Europa's southern hemisphere . . . . . . . . . . . . . . . . . . . . .
The Thera and Thrace maculae on Europa . . . . . . . . . . . . . . . . . . . . . . . . . . .
Galileo observed Saturn, and Europa glowing in `Jupiter shine' . . . . . . . . . . .
A recently erupted lava flow at Pillan Patera on Io . . . . . . . . . . . . . . . . . . . . .
Pits and domes near Pillan Patera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A fire curtain in one of the calderas of the Tvashtar catena on Io . . . . . . . . . .
Nicholson Regio on Ganymede . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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286
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Illustrations

xi

Harpagia Sulcus on Ganymede . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The trajectories of Galileo and Cassini in the Jovian system . . . . . . . . . . . . . .
The 500-km-tall plume of Thor on Io . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tohil Patera, Radegast Patera and Tohil Mons on Io . . . . . . . . . . . . . . . . . . .
The volcanic Tvashtar catena on Io. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The ESRO ion-propelled out-of-ecliptic spacecraft . . . . . . . . . . . . . . . . . . . . .
The canceled NASA contribution to the out-of-ecliptic mission . . . . . . . . . . . .
ESA's Ulysses spacecraft depicted on a Centaur G-prime stage . . . . . . . . . . . .
The Ulysses spacecraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The cosmic dust analyzer on Ulysses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The trajectory of Ulysses through the Jovian system . . . . . . . . . . . . . . . . . . . .
The heliocentric trajectory of the Ulysses spacecraft . . . . . . . . . . . . . . . . . . . .
The Mars Observer spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mars Observer on a TOS stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A long-range view of Mars by Mars Observer . . . . . . . . . . . . . . . . . . . . . . . . .
A concept for the Mars Rover and Sample Return spacecraft . . . . . . . . . . . . .
The MRSR orbiter and lander in their aerocapture shell . . . . . . . . . . . . . . . . .
The architecture of the MRSR mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Robby, the three-bodied wheeled prototype rover for MRSR . . . . . . . . . . . . .

300
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305
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321
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330
331
332
336
338
339
341

Chapter 6
The Russian Regatta satellite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Venus Multiprobe proposal for the Discovery program . . . . . . . . . . . . . .
The US Department of Defense's Clementine spacecraft . . . . . . . . . . . . . . . . .
A view of Clementine's array of cameras. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Radar observations of Geographos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Radar images of Toutatis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Near-Earth Asteroid Rendezvous spacecraft . . . . . . . . . . . . . . . . . . . . . . .

Comet Hyakutake as seen by the NEAR spacecraft . . . . . . . . . . . . . . . . . . . .
Mathilde viewed by NEAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The south polar regions of Earth and the Moon viewed by NEAR . . . . . . . . .
Eros viewed by NEAR during its December 1998 flyby. . . . . . . . . . . . . . . . . .
A close up view of Eros by NEAR on 3 March 2000 . . . . . . . . . . . . . . . . . . .
The crater Psyche on Eros. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A view of Eros facing Himeros in shadow. . . . . . . . . . . . . . . . . . . . . . . . . . . .
NEAR's low-altitude flyover of Eros. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The last four images of NEAR's descent to Eros. . . . . . . . . . . . . . . . . . . . . . .
Views of Pluto by the Hubble Space Telescope . . . . . . . . . . . . . . . . . . . . . . . .
The small Pluto±Kuiper Express spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mars Global Surveyor in its mapping configuration . . . . . . . . . . . . . . . . . . . .
The camera for Mars Global Surveyor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The thermal emission spectrometer for Mars Global Surveyor. . . . . . . . . . . . .
A press conference showing a chip of meteorite ALH84001. . . . . . . . . . . . . . .
Mars Global Surveyor during ground preparations . . . . . . . . . . . . . . . . . . . . .
The launch of Mars Global Surveyor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

348
351
354
355
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361
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367

369
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391


xii

Illustrations

A long-range view of Mars by Mars Global Surveyor . . . . . . . . . . . . . . . . . . .
The aerobraking configuration of Mars Global Surveyor . . . . . . . . . . . . . . . .
Layering in the wall of western Candor Chasma on Mars . . . . . . . . . . . . . . . .
Mars Global Surveyor's first phase of aerobraking . . . . . . . . . . . . . . . . . . . . .
A view of Nanedi Vallis by Mars Global Surveyor . . . . . . . . . . . . . . . . . . . . .
A close-up of Phobos taken by Mars Global Surveyor . . . . . . . . . . . . . . . . . .
Mars Global Surveyor discovered magnetic stripes on Mars . . . . . . . . . . . . . .
Mars Global Surveyor's second phase of aerobraking . . . . . . . . . . . . . . . . . . .
The cliff-bench terrain in southwestern Candor Chasma . . . . . . . . . . . . . . . . .
Gullies on the wall of a small unnamed crater on Mars. . . . . . . . . . . . . . . . . .
A field of dark, horn-shaped dunes on Mars . . . . . . . . . . . . . . . . . . . . . . . . . .
Streaks left by dust devils on Argyre Planitia . . . . . . . . . . . . . . . . . . . . . . . . .

Mars Global Surveyor caught a dust devil in the act . . . . . . . . . . . . . . . . . . . .
`Swiss cheese' terrain near the south polar cap of Mars . . . . . . . . . . . . . . . . . .
A remarkable heart-shaped pit in Acheron Catena . . . . . . . . . . . . . . . . . . . . .
Dust in the Martian atmosphere during the global storm of 2001 . . . . . . . . . .
Mars Global Surveyor noted changes to gullies on Mars . . . . . . . . . . . . . . . . .
A fresh crater on the flank of Ulysses Patera. . . . . . . . . . . . . . . . . . . . . . . . . .
A VNII Transmash prototype Marsokhod . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Soviet Mars Sample Return lander . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Mars orbiter proposal using the UMVL bus . . . . . . . . . . . . . . . . . . . . . . . .
The Argus scan platform of the Russian Mars 94/96 spacecraft. . . . . . . . . . . .
The German Mars 94/96 wide-angle camera . . . . . . . . . . . . . . . . . . . . . . . . . .
A mockup of the Russian `small station' lander for Mars . . . . . . . . . . . . . . . .
The descent profile of a Russian `small station' lander . . . . . . . . . . . . . . . . . .
A cutaway of the Russian penetrator for Mars . . . . . . . . . . . . . . . . . . . . . . . .
The Russian Mars 96/98 orbiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The landing profile of the Mars 96/98 rover and balloon. . . . . . . . . . . . . . . . .
The Russian Mars 8 spacecraft in Lavochkin's integration hall . . . . . . . . . . . .
Line drawings of the Mars 8 spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Don Bickler's first `Rocky' rover prototype of 1989 . . . . . . . . . . . . . . . . . . . .
A view of the front of Sojourner during ground preparations . . . . . . . . . . . . .
The Mars Pathfinder airbags during ground tests . . . . . . . . . . . . . . . . . . . . . .
Sojourner about to be sealed inside Mars Pathfinder . . . . . . . . . . . . . . . . . . . .
Mating Mars Pathfinder with its propulsive stage . . . . . . . . . . . . . . . . . . . . . .
A mosaic taken by Mars Pathfinder shortly after landing . . . . . . . . . . . . . . . .
Mars Global Surveyor imaged the Mars Pathfinder landing site . . . . . . . . . . .
A view of Mars Pathfinder by Sojourner . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Sojourner view of the Yogi boulder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sojourner imaged one of its hazard-detection laser stripes . . . . . . . . . . . . . . . .
A panorama of the Mars Pathfinder landing site . . . . . . . . . . . . . . . . . . . . . . .
Sojourner's view of dunes beyond the Rock Garden . . . . . . . . . . . . . . . . . . . .

The MARSNET semi-hard lander . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

392
393
395
397
399
400
401
402
405
406
408
409
410
412
413
417
420
422
425
426
427
429
430
432
433
435
436
438

440
441
444
446
447
449
451
454
455
457
457
458
459
461
463


Tables
Chronology of solar system exploration 1983±1996 . . . . . . . . . . . . . . . . . . . . . 477
Planetary launches 1983±1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479
Galileo orbits and encounters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481


Foreword
The series Robotic Exploration of the Solar System by P. Ulivi and D. M. Harland is,
first of all, a monumental chronicle of the amazing adventure that in the last 50 years
allowed mankind to visit and understand the immense and eerie domain of the solar
system, with its hidden nooks and unexpected peculiarities, providing data, images
and in some cases samples. The story is told with an extraordinary amount of factual
and technical details, mostly arranged to trace each project from its conception to

engineering design, to construction of the spacecraft, execution of the actual mission,
data analysis and, finally, publication of the results. Most of these details are not
known even to the communities of experts: temporary reports, especially if technical,
are seldom published and are easily forgotten or lost. The style of this series is one of
first class journalism: the story unfolds in a fascinating and easy-going way, without
difficult digressions at the physical and engineering level. But the content is in no
way superficial or vague: the accuracy of the information is confirmed not only by its
exhaustive quantitative level, but also by the supporting primary documents quoted
in the bibliography. Any future historical study of space exploration will have to be
based on this chronicle. Much of its content refers to details of the instrumentation
on each spacecraft, and to the manner in which the mission was accomplished. The
design, making and testing of instruments for use in space is not an easy task.
Conditions in space are often prohibitive, as, for instance, near the Sun, owing to its
radiation and solar wind. Systems must reliably function for years without any check
and repair. Extraordinary sensitivities for various physical quantities, like very weak
magnetic fields and high-energy particles, are required. The possibility of storing on
board very large amounts of data, processing it and sending it back to Earth is an
essential condition for success. To reproduce space conditions on the ground to test
systems is difficult, if not impossible.
I have been a Principal Investigator of the Ulysses mission, which is described in
this volume. Launched in 1990, it conducted for the first time a deep exploration of
the solar system environment outside the ecliptic plane in which most of the planets
orbit the Sun ± with outstanding results, as announced in the journal Nature on 3
July 2008. In the near future, after 18 years, its operation will terminate, not because
of instrument problems, but because its radioisotope fuel is nearly exhausted.


xvi

Foreword


The word `robotic' in the title of this series points to an important controversy in
space exploration: is direct human involvement necessary, or even advisable? For
example, is the International Space Station commendable from the scientific point of
view? I am clear on this point: the extraordinary developments in remote-sensing,
software and control make a human presence on an orbiting machine for exploration
useless for most of the time, costly and dangerous. Even when the round-trip time of
a radio signal from Earth takes hours ± such as in the descent of the Huygens probe
to the surface of Titan, Saturn's large satellite (a mission that will be discussed in the
next volume of this series) ± an unmanned probe can work very well, even though the
control from Earth is delayed and an immediate reaction to unforeseen conditions
impossible. The system on Huygens, on the basis of pre-planned choices, was able to
decide autonomously which actions to take on the basis of the physical conditions it
encountered in the descent.
The word `exploration', usually romantically understood as the strenuous efforts
of daring and often irresponsible people to survey unknown lands and civilizations,
has acquired another meaning: instruments provide us with eyes and sensors far
more powerful and penetrating than our own senses, supported by a vast memory
capacity. The accounts in this series impressively confirm this view. This leads me to
my final topic: the use of robotic space probes in the solar system to understand the
structure of space and time. As the Oxford English Dictionary explains, the primary
meaning of the verb `to explore' is to investigate; to survey an unknown land is
secondary. Most emphatically, the main purpose of the exploration of the solar
system is not the sheer collection and cataloguing of images and data in very great
quantities; it is the rational understanding of the structure, the history and the
functioning of the physical objects that they refer to. In 1958, at the beginning of
space exploration of the solar system, the conceptual framework was already set up
and well accepted: first, planets and other large bodies move according to the laws of
gravitation devised by Isaac Newton and applied to an exceedingly refined degree by
mathematicians in France and England in the nineteenth and twentieth centuries;

secondly, the origin of the planetary system in the collapse of a rotating interstellar
cloud of gas and dust, at the centre of which the Sun began to shine 4.56 billion years
ago, was a well established scenario. Space exploration did not change this general
framework, but it opened up unexpected windows and led to extraordinary
discoveries, two of which I shall quote. Planets and their satellites are not point-like,
as assumed in the Newtonian model; their finite size gives rise to new forces and tidal
effects that significantly influence the evolution of the system, and these have been
extensively investigated with space probes. In 1979 Voyager 1 discovered a few active
volcanoes on Io, one of Jupiter's moons. In fact, their existence had been predicted
by S.J. Peale and his collaborators at the University of California at Santa Barbara,
on the basis of tidal forces exerted on Io by the nearby moons Europa and
Ganymede. Space probes have also allowed immense progress in the investigation of
planetary atmospheres, in particular on their composition, their evolution, and how
they are maintained or replenished in spite of their continuous loss to space. Again,
the traditional laws of chemistry and physics are not under question here; but no
theory can predict or even explain the wealth of interlocking phenomena and


Foreword

xvii

complex behaviours, which often can be revealed and understood only with in-situ
observations. A striking example is the recent discovery of extensive water activity
on the surface of Mars in the geological past; of course, this has a bearing on the
possible presence of life. But acceptance of physical laws can never be uncritical;
indeed, the statement that a natural law is correct is idle and logically inconsistent, as
there is no way to test it; one can only say, in the negative, that a given physical law is
self-contradictory or conceptually inadequate, or that it disagrees with observations.
It is well known, for example, that the Newtonian law of gravity works very well in

most cases, but on both counts it is unacceptable. Minor anomalies in the motions of
planets and the propagation of light in the solar system that are inexplicable by it are
a quantitative consequence of the theory of general relativity announced by Albert
Einstein in 1915; this theory is the currently accepted framework. The large
computer programs used to predict and control the motions of interplanetary probes
are in fact based on a fully relativistic mathematical scheme, and they include as an
essential part the appropriate corrections to Newtonian theory to take account of
relativity. A major question faced by theoretical physicists is: how, and at what
quantitative level is general relativity violated? Space probes play a very important
role in addressing this fundamental issue. They orbit the Sun at very large distances
in an environment which is practically empty, and free from Earth's gravity and
mechanical disturbances like microseisms. The sophistication of measurements using
space probes of time intervals, distances and relative velocities is improving all the
time, and such measurements have allowed the predictions of general relativity to be
tested to a very high degree of accuracy. Remarkably, more than 90 years after its
discovery, Einstein's theory is still unchallenged; but the assault is mounting, with a
number of new missions in preparation to explore the deep nature of gravitation. An
important experiment was carried out in 2002 by the Cassini spacecraft, which was
cruising through interplanetary space to Saturn. Its radio system and a specially built
antenna at NASA's Deep Space Network complex at Goldstone, California, enabled
the relative velocity between them to be measured to an unprecedented accuracy, and
made possible a new test of a relativistic effect of the Sun's gravitational field on the
propagation of radio waves. No discrepancy from the prediction of general relativity
was detected. It is quite remarkable that space probes are able not only to explore the
mechanisms by which the objects in the solar system work, but also to investigate the
very nature of space and time.
Bruno Bertotti
Dipartimento di Fisica Nucleare e Teorica
UniversitaÁ di Pavia (Italy)



Author's preface
The first part of Robotic Exploration of the Solar System ended with launches in
1981, but related missions in flight at that time through to their completion. This
second part covers missions launched between 1983 and 1996, employing the same
``spotter's guide to planetary spacecraft'' approach. While the period covered is
short, and was marked by a frustrating hiatus with rare missions, it saw the debut of
new players, the decline of another, and a number of triumphs and failures. It was
also marked by the `Christmas tree' approach to planetary exploration which on the
one hand caused a dearth of planetary missions and on the other hand a number of
missions that produced an overwhelming return of results, not all of which were able
to be included in this book. The period was also shaped by some peculiar external
conditions: the American emphasis on human spaceflight and Shuttle flights, which
deprived planetary missions of badly needed funds; the Challenger accident which
derailed those few projects that had managed to survive; and finally the Strategic
Defense Initiative, which provided technology for the low-cost revolution in deepspace missions of the 1990s. The low-cost approach, too, would soon dramatically
show its shortcomings, but these will be left to future volumes in the series.
Paolo Ulivi
Milan, Italy
July 2008


Acknowledgments
As usual, there are many people that I must thank. First, I must thank my family for
their support and help. I found invaluable support from the library of the aerospace
engineering department of Milan Politecnico, and the Historical Archives of the
European Union, as well as members of the Internet forums in which I participate.
Special thanks go to all of those who provided documentation, information, and
images for this volume, including Giovanni Adamoli, Nigel Angold, Luciano
Anselmo, Bruno Besser, Michel Boer, Bruno Bertotti, Robert W. Carlson, Dwayne

Day, David Dunham, Kyoko Fukuda, James Garry, Giancarlo Genta, Olivier
Hainaut, Brian Harvey, Ivan A. Ivanov, Viktor Karfidov, Jean-FrancËois Leduc,
John M. Logsdon, Richard Marsden, Sergei Matrossov, Don P. Mitchell, Jason
Perry, Patrick Roger-Ravily, Jean-Jacques Serra, Ed Smith, Monica Talevi and
David Williams; I apologize if I have inadvertently left out anyone. I also thank all
of my friends. In addition to all of those already mentioned in the first volume, I
must add my work colleagues Attilio, Claudio, Erika, Ilaria, Massimiliano, Paolo,
Rosa and Teresa. I particularly thank Giorgio B., whose enthusiasm makes me feel
like there are people out there still interested in these subjects.
I must thank David M. Harland for his support in reviewing and expanding the
subject, and Clive Horwood and John Mason at Praxis for their help and support. I
must thank Bruno Bertotti for sharing with me some of his recollections of working
as scientist on these missions and for writing the Foreword. And I am grateful to
David A. Hardy of www.astroart.org for the cover art, which was originally made
for the Particle Physics and Astronomy Research Council of the UK government.
Although I have managed to identify the copyright holders of most of the drawings
and photographs, in those cases where this has not been possible and I deemed an
image to be important in illustrating the story, I have used it and attributed as full a
credit as possible; I apologise for any inconvenience this may create.
The most special thank-you of course goes to Paola, the wonderful brown-eyed
planet of which I am the sputnik.


4
The decade of Halley

THE CRISIS
By the end of the 1970s the American program of solar system exploration was in
disarray. After the success of Viking, and with the Voyager and Pioneer Venus
missions underway, it appeared to some that planetary exploration had achieved its

goals and, consequently, there was little left to do. In addition, many other factors
conspired against launching further missions. Chief among them was the fact that
the National Aeronautics and Space Administration (NASA) spent so much of its
budget on human spaceflight, and in particular the Space Shuttle, which the agency
had `sold' to Congress by promising that the high development cost (projected at
about $5 billion) would be offset in service by partial re-usability and the high rate of
flights (as many as 60 per year). In fact, the Shuttle cost almost twice as much to
develop and proved to be capable of at most a dozen flights per year, and the actual
degree of re-usability and turn-around time left a great deal to be desired. To cover
the Shuttle overruns, NASA cut into the budgets of its scientific programs, creating
such havoc that these took almost a decade to recover. Another reason for the crisis
in the planetary exploration program was America's deÂtente with the Soviet Union,
which fostered cooperation rather than competition in space. But planetary science
gained little if any advantage from it, and the rapprochement declined in the early
1980s. In the meantime, NASA shifted its scientific focus away from planetary
exploration towards terrestrial studies and astronomy, in particular approving the
development of the Large Space Telescope, which would later become the Hubble
Space Telescope, as the first in a series of space-based `Great Observatories' that
would, between them, cover the electromagnetic spectrum from the far infrared to
gamma-ray wavelengths. Finally, in the face of budgetary austerity, Congress was
unsympathetic to proposals for planetary missions costing $500 million ± although at
that time this was less than the procurement cost of almost any program by the
Department of Defense. Thus, as the 1980s began, NASA and the Jet Propulsion
Laboratory (JPL) of the California Institute of Technology, which as a result of a
NASA reorganization remained the only facility building planetary probes, had just


2

The decade of Halley


three missions approved for development, none of which was on a particularly solid
financial footing. They were the Venus Orbiting Imaging Radar, the Galileo Jupiter
orbiter and probe, and the out-of-ecliptic International Solar Polar Mission. In the
meantime, the principal source of fresh data would be the `Grand Tour' which
Voyager 2 would conduct, with terrestrial observatories filling in gaps, for example
by serendipitously discovering that Uranus possesses a ring system. Still, the hiatus
would mean that, for the first time in 18 years, no fresh data on the solar system
would be collected in 1982; and unless things changed nor would there be any in
1983, 1984 or 1985.1
The situation worsened when Ronald Reagan became US president in 1981 and
promptly sought to cut federal spending in many areas, including civilian space. As a
result, one of the planetary missions in development was scaled back, another was
canceled, and consideration was given to closing JPL's Deep Space Network, the
worldwide network of antennas that provided communication with all probes in
deep space ± which would in turn mean ending the Voyager 2 mission at Saturn.
James Beggs, the incumbent NASA administrator, pointed out that ``elimination of
the planetary exploration program [would] make the JPL in California surplus to our
needs''. At the same time, Reagan's science adviser, George Keyworth, floated the
suggestion of completely eliminating planetary missions for 10 years so as to enable
NASA to focus on getting the Shuttle into service and then using this to conduct a
variety of more worthwhile missions. The proponents of such a myopic viewpoint
were unconcerned by the difficulty JPL would face in maintaining its institutional
knowledge of how to design, build and operate a planetary spacecraft, in order to
enable it to pick up the program after a decade of inactivity.2,3

As NASA and JPL struggled to keep alive those planetary missions which were
underway, and to fend off threats to the budgets for the development of new ones,
the Soviet Union continued its own program. The exploration of Venus, which had
proved to be within the capabilities of the relatively unreliable but rugged Soviet

technology would continue, at least in the short term, while an effort was underway
to resume missions to Mars ± which had been abandoned after a secret `War of the
Worlds' debate in the 1970s. Of course, by this time, the Superpowers had come to
realize that planetary missions no longer had the propaganda value which they had
delivered in the early 1960s.4 Nevertheless, such activities remained popular with the
public.
Finally, new entrants in the space arena were set to steal the show from both the
financially strapped United States and the technically limited Soviet Union. After 20
years of considering possible deep-space missions, Europe was gearing up to fly one.
This program capitalized on the cooperative programs between the individual
member nations (France, Germany, the United Kingdom, Italy, Austria, etc) with
both of the Superpowers. And ever since launching its first satellite in 1970 Japan
had also been studying possible deep-space missions, and now had the capability to
join in.


The face of Venus

3

THE FACE OF VENUS
Having successfully imaged Venus at ground level using Veneras 9, 10, 13 and 14, the
logical next step for the Soviet Union was to place a spacecraft into orbit to use an
imaging radar to observe the surface through the enshrouding clouds and create a
topographical map.5
Imaging radars, or synthetic-aperture radars (SAR) as they are more correctly
called, record the Doppler shift and time delay of returned echoes of short pulses of
microwave energy from a surface, and combine them to produce a high-resolution
`image', with each picture element (pixel) assigned a brightness proportional to the
energy returned by the particular combination of Doppler shift and time delay for

that point. The returned energy is influenced by surface slope, degree of roughness
on the scale of the wavelength of the illumination pulse, and dielectric properties of
the surface material. By extensive computer processing, the points collected as the
spacecraft travels along its trajectory can be used to synthesize (hence the name) or
simulate the observations of a much larger antenna. The illuminated `footprint' is
offset to one side of the ground track, because otherwise it would not be possible to
discriminate between echoes coming from the left side and those from the right side.
Such was the computing power needed to process SAR data, however, that when
NASA's first radar satellite, named Seasat, was launched in 1978, it was predicted
that it would take 75 years to process all the data from the planned 3-year mission.
Compared to other applications, the analysis of the data from a spaceborne radar
had to take into account a number of additional factors, including the fact that
orbital motion and ionospheric effects introduced Doppler shifts and phase
scintillations.6,7

In the early 1970s two teams, one at Ames Research Center, the other at JPL,
started to study a dedicated Venus mapping-radar mission. Ames proposed to adapt
the Pioneer Venus spacecraft that it was developing, while the JPL proposal, which
was named VOIR (Venus Orbiting Imaging Radar, but also ``to see'' in French),
envisaged a new spacecraft using a radar system equipped with a large parabolic
antenna such as on the Pioneers and Voyagers which were to explore the outer solar
system, or alternatively a linear phased-array antenna. To minimize the orbitinsertion burn Ames intended to put its spacecraft into an elliptical orbit, but JPL
wanted a circular orbit so that all the data would be collected at the same altitude
and thus simplify the data reduction and analysis, even although this would greatly
complicate the orbit-insertion process and would require the craft to have larger
propellant tanks. Although some scientists argued that terrestrial radio-telescopes
would soon be able to obtain data similar to that expected from an orbiting radar, at
a much lower cost, in 1977 NASA adopted the VOIR proposal. In fact, the Arecibo
telescope in Puerto Rico had recently achieved a resolution as fine as 100 meters in a
few selected areas of the planet.

Meanwhile, a series of experiments were conducted to refine the SAR concept.
Between 1977 and 1980, JPL tested its planetary synthetic-aperture radar by flying
NASA's Convair 990 `Galileo II' aircraft over the forests of Guatemala and Belize,
and demonstrated that the radar could penetrate the foliage to reveal ancient roads,


4

The decade of Halley

A Seasat synthetic-aperture radar image of a section of the Cascade Range in the
western United States featuring Mount St. Helens. The Venus Orbiting Imaging Radar
(VOIR) would have returned images of Venus at a comparable resolution. (JPL/NASA/
Caltech)

stone walls, terraces and agricultural canals, in the process providing insights into
the Mayan civilization and its economic structure (and, by coincidence, furthering
the centuries-old association between the Mayans and the planet Venus).8,9 JPL's
Seasat, which was America's first civilian radar imaging satellite, was launched in
June 1978 but it was crippled after 105 days by the failure of an electrical slip-ring
connector. Nevertheless, its data greatly impressed oceanographers. It also showed
why SAR was popular with the military: Seasat was reputedly capable of detecting
the bow shocks of submerged submarines and also of the prototypes of `stealth'
airplanes crossing water.10 Meanwhile, the Pioneer Venus Orbiter was compiling a
preliminary radar map of Venus with a resolution of 150 km.11


The face of Venus

5


The antenna of JPL's prototype planetary radar protruding from the rear fuselage of the
`Galileo II' airplane prior to a NASA flight over the Guatemalan forest.

In early 1980 Martin Marietta Aerospace, Hughes Aircraft and Goodyear
Aerospace submitted proposals for the development of the VOIR spacecraft and its
synthetic-aperture radar, and the project was included in the agency's 1981 budget ±
although with its launch postponed from May±June 1983 to May 1986.12 Martin
Marietta was eventually selected to build the spacecraft, while Hughes, which had
worked in an analogous role on the Pioneer Venus Orbiter, would supply the radar.
The plan was for VOIR to be launched by the Space Shuttle and released in low
Earth orbit, then boosted by a Centaur stage on a trajectory that would reach Venus
in November 1986, whereupon the spacecraft would enter orbit and undertake a 5month survey mission that would map the entire surface at 600 meters resolution and
certain areas at somewhat higher resolution, and provide a global topographic and
gravimetric map. The result would hopefully be a leap in knowledge of Venus to
match that of Mars after Mariner 9. This would provide context for the pictures
taken at ground level by the Venera landers, and the geological analyses derived
from them, and would identify processes that were not evident in the low-resolution
radar map provided by the Pioneer Venus Orbiter. In fact, transferred to Earth, the
resolution of the Pioneer Venus Orbiter's radar would have missed the largest river
basins, including the Mississippi and the Amazon; would have washed out some of
the most geologically important mountain ranges, including the American Rockies,
the Alps and Mount Everest in the Himalayas; and, even worse, would not have
shown the continental margins, knowledge of which is the key to understanding the
processes which have shaped the terrestrial crust. To minimize its cost, VOIR was to
reuse as many components from previous missions as possible: the solar panels were


6


The decade of Halley

spares left over from Mariner 10, the electronics were from Voyager, the radar
altimeter was from the Pioneer Venus Orbiter and the imaging radar from Seasat.
Compared to synthetic-aperture radars carried by aircraft, that of VOIR operated at
the longer wavelength of about 25 cm, which was better able to penetrate the dense
Venusian atmosphere without substantial attenuation of the signal. The spacecraft
was to carry several other instruments. One was to be a microwave radiometer to
measure the amount of energy radiating from various depths in the atmosphere and
to determine the temperature and how much sulfur dioxide, sulfuric acid and water
vapor were present. An airglow spectrometer and photometer would observe the
upper atmosphere and ionosphere to study the circulation of the atmosphere in this
region. A Langmuir probe would measure the temperature and distribution of ions
and electrons in the ionosphere, as a quadrupole mass spectrometer monitored the
composition, temperature and concentration of neutral gases. The final instrument
would measure the temperature and density of ions in the ionosphere. On reaching
the planet, the spacecraft would first enter an elliptical polar orbit, then circularize
this by using either a conventional engine or the novel technique of aerobraking in
which it would fire its engine to lower the periapsis of its orbit into the fringe of the
atmosphere and then exploit atmospheric drag on successive passes to lower its
apogee to the desired altitude. Although this technique had been pioneered in Earth
orbit by the Atmospheric Explorer C satellite in 1973, it was nevertheless a risky
maneuver. Its attraction was that it would enable the mass of the VOIR spacecraft to
be limited to 850 kg. After circularization, by January or February 1987, VOIR
would jettison the aerobraking shield, raise its periapsis from the atmosphere and
start its primary mission, using its high-gain antenna to relay the data in real-time at
1 Mbps; fully 500 times the data rate of the Pioneer Venus Orbiter. The resulting
map would provide almost global coverage, including one of the poles. In addition,
for about 30 seconds on each orbit the radar would image a swath 10 km wide and
200 km long at higher resolution. In total, such `spot data' would cover about 2 per

cent of the surface. The primary mission was to last 120 days, or half of a Venusian
day. An extension of up to a year was possible, so as to fill in gaps in the coverage
and map the other pole, and to provide a detailed gravimetric survey which would
enable geophysicists to estimate the thickness of the crust and place constraints on
the size of the planet's core (if any) and on the rigidity of the mantle.13,14,15 Overall, it
promised to be a tremendous mission.
But in 1981 the incoming Reagan administration decided to scale down federal
spending, and NASA was told to cancel one major program. The cost of VOIR was
then estimated at $680 million, and the launch had been slipped again, this time to
March 1988, so NASA reluctantly canceled it.16
In the Soviet Union the Lavochkin bureau, which had specialized in planetary
and lunar missions since 1965, had in 1976 started work on a Venus orbiter that
would carry a synthetic-aperture radar to map the radio reflectivity and topography
of the surface. In 1977 further studies were supported by the Academy of Sciences,
the Ministry of General Machine Building (a vast organization whose innocuous
name `hid' the space industry) and the Ministry of Radio Production, and contracts
were awarded to make a suitable radar system. Although the `Kometa' bureau led by


The face of Venus

Impressions of the Venus Orbiting Imaging Radar (VOIR) spacecraft showing its
deployment by Shuttle, and (inset) ignition to leave Earth orbit.

7