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to make a close approach, or from the interplanetary magnetic field. For this field to play an important role, its energy density must be comparable to the kinetic energy density of the comet. (We do not even worry about whether the comet has a distribution of charges and fields which will permit it to respond to the imposed field.) Thus, the condition is

where B is the magnetic field strength in gauss, R is the radius of the comet, m its mass, v its velocity and ρ its density. We note that the condition is independent of the mass of the comet. Taking a typical cometary velocity in the inner solar system of about 25 km/sec, and ρ as the density of Venus, about 5 gm/cm3, we find that a magnetic field strength of over 10 million gauss is required. (A similar value in electrostatic units would apply if the circularization is electrical rather than magnetic.) Earth’s equatorial surface field is about 0.5 gauss. The fields of Mars and Venus are less than 0.01 gauss. The Sun’s field is several gauss, ranging up to several hundred gauss in sunspots. Jupiter’s field as measured by Pioneer 10 is less than 10 gauss. Typical interplanetary fields are 10−5 gauss. There is no way to generate anything approaching a 10 megagauss field on a large scale in the solar system. And there is no sign that such a field was ever experienced in the vicinity of Earth. We recall that the magnetic domains of molten rock in the course of refreezing are oriented by the prevailing field. Had Earth experienced, even fairly briefly, a 10 Mg field 3,500 years ago, rock magnetization evidence would show it clearly. It does not.

The New York Times Bestseller

COMET

by

Carl Sagan and Ann Druyan

What are these graceful visitors to our skies? We now know that they bring both life and death and teach us about our origins. In Comet, Pulitzer Prize-winning astronomer Carl Sagan and writer Ann Druyan explore the origin, nature, and future of comets, and the exotic myths and portents attached to them. Lavishly illustrated and including new material, this edition of Comet is indispensable for anyone who has ever gazed up at the heavens and wondered why.

COMET

by Carl Sagan and Ann Druyan

Published by The Random House Publishing Group.

Available in bookstores everywhere.

Carl Sagan and acclaimed author Ann Druyan show us the roots of the human species in

SHADOWS OF

FORGOTTEN

ANCESTORS

A thrilling account of how humans got to be the way we are, Shadows of Forgotten Ancestors begins its saga with the origin of the Earth and shows that many of our key traits—self-awareness, family ties, reason, ethics, submission to authority, and hatred for those who are different—are rooted in the deep past and illuminated by our kinship with other animals.

SHADOWS OF FORGOTTEN ANCESTORS

by Carl Sagan and

Ann Druyan

Published by The Random House Publishing Group.

Available wherever books are sold.

The New York Times Bestseller

BILLIONS

&

BILLIONS

by

Carl Sagan

In the final book of his astonishing career, Carl Sagan brilliantly examines the burning questions of our lives, our world, and the universe around us. These luminous, entertaining essays travel both the vastness of the cosmos and the intimacy of the human mind. Here, too, is a rare private glimpse of Sagan’s thoughts about love, death, and spirituality as he struggled with fatal disease. Ever forward-looking and vibrant with the sparkle of his unquenchable curiosity, Billions & Billions is a testament to one of the great scientific minds of our day.

BILLIONS & BILLIONS

by Carl Sagan

Published by The Random House Publishing Group.

Available in bookstores everywhere.