From: baalke@kelvin.jpl.nasa.gov (Ron Baalke)
Subject: Pioneer Venus Closes In On Venus
News-Software: VAX/VMS VNEWS 1.3-4   
Keywords:  Pioneer Venus
Reply-To: baalke@kelvin.jpl.nasa.gov
Organization: Jet Propulsion Laboratory
Date: Thu, 3 Sep 1992 01:26:16 GMT

Paula Cleggett-Haleim
Headquarters, Washington, D.C.            September 1, 1992
(Phone:  202/358-1547)

Pete Waller
Ames Research Center, Mountain View, Calif.
(Phone:  415/604-3938)

RELEASE:  92-140

NASA'S PIONEER VENUS ORBITER CLOSES IN ON VENUS

     Between now and year's end, the Pioneer Venus Orbiter
will skim closer to Venus than ever before, returning
significant new knowledge of the planet.

     It will explore upper atmosphere regions never before
reached, helping answer questions about Venus' environment
-- whether there is lightning on the planet and whether
Venus once had oceans.  The orbiter then will enter Venus'
atmosphere at hypersonic speed and burn.  This is expected
to happen in October, November or December.

     "Completing the picture of Venus' outer environment
will be a major achievement," said Richard Fimmel of NASA's
Ames Research Center, Mountain View, Calif.  "It will
culminate 14 years of receiving information since the
spacecraft was launched in 1978."

     Pioneer Venus provided data for the first topographic
maps of 90 percent of the planet's previously unknown
surface.  It identified mountain ranges, plateaus, plains
and deep depressions.

     The expected findings about Venus' outer environment,
Fimmel said, relate to atmosphere/plasma interactions.

     Plasmas (electrically charged gases) are the most
common form of matter in the universe.  The million-mile-an-
hour solar wind is an ionized gas which continuously batters
Venus' atmosphere .  The solar wind and Venus' atmosphere
interact in many ways.

     Studying these complex interactions on Venus may
increase knowledge of similar interactions in the
atmospheres of comets, planets, moons and star systems as
well as processes in the early solar system and interstellar
gas clouds.

     Researchers also hope to find out more about Venus'
water-loss mechanisms by measuring for the first time the
top of Venus' "mixed" atmosphere and the densest part of the
planet's ionosphere.

     In the final phase, gravity will bring Pioneer's orbit
down into Venus' upper atmosphere.  With Pioneer about 84
miles above the planet, NASA Ames officials will raise the
low point of its orbit by firing thrusters early on Sept. 7
and then every 5 days thereafter until the estimated four
pounds of remaining hydrazine propellant is gone.  Then, the
spacecraft will plunge into the Venusian atmosphere and
burn.

     The Pioneer Venus mission has had three phases.  NASA
researchers held the orbit's lowest point, known as
"periapsis," at 93 miles above Venus' surface to map and
study the ionosphere and atmosphere during the first 19
months at Venus.  In phase two, with only 10 percent of
propellant left, they allowed solar gravity to gradually
raise periapsis to 1,500 miles and then return it to low
altitude.

     By June of this year, with periapsis at 125 miles, the
orbiter's instruments began to measure Venus' ionosphere
again.  By Sept. 7, the sun will pull periapsis down to 82
miles.  If enough hydrazine remains, thrusters will be fired
to raise periapsis eight more times.

     By Nov. 22, solar gravity will briefly raise periapsis
to about 98 miles, but will pull it down to about 87 miles
by Dec. 10.  Officials then plan to fire any remaining fuel
to obtain more data from farther into Venus' "day side"
before the orbiter's final entry into the atmosphere.

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    /_ /|  /____/ \  /_ /|     Ron Baalke         | baalke@kelvin.jpl.nasa.gov
    | | | |  __ \ /| | | |     Jet Propulsion Lab |
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