Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                                     
May 18, 1993
(Phone:  202/358-1547)

Franklin O'Donnell
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone:  818/354-5011)


RELEASE:  93-89

MAGELLAN TO TEST AEROBRAKING MANEUVER IN VENUS ATMOSPHERE

   NASA's Magellan spacecraft will dip into the atmosphere of 
Venus beginning May 25 in a first-of-its-kind "aerobraking" 
maneuver, lowering the spacecraft's orbit to start a new 
experiment.

   The aerobraking technique will use the drag created by Venus' 
atmosphere to slow the spacecraft and circularize Magellan's 
orbit.  Currently Magellan is looping around Venus in a highly 
elliptical orbit.

   "This aerobraking technique has never been used before on a 
NASA planetary mission," said Douglas Griffith, Magellan project 
manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

   "Magellan has been highly successful in completing all of its 
primary mission goals," said Alphonso V. Diaz, Deputy Associate 
Administrator for NASA's Office of Space Science.  "The new orbit 
will enhance the scientific return from what is already one of 
NASA's most successful space science missions."

   According to Griffith, aerobraking is the only way to make such 
a large change in Magellan's orbit because the spacecraft does not 
have enough thruster fuel onboard for the change.  "Although 
aerobraking creates some risk of losing the spacecraft, the 
scientific benefits make the risk worthwhile," he said.

   The benefit of changing the orbit is to make possible better 
measurements of Venus's gravity field, particularly at latitudes 
near the planet's poles, said Dr. R. Stephen Saunders of JPL, the 
Magellan Project Scientist.


                             - more -
                                      
                              - 2 -


   For the past 8 months, Magellan has been collecting data on 
Venus' gravity.  However, measurements from the current elliptical 
orbit are blurred at high latitudes by the height of the 
spacecraft above the surface -- about 1,300 miles (2,100 
kilometers) near the north pole and 1,700 miles (2,800 kilometers) 
near the south pole.

   Scientists also hope to study Venus's atmosphere using data 
collected during the aerobraking experiment itself.  And another 
objective is to gain the engineering experience that may allow 
future missions to use aerobraking to enter planetary orbit or to 
change orbit without using large thrusters.

   Launched in May 1989, Magellan will complete its fourth 243-day 
orbital cycle at Venus on May 25.  During each of the 8-month 
cycles, Magellan orbits from north to south while the planet turns 
once underneath the spacecraft.

   During earlier cycles, Magellan used its radar to map Venus's 
surface with a resolution as fine as 250 feet (75 meters).  Data 
was obtained on the elevation, slope, radar reflectivity and radar 
emissivity over 98 percent of the planet.

   In the upcoming maneuver, flight controllers hope to lower the 
spacecraft from a low point near 100 miles (170 kilometers) and 
high point of 5,300 miles (8,500 kilometers).  The target orbit is 
125 by 375 miles (200 by 600 kilometers).  This would alter orbit 
time from 3-1/4 hours to 90 minutes.

   The aerobraking experiment will start at 1:30 p.m. EDT May 25, 
when the spacecraft makes the first maneuver.  By controlling the 
orbit altitude, the drag and heat generated on the spacecraft will 
be kept within tolerable limits.

   Completing the change will take about 80 days.  The short 
period of drag on each orbit, a few minutes at the start to about 
20 minutes near the end, will lower the orbital high point by 
about 6 miles (10 kilometers) on every orbit.

   Measuring Venus's gravity field permits scientists to measure 
the pattern of heavier and lighter regions under the planet's 
surface.  It is the only technique currently possible to look 
inside Venus and provides information like that gained using 
seismometers to probe inside a planet.  Similar measurements on 
Earth helped reveal plate tectonics, Earth's fundamental geologic 
process.

   "Without better measurements from a lower orbit, it would 
remain very hard to understand Venus's internal geology and why it 
is so different from Earth," said Saunders.

   JPL manages the Magellan mission for NASA's Office of Space 
Science, Headquarters, Washington, D.C.
                              -end-