spacetoday.net: News briefs: March 16-17

Posted: Mon, Mar 18, 2002, 8:17 AM ET (1317 GMT)

Radar observations of the planet Mercury have revealed a large crater on its surface, Sky and Telescope reported. The crater, about 85 km across, looks similar in appearance to the lunar crater Tycho; planetary scientists reporting the discovery say it could be younger than the 109-million-year old Tycho. Almost half of the planet was never observed by Mariner 10, the only spacecraft to date to visit the planet, making radar observations the only way to map the planet until spacecraft visit the planet again later this decade.
Astronomers are finding their estimates of the age of pulsars in question after recent discoveries. Astronomers studying the motion of one pulsar from the center of the supernova remnant that formed it measured it to be 64,000 years old, compared to the 107,000-year "characteristic age" derived from its rotation rate. The previous discovery of a pulsar older than its characteristic age is leading astronomers to question the assumptions they make when generating those age estimates.
Planetary scientists have encountered difficulty selecting landing sites for two Mars rovers scheduled for launch next year, SPACE.com reported Friday. Four sites are under consideration, but all have problems ranging from cold temperatures to rocky terrain to dust devils. The final selection of two landing zones is planned for this May.