Paula Cleggett-Haleim
Headquarters, Washington, D.C.
(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 poles, said Dr. R. Stephen Saunders of JPL, the Magellan Project 
Scientist.

	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.