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Voyager 2's encounter with Neptune
Vistas in Astronomy, Voh 33, pp. 1-20, 1990 Printed in Great Britain.
0083--6656/90 $0.00 + .50 1990 Pergamon Press plc.
V O Y A G E R 2'S E N C O U N T E R WITH NEPTUNE William I. McLaughlin Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.
Voyager 2 completed its tour of the four gas giants -Jupiter,
Saturn, Uranus,
and Neptune -- with a successful flyby
of the Neptunian system in August 1989, 12 years after launch. After its encounter with Saturn in 1981, the spacecraft was extensively reconfigured to prepare it for the rigors of the outer solar system.
A damaged gear train for the scan platform,
on which the remote-sensing instruments are located, was analyzed by means of replicas constructed on Earth, and a safe domain of operating conditions was established.
The increased
telecommunications distance, with its inference of decreased data rates, was addressed through development of onboard datacompression techniques and the arraying of antennas on Earth. The low light levels at Uranus and Neptune would require longer exposure times for imaging, with the possibility for smeared images due to unwanted motions while the camera shutter was open. Systematic motions were reduced by devising methods to "pan" the camera so as to null-out apparent relative motion between spacecraft and target.
"Random" motions of the three-axis
stabilized spacecraft were reduced more than twofold by reducing
W. I. McLaughlin the strength of the bursts of hydrazine
from its attitude-control
thrusters. The preparation dividends
of Voyager 2 as an observing
at Neptune with a step-function
of planet,
magnetosphere,
rings,
Blue Neptune derives wavelengths
gain in our knowledge
its color from absorption
Unlike Uranus,
of red
in the hydrogen/helium
the Neptunian
atmosphere
variety of features visible to the imaging system. prominent
is the Earth-sized
to Jupiter's changing
"Great Dark Spot"
"Great Red Spot".
formations
paid
and satellites.
by a small amount of methane
atmosphere.
instrument
supports Most
(GDS), analogous
The GDS is accompanied
by rapidly
of white cirrus clouds of methane.
A methane
cycle probably operates radiation transforming
in the atmosphere with solar ultraviolet methane
in the stratosphere
into compounds
such as ethane and acetylene which sink into the troposphere eventually upwell
a
into the stratosphere,
transformed
and
once more
into methane. Radio observations
determined
Neptune to be 16 hr 03 min
the deep rotation rate of
(± 04 min),
and, combined with visual
observations,
this datum permitted wind velocity curves to be
constructed.
The highest retrograde
any planetary
atmosphere
m/s.
Neptune,
investigated
structures, relatively
wind speeds of
by Voyager were seen:
like Jupiter and Saturn,
source and a complex relationship and latitude.
(westward)
has an internal heat
between atmospheric wind speed
These three planets show significant
while Uranus,
350
atmospheric
with no internal heat source and a
simple wind-speed
curve,
presents
a visually bland
atmosphere. The Neptunian magnetic
field was a surprise.
It had been
felt that the offset dipole observed at Uranus was atypical
and
Voyager 2's Encounter with Neptune
Fig.
I.
3
N e p t u n e ' s G r e a t D a r k S p o t d o m i n a t e s t h i s v i e w of t h e planet, t a k e n w i t h V o y a g e r 2's n a r r o w - a n g l e c a m e r a at a r a n g e of 6.1 m i l l i o n km. NASA/JPL
4
W . I . McLaughlin
Fig.
2
T h e rapid e v o l u t i o n of m e t h a n e c i r r u s c l o u d s n e a r N e p t u n e ' s G r e a t D a r k S p o t is s h o w n in this image w h i c h has a r e s o l u t i o n of a b o u t i00 km. A period of 18 h o u r s s e p a r a t e s e a c h panel. NASA/JPL
Fig.
3.
H i g h c l o u d s in the s o u t h - p o l a r r e g i o n , 68 ° s o u t h l a t i t u d e , of N e p t u n e cast s h a d o w s on l o w e r l a y e r s of c l o u d s . T h i s i m a g e w i t h t h e n a r r o w - a n g l e c a m e r e w a s t a k e n on A u g u s t 23, 1989 f r o m a r a n g e o f 25 m i l l i o n km, yielding a resolution of 45 km. T h e s e a r e the f i r s t c l o u d s h a d o w s s e e n b y V o y a g e r on a n y p l a n e t . NASA/JPL
N
Z
O
bo
-i
O
<
Fig.
VOYAGER 2
,PLASMASPHERE
MAGNETIC AXIS
4. N e p t u n e ' s m a g n e t o s p h e r e , like that of Uranus, is s t r u c t u r e d by a m a g n e t i c field w h o s e axis is s i g n i f i c a n t l y o f f s e t from the r o t a t i o n a l axis of the planet. V o y a g e r 2 s k i m m e d o v e r the N o r t h Pole at an a l t i t u d e of about 5000 km above the c l o u d t o p s of N e p t u n e b e f o r e m a k i n g its c l o s e s t a p p r o a c h to T r i t o n five hours later. NASA/JPL
BOW SHOCK
MAGNETOPAU
0D
0%
Voyager 2's Encounter with Neptune
Fig.
5.
T h e two m a i n rings of N e p t u n e are l o c a t e d at about 54 t h o u s a n d and 62 t h o u s a n d km from the c e n t e r of the planet. C l u m p i n g of r i n g m a t e r i a l is a p p a r e n t in this llls w i d e - a n g l e c a m e r a image t a k e n at a range of i.i m i l l i o n km. NASA/JPL
7
W. I. McLaughlin
Fig. 6.
The four rings of Neptune are captured in a 591s exposure with the wide-angle camera of Voyager 2 at a distance of 280 thousand km behind the planet. In addition to the two main rings (62 thousand and 54 thousand km from the center of the planet), a faint inner ring is visible at about 42 thousand km, and a broad plateau of material extends inward from halfway between the two bright rings. NASA/JPL
Voyager 2's Encounter with Neptune
Fig.
7.
This photomosaic of Triton, assembled from 14 individual frames, shows the great variety of its surface features, which are sculpted in ice. At bottom is the south polar cap, including dark streaks pointing in a northeastward direction and which are remnants of "icy volcanism". The "cantaloupe" terrain in the northwest is interlaced by fractures. The densest cratering of the surface is seen in the plains of the northeast sector. NASA/JPL
9
Fig.
8. An active g e y s e r is seen e r u p t i n g from the s o u t h p o l a r region of Triton. The nitrogen, d a r k l y t i n t e d w i t h o r g a n i c m o l e c u l e s , spurts 8 km above the s u r f a c e and then is blown w e s t w a r d by w i n d s in the thin n i t r o g e n a t m o s p h e r e of the satellite. NASA/JPL
_---.
Voyager 2's Encounter with Neptune
Fig.
9.
The field of v i e w is a p p r o x i m a t e l y i000 k m a c r o s s and shows d a r k i r r e g u l a r areas s u r r o u n d e d by b r i g h t aureoles. NASA/JPL
11
12
W . I . McLaughlin
Fig.
i0.
T h i s large, flooded b a s i n on T r i t o n is 400 x 200 km in size. Some s l i g h t r o u g h n e s s of the flood p l a i n m a y be due to ice rafts a s s o c i a t e d w i t h the initial event. NASA/JPL
Voyager 2's Encounter with Neptune
Fig.
ii.
D e t a i l s as small as 2.5 km are v i s i b l e in this v i e w of T r i t o n ' s s u r f a c e t h a t is d o m i n a t e d by a long linear feature r u n n i n g v e r t i c a l l y t h r o u g h the image. P r o b a b l y a graben, a n a r r o w d o w n - d r o p p e d fault b l o c k about 35 km across, a c e n t r a l ridge of u p w e l l i n g ice is also v i s i b l e for m o s t of its length. NASA/JPL
13
14
W. 1. McLaughlin
i ?i!i iii
Fig.
12.
i
•i
A layer of h a z e e x t e n d s a b o u t 6 km s u r f a c e in the n i t r o g e n a t m o s p h e r e image was t a k e n from a d i s t a n c e of 170 t h o u s a n d km.
i i~
ii~ i i !i i iii/ !i i!!iii II~
a b o v e the of Triton. The approximately NASA/JPL
Voyager
Fj_g. 13.
2’s Encounter
with
Neptune
Six new satellites were discovered by Voyager 2 at Neptune, to join previously known Triton and Nereid. The largest of the new satellites, 1989N1, is approximately 400 km in diameter and exhibits a distinctly non-spherical shape. Resolution is about 2.7 km at a range of 146 thousand km. NASA/JPL
15
17
Voyager 2's Encounter with Neptune probably related to events that led to the planet "lying on its side".
However,
the Neptunian dipole is also skewed, being
oriented approximately 50 ° from the planet's spin axis about 60 ° for Uranus) of mass.
(versus
and significantly removed from the center
The population of charged particles in the
magnetosphere
is considerably reduced from the other gas giants,
e.g., the flux of 50 keV electrons was only about i/i0 of t h a t seen at Uranus.
Dynamo electrical currents in Neptune's
subsurface region of gas and melted ice are probably responsible for the magnetic field.
The low density of charged particles may
be due to sweeping by satellites and ring particles:
a process
facilitated by the tilt and offset of the dipole. Auroral activity is present and widespread over the surface of Neptune rather than being concentrated as is more typical of planetary auroras.
This phenomenon is the result of a
corresponding pattern of charged-particle impingement dictated by the complicated magnetic field. Partial ring arcs had been hypothesized to explain a series of earlier Earth,based observations.
What was found were two
main rings with significant clumping of material into three arcs in the outer ring, the planet.
located at 62 thousand km from the center of
The three ring arcs, which represent the optically
thick parts of the outer ring, are most likely responsible for all of the occultation events observed from Earth, with the exception of one.
That one seems to have been caused by a chance
occultation of the test star by a then unknown satellite of Neptune
(1989N2)!
observed.
Two other rings, more difuse, were also
The Neptunian rings contain a larger fraction of small
particles than their Uranian counterparts.
18
W.I. McLaughlin Six new satellites were discovered,
Nereid.
One,
joining Triton and
1989NI, with a diameter of about 400 km,
is larger
than Nereid but was previously undetected due to its proximity to the bright planetary disk. Triton,
like Io at Jupiter and Miranda at Uranus,
the centerpiece for the encounter
(the Saturnian rings fulfilled
that function at the sixth planet).
The terrain of this 2720 km-
diameter satellite is extremely varied with fractures, basins,
some craters,
provided
flooded
a southern polar cap, and features which
represent "icy volcanism".
The surface is principally composed
of water ice with an overlay of nitrogen and methane frosts, as at the polar cap.
such
The overlaid ices are sublimated and
condensed in response to seasonally induced thermal cycles,
while
the water ice serves as the primary geological material of Triton.
At these very cold temperatures
it possesses a
mechanical strength comparable to that of rock on Earth.
With a
density of approximately 2.0 g cm -3, Triton clearly has a rocky component in its interior. At the time of the encounter in August, Soderblom,
geologist Lawrence
a member of the Voyager Imaging Team,
speculated that
some 50 dark streaks on the southern polar cap might be the result of wind-borne deposits of erupting material: volcanism".
"icy
On October 2, it was announced that p o s t - e n c o u n t e r
analysis of the imaging data had indeed detected a geyser in the act of spouting with, the material downwind.
as hypothesized by Soderblom,
conveyance of
The bulk of the ejected material
is
thought to be nitrogen with the dark hue coming from an admixture of organic molecules.
The presence of geysers is surprising,
particularly since so little energy is available to activate the subsurface reservoirs of nitrogen;
at Triton's distance,
the Sun
19
Voyager 2's Encounter with Neptune is only 1/900 of its brightness at Earth. Triton is colder and smaller than had been expected because its albedo,
averaging 70%, is larger than anticipated.
Thus, the
lakes of liquid nitrogen that some had foreseen as possible surface features of the satellite were,
indeed, not possible
since nitrogen cannot exist as a liquid on the surface of this cold body
(the temperature in the lower atmosphere is
approximately 37 ° K.).
The smaller size of Triton posed a
challenge to Voyager's Navigation Team in its
(successful)
quest
to thread the spacecraft's trajectory through the constricted zone which would guarantee a dual occultation of Earth and Sun as seen from the vehicle.
The occultation of the Earth was used in
the investigation of atmospheric properties by refracting radio waves through the thin, for reception at Earth.
10-microbar nitrogen atmosphere of Triton The occultation of the Sun similarly
yielded atmospheric information when observed with the spacecraft's ultraviolet spectrometer. Triton is the coldest body yet observed in the solar system and probably represents our best idea of what Pluto is like until a flyby of that planet can be accomplished. Voyager 1 is now the most distant spacecraft from the Sun and is receding from it, northward of the ecliptic, A.U. per year.
at about 3.5
Voyager 2 is proceeding in a southerly course at
about 3.4 A.U. per year.
These spacecraft will probably continue
to return data from the outer solar system, possible crossing of the heliopause,
including the
for around 25 years, until
their radioactively-driven power sources decay to a level that can no longer support a meaningful set of scientific experiments.
20
W . I . McLaughlin The work described in this paper was carried out by the Jet
Propulsion Laboratory,
California Institute of Technology,
under
contract with the National Aeronautics and Space Administration.
November 3, 1989
0083--6656/90 $0.00 + .50 1990 Pergamon Press plc.
V O Y A G E R 2'S E N C O U N T E R WITH NEPTUNE William I. McLaughlin Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.
Voyager 2 completed its tour of the four gas giants -Jupiter,
Saturn, Uranus,
and Neptune -- with a successful flyby
of the Neptunian system in August 1989, 12 years after launch. After its encounter with Saturn in 1981, the spacecraft was extensively reconfigured to prepare it for the rigors of the outer solar system.
A damaged gear train for the scan platform,
on which the remote-sensing instruments are located, was analyzed by means of replicas constructed on Earth, and a safe domain of operating conditions was established.
The increased
telecommunications distance, with its inference of decreased data rates, was addressed through development of onboard datacompression techniques and the arraying of antennas on Earth. The low light levels at Uranus and Neptune would require longer exposure times for imaging, with the possibility for smeared images due to unwanted motions while the camera shutter was open. Systematic motions were reduced by devising methods to "pan" the camera so as to null-out apparent relative motion between spacecraft and target.
"Random" motions of the three-axis
stabilized spacecraft were reduced more than twofold by reducing
W. I. McLaughlin the strength of the bursts of hydrazine
from its attitude-control
thrusters. The preparation dividends
of Voyager 2 as an observing
at Neptune with a step-function
of planet,
magnetosphere,
rings,
Blue Neptune derives wavelengths
gain in our knowledge
its color from absorption
Unlike Uranus,
of red
in the hydrogen/helium
the Neptunian
atmosphere
variety of features visible to the imaging system. prominent
is the Earth-sized
to Jupiter's changing
"Great Dark Spot"
"Great Red Spot".
formations
paid
and satellites.
by a small amount of methane
atmosphere.
instrument
supports Most
(GDS), analogous
The GDS is accompanied
by rapidly
of white cirrus clouds of methane.
A methane
cycle probably operates radiation transforming
in the atmosphere with solar ultraviolet methane
in the stratosphere
into compounds
such as ethane and acetylene which sink into the troposphere eventually upwell
a
into the stratosphere,
transformed
and
once more
into methane. Radio observations
determined
Neptune to be 16 hr 03 min
the deep rotation rate of
(± 04 min),
and, combined with visual
observations,
this datum permitted wind velocity curves to be
constructed.
The highest retrograde
any planetary
atmosphere
m/s.
Neptune,
investigated
structures, relatively
wind speeds of
by Voyager were seen:
like Jupiter and Saturn,
source and a complex relationship and latitude.
(westward)
has an internal heat
between atmospheric wind speed
These three planets show significant
while Uranus,
350
atmospheric
with no internal heat source and a
simple wind-speed
curve,
presents
a visually bland
atmosphere. The Neptunian magnetic
field was a surprise.
It had been
felt that the offset dipole observed at Uranus was atypical
and
Voyager 2's Encounter with Neptune
Fig.
I.
3
N e p t u n e ' s G r e a t D a r k S p o t d o m i n a t e s t h i s v i e w of t h e planet, t a k e n w i t h V o y a g e r 2's n a r r o w - a n g l e c a m e r a at a r a n g e of 6.1 m i l l i o n km. NASA/JPL
4
W . I . McLaughlin
Fig.
2
T h e rapid e v o l u t i o n of m e t h a n e c i r r u s c l o u d s n e a r N e p t u n e ' s G r e a t D a r k S p o t is s h o w n in this image w h i c h has a r e s o l u t i o n of a b o u t i00 km. A period of 18 h o u r s s e p a r a t e s e a c h panel. NASA/JPL
Fig.
3.
H i g h c l o u d s in the s o u t h - p o l a r r e g i o n , 68 ° s o u t h l a t i t u d e , of N e p t u n e cast s h a d o w s on l o w e r l a y e r s of c l o u d s . T h i s i m a g e w i t h t h e n a r r o w - a n g l e c a m e r e w a s t a k e n on A u g u s t 23, 1989 f r o m a r a n g e o f 25 m i l l i o n km, yielding a resolution of 45 km. T h e s e a r e the f i r s t c l o u d s h a d o w s s e e n b y V o y a g e r on a n y p l a n e t . NASA/JPL
N
Z
O
bo
-i
O
<
Fig.
VOYAGER 2
,PLASMASPHERE
MAGNETIC AXIS
4. N e p t u n e ' s m a g n e t o s p h e r e , like that of Uranus, is s t r u c t u r e d by a m a g n e t i c field w h o s e axis is s i g n i f i c a n t l y o f f s e t from the r o t a t i o n a l axis of the planet. V o y a g e r 2 s k i m m e d o v e r the N o r t h Pole at an a l t i t u d e of about 5000 km above the c l o u d t o p s of N e p t u n e b e f o r e m a k i n g its c l o s e s t a p p r o a c h to T r i t o n five hours later. NASA/JPL
BOW SHOCK
MAGNETOPAU
0D
0%
Voyager 2's Encounter with Neptune
Fig.
5.
T h e two m a i n rings of N e p t u n e are l o c a t e d at about 54 t h o u s a n d and 62 t h o u s a n d km from the c e n t e r of the planet. C l u m p i n g of r i n g m a t e r i a l is a p p a r e n t in this llls w i d e - a n g l e c a m e r a image t a k e n at a range of i.i m i l l i o n km. NASA/JPL
7
W. I. McLaughlin
Fig. 6.
The four rings of Neptune are captured in a 591s exposure with the wide-angle camera of Voyager 2 at a distance of 280 thousand km behind the planet. In addition to the two main rings (62 thousand and 54 thousand km from the center of the planet), a faint inner ring is visible at about 42 thousand km, and a broad plateau of material extends inward from halfway between the two bright rings. NASA/JPL
Voyager 2's Encounter with Neptune
Fig.
7.
This photomosaic of Triton, assembled from 14 individual frames, shows the great variety of its surface features, which are sculpted in ice. At bottom is the south polar cap, including dark streaks pointing in a northeastward direction and which are remnants of "icy volcanism". The "cantaloupe" terrain in the northwest is interlaced by fractures. The densest cratering of the surface is seen in the plains of the northeast sector. NASA/JPL
9
Fig.
8. An active g e y s e r is seen e r u p t i n g from the s o u t h p o l a r region of Triton. The nitrogen, d a r k l y t i n t e d w i t h o r g a n i c m o l e c u l e s , spurts 8 km above the s u r f a c e and then is blown w e s t w a r d by w i n d s in the thin n i t r o g e n a t m o s p h e r e of the satellite. NASA/JPL
_---.
Voyager 2's Encounter with Neptune
Fig.
9.
The field of v i e w is a p p r o x i m a t e l y i000 k m a c r o s s and shows d a r k i r r e g u l a r areas s u r r o u n d e d by b r i g h t aureoles. NASA/JPL
11
12
W . I . McLaughlin
Fig.
i0.
T h i s large, flooded b a s i n on T r i t o n is 400 x 200 km in size. Some s l i g h t r o u g h n e s s of the flood p l a i n m a y be due to ice rafts a s s o c i a t e d w i t h the initial event. NASA/JPL
Voyager 2's Encounter with Neptune
Fig.
ii.
D e t a i l s as small as 2.5 km are v i s i b l e in this v i e w of T r i t o n ' s s u r f a c e t h a t is d o m i n a t e d by a long linear feature r u n n i n g v e r t i c a l l y t h r o u g h the image. P r o b a b l y a graben, a n a r r o w d o w n - d r o p p e d fault b l o c k about 35 km across, a c e n t r a l ridge of u p w e l l i n g ice is also v i s i b l e for m o s t of its length. NASA/JPL
13
14
W. 1. McLaughlin
i ?i!i iii
Fig.
12.
i
•i
A layer of h a z e e x t e n d s a b o u t 6 km s u r f a c e in the n i t r o g e n a t m o s p h e r e image was t a k e n from a d i s t a n c e of 170 t h o u s a n d km.
i i~
ii~ i i !i i iii/ !i i!!iii II~
a b o v e the of Triton. The approximately NASA/JPL
Voyager
Fj_g. 13.
2’s Encounter
with
Neptune
Six new satellites were discovered by Voyager 2 at Neptune, to join previously known Triton and Nereid. The largest of the new satellites, 1989N1, is approximately 400 km in diameter and exhibits a distinctly non-spherical shape. Resolution is about 2.7 km at a range of 146 thousand km. NASA/JPL
15
17
Voyager 2's Encounter with Neptune probably related to events that led to the planet "lying on its side".
However,
the Neptunian dipole is also skewed, being
oriented approximately 50 ° from the planet's spin axis about 60 ° for Uranus) of mass.
(versus
and significantly removed from the center
The population of charged particles in the
magnetosphere
is considerably reduced from the other gas giants,
e.g., the flux of 50 keV electrons was only about i/i0 of t h a t seen at Uranus.
Dynamo electrical currents in Neptune's
subsurface region of gas and melted ice are probably responsible for the magnetic field.
The low density of charged particles may
be due to sweeping by satellites and ring particles:
a process
facilitated by the tilt and offset of the dipole. Auroral activity is present and widespread over the surface of Neptune rather than being concentrated as is more typical of planetary auroras.
This phenomenon is the result of a
corresponding pattern of charged-particle impingement dictated by the complicated magnetic field. Partial ring arcs had been hypothesized to explain a series of earlier Earth,based observations.
What was found were two
main rings with significant clumping of material into three arcs in the outer ring, the planet.
located at 62 thousand km from the center of
The three ring arcs, which represent the optically
thick parts of the outer ring, are most likely responsible for all of the occultation events observed from Earth, with the exception of one.
That one seems to have been caused by a chance
occultation of the test star by a then unknown satellite of Neptune
(1989N2)!
observed.
Two other rings, more difuse, were also
The Neptunian rings contain a larger fraction of small
particles than their Uranian counterparts.
18
W.I. McLaughlin Six new satellites were discovered,
Nereid.
One,
joining Triton and
1989NI, with a diameter of about 400 km,
is larger
than Nereid but was previously undetected due to its proximity to the bright planetary disk. Triton,
like Io at Jupiter and Miranda at Uranus,
the centerpiece for the encounter
(the Saturnian rings fulfilled
that function at the sixth planet).
The terrain of this 2720 km-
diameter satellite is extremely varied with fractures, basins,
some craters,
provided
flooded
a southern polar cap, and features which
represent "icy volcanism".
The surface is principally composed
of water ice with an overlay of nitrogen and methane frosts, as at the polar cap.
such
The overlaid ices are sublimated and
condensed in response to seasonally induced thermal cycles,
while
the water ice serves as the primary geological material of Triton.
At these very cold temperatures
it possesses a
mechanical strength comparable to that of rock on Earth.
With a
density of approximately 2.0 g cm -3, Triton clearly has a rocky component in its interior. At the time of the encounter in August, Soderblom,
geologist Lawrence
a member of the Voyager Imaging Team,
speculated that
some 50 dark streaks on the southern polar cap might be the result of wind-borne deposits of erupting material: volcanism".
"icy
On October 2, it was announced that p o s t - e n c o u n t e r
analysis of the imaging data had indeed detected a geyser in the act of spouting with, the material downwind.
as hypothesized by Soderblom,
conveyance of
The bulk of the ejected material
is
thought to be nitrogen with the dark hue coming from an admixture of organic molecules.
The presence of geysers is surprising,
particularly since so little energy is available to activate the subsurface reservoirs of nitrogen;
at Triton's distance,
the Sun
19
Voyager 2's Encounter with Neptune is only 1/900 of its brightness at Earth. Triton is colder and smaller than had been expected because its albedo,
averaging 70%, is larger than anticipated.
Thus, the
lakes of liquid nitrogen that some had foreseen as possible surface features of the satellite were,
indeed, not possible
since nitrogen cannot exist as a liquid on the surface of this cold body
(the temperature in the lower atmosphere is
approximately 37 ° K.).
The smaller size of Triton posed a
challenge to Voyager's Navigation Team in its
(successful)
quest
to thread the spacecraft's trajectory through the constricted zone which would guarantee a dual occultation of Earth and Sun as seen from the vehicle.
The occultation of the Earth was used in
the investigation of atmospheric properties by refracting radio waves through the thin, for reception at Earth.
10-microbar nitrogen atmosphere of Triton The occultation of the Sun similarly
yielded atmospheric information when observed with the spacecraft's ultraviolet spectrometer. Triton is the coldest body yet observed in the solar system and probably represents our best idea of what Pluto is like until a flyby of that planet can be accomplished. Voyager 1 is now the most distant spacecraft from the Sun and is receding from it, northward of the ecliptic, A.U. per year.
at about 3.5
Voyager 2 is proceeding in a southerly course at
about 3.4 A.U. per year.
These spacecraft will probably continue
to return data from the outer solar system, possible crossing of the heliopause,
including the
for around 25 years, until
their radioactively-driven power sources decay to a level that can no longer support a meaningful set of scientific experiments.
20
W . I . McLaughlin The work described in this paper was carried out by the Jet
Propulsion Laboratory,
California Institute of Technology,
under
contract with the National Aeronautics and Space Administration.
November 3, 1989