Magellan: a fortuitous start to Venus.

Magellan: A fortuitous start to Venus

If Magellan, the first U.S. interplanetary spacecraft sent from Earth since 1978, orbits Venus next year as successfully as it took off on May 4, a stroke of scientific luck may emerge from the six-day delay in launching the space shuttle Atlantis.

The relative motions of Venus and Earth meant that as late as May 5 the radar mapper's approach to Venus would be slower for each day the launch was postponed, reducing the amount of hydrazine propellant needed to slow the craft enough to settle into its correct orbit. The "launch window" opened April 28, and mission officials had planned to launch that day, because unforeseen delays lasting more than a month would have forced Magellan to wait nearly two years for another try. May 5 would have proved best from a fuel-economy point of view, says Elliott Cutting, head of the Magellan Mission Planning Team at the Jet Propulsion Laboratory in Pasadena, Calif., but May 4 was "awfully close."

That launch date probably saved about 5 of Magellan's 132 kilograms of hydrazine, Cutting says. If everything else about the craft behaves as planned, this could provide a substantial fuel reservoir for extending the basic 243-day mission. Each additional 243-day cycle -- the time needed for Magellan to map its way once around the planet -- should use about 2 kilograms of hydrazine. The excess could thus be enough to hold the craft steady for 2-1/2 more cycles.

What scientists want most from an extended mission is to map any areas Magellan misses the first time. Such gaps may be created when the sun blocks the craft's radio beam to Earth.

Scientists also hope the craft will have time (and hydrazine) to map Venus' south polar region, not seen at all by the Soviet Venera 15 and 16 orbiters, the most recent craft to go there. In addition, the mission team is studying the possibilities of remapping some of the surface with the radar beam aimed at a different angle and polarized differently -- two ways to extract more information from a global Venus database that may be the last for years.

A particularly exciting possibility calls for Magellan to make its radar maps in stereo, by electronically adjusting its antenna to add what amounts to a "left-eyed" view of areas where it gathered "right-eyed" data during a previous orbit. Another use for some of the hydrazine would be to lower the orbit's lowest point at some time from 250 to 200 kilometers, enabling the craft to produce maps with better contrast and more detail.