Twenty years of physics: The solar system

BOUNCING RADAR SIGNALS off the moon was considered quite a feat 20 years ago. Today they are being bounced off planets on the other side of the sun. This accomplishment is symbolic of the enormous expansion in our knowledge of the solar system that has taken place in the last two decades. The expansion has resulted jointly from the development of new technology and from the large investment in the space program and in geophysical and astrophysical research.

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