Its ONLY Rocket Science – Lucy Rogers (1)

The inter- planetary system would provide better navigation capabilities and more precise location and target information. It would also pro- vide an improved communication capability, enabling easier and quicker transfer of both scientific and operational data. Time Dilation and Relativistic Effects Most, if not all, satellites carry an accurate clock that is synchro- nised with an Earth-based clock. This is used for many things, including digital processing and determination of the location of the spacecraft.

However, no matter how accurate the clocks are, after a while there will be a discrepancy between those on the Earth and those in space. This is because time passes more slowly if you are travelling very fast. Unfortunately, this is not just a case of “if you’re having a really nice time, ten minutes seems to go by in an instant, whereas if you are waiting for something special, ten min- utes can drag forever.”

It is a bit more complicated. Sufficient to say that in 1905, Albert Einstein showed in his Special Theory of Relativity that time runs slower during very fast movements. This is known as time dilation and was proved empirically in October 1971, when four atomic clocks were flown around the world. Com- pared to identical clocks that stayed at the US Naval Observatory, the clocks on the aeroplane showed a different time when they got back.

A clock on a satellite travelling around the Earth at about 3.8 kilometres per second, which is the speed of the GPS constel- lation of satellites, runs slower when viewed from the Earth. This leads to an inaccuracy of time of approximately 7.2 microseconds or 0.0000072 seconds a day, or 0.0026 seconds a year.

This does not sound much, but the GPS timing signal is typically accurate to ten nanoseconds or 0.01 microseconds, which is 0.00000001 seconds. Eleven years after his Special Theory of Relativity, Albert Ein- stein showed in his General Theory of Relativity that time moves slower in a stronger gravitational field.

It’s ONLY Rocket Science An Introduction in Plain English Dr. Lucy Rogers CEng MIMechE FRAS Isle of Wight, UK. www.itsonlyrocketscience.com ISBN 978-0-387-75377-5 e-ISBN 978-0-387-75378-2 DOI: 10. 1007/978-0-387-75378-2 Library of Congress Control Number: 2007939660 © 2008 Springer Science+Business Media, LLC All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis.

Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.

9 8 7 6 5 4 3 2 1 Springer Science + Business Media springer.com For Laura and Hannah Acknowledgements There are many people and organizations that have helped, either directly or indirectly, to make this book a reality. I would first like to thank the British Association for the Advancement of Science and the Guardian Newspaper, for encouraging me, as an engineer, to become involved with the media. I was fortunate enough to be awarded one of the BA’s Media Fellowships at the Guardian news- paper, and this scheme opened my eyes to the possibility of sharing science with everyone, and not just limiting it to academia and industry.

Without this scheme and the wise words of Tim Radford, the then science editor at the Guardian, I would never have started writing. I would also like to thank John Thomson and West Didsbury Astronomical Society for rekindling my interest in astronomy, and to Starchaser Industries for the opportunity to actually play, I mean work, with rockets through all stages of development. My thanks also go to the team at Springer, particularly John Watson, who first believed in the project, and also to Harry Blom, my editor, and his assistant editor Chris Coughlin.

I have been constantly amazed at the generosity of the many people who have taken the time to answer my questions and explain facets of rocket science that, at the start of the project, I did not even know were involved. These include the staff at NASA, in particular Kylie Clem, Allard Beutel and Jennifer Ross-Nazzal and also to Colin R.

McInnes, A.D. King, Russell Eberst, Claude Phipps, Gregory Benford and also the members of the HearSat email list.

This is a short excerpt from the opening of “” by Unknown, quoted for review and introduction purposes. All rights belong to the copyright holders.

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