SETI Pioneer 10 Status Report - July 6, 1999


Larry Klaes (lklaes@bbn.com)
Wed, 07 Jul 1999 09:32:50 -0400


http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNStat.html

STATUS UPDATED: 6 July 1999

                        Pioneer 10

(Launched 2 March 1972)

Distance from Sun (1 July 1999): 72.94 AU Speed relative to
the Sun: 12.24 km/sec (27,380 mph)
Distance from Earth: 11.06 billion kilometers (6.870 billion miles)
Round-trip Light Time: 20 hours 28 minutes

The reduced tracking continues to provide good support,
and the RTG power is still sufficient for operations
and limited science. The bus voltage is around 27.5 volts
(nominal value = 28 volts). Tracking is on the 70
meter antennas only. The signal level is between -177 to
-179 dbm, with SNR from 0.8 to 1.5 dB. The telemetry
quality is still good with the deletion rate running from
0 to 50% (small elevation angles). It looks like it will be
possible to operate Pioneer 10 into September.

The mission formally ended on 31 March 1997 when funding
ended in favor of more scientifically productive Heliospheric missions.
However, a waiver was given to operate Pioneer 10
as part of the Lunar Prospector controller training program
as long as other NASA missions were not interfered with.

Pioneer 10 has continued at a much reduced activity level
under those guidelines. We are deeply grateful for the
gracious way that the Lunar Prospector staff and the DSN have
managed this extra burden on their time.

The spacecraft is at a distance of >6.8 Billion miles (>72
AU's) and is the farthest out in the opposite direction to
which the Sun moves. Voyager 1 passed Pioneer 10 in mileage
out of the Solar System on 17 February 1998 but is travelling
in the opposite direction.

The low-power Geiger-Tube-Telescope (GTT) instrument still
yields valuable scientific data. We also receive data from
the Charged Particle Instrument but only for a few hours
each week to conserve battery power on Pioneer 10.

POSSIBLE HELIOPAUSE DATA

Continuing GTT data from Pioneer 10 during the first part
of 1999 will be of special importance in determining
whether or not Pioneer 10 is still interior the heliopause.

Neutron monitors on Earth (e.g., at Climax and Goose Bay)
recorded a marked and rapid decrease in cosmic ray intensity
of about 4 % during April and early May of 1998. If Pioneer
10 is still inside the heliopause, we can expect a decrease
in cosmic-ray intensity at Pioneer 10 to occur during early
1999. The approximate 9 month delay from Earth to Pioneer 10
corresponds to the distance of ~72 AU covered by the solar wind assuming a
speed of 450 km/s. If Pioneer10 has passed outside
the heliopause into interstellar space, then the decrease
in cosmic intensity will not be observed at Pioneer 10.

The battery reading is very low - perhaps at a minimum.
Pioneer 10 persists longer than ever conceived or
expected. Stay tuned!

Pioneer 10 will continue into interstellar space, heading
generally for the red star Aldebaran, which forms the eye
of Taurus (The Bull). Aldebaran is about 68 light years
away and it will take Pioneer over 2 million years to
reach it.

SUNWARD PULL!?(See the December 1998 issue of Scientific American)

A team of planetary scientists and physicists led by
John Anderson (Pioneer 10 Principal Investigator for
Celestial Mechanics) has identified a tiny unexplained
acceleration towards the sun in the motion of the
Pioneer 10, Pioneer 11 and Ulysses spacecraft. The
anomalous acceleration - about 10 billion times smaller
than the acceleration we feel from Earth's gravitational
pull - was identified after detailed analyses of radio data
from the spacecraft. A variety of possible causes were
considered including: perturbations from the gravitational
attraction of planets and smaller bodies in the solar system; radiation
pressure, the tiny transfer of momentum when
photons impact the spacecraft; general relativity;
interactions between the solar wind and the spacecraft;
possible corruption to the radio Doppler data; wobbles
and other changes in Earth's rotation; outgassing or
thermal radiation from the spacecraft; and the possible
influence of non-ordinary or dark matter. After exhausting
the list of explanations deemed most plausible, the
researchers examined possible modification to the force of
gravity as explained by Newton's law with the sun being
the dominant gravitational force. "Clearly, more analysis,
observation, and theoretical work are called for," the
researchers concluded. The scientists expect the explanation
when found will involve conventional physics.

Pioneer 11

(Launched 5 April 1973)

The Mission of Pioneer 11 has ended. Its RTG power source is exhausted.

The last communication from Pioneer 11 was received in
November 1995, shortly before the Earth's motion
carried it out of view of the spacecraft antenna.

The spacecraft is headed toward the constellation of Aquila
(The Eagle), Northwest of the constellation of
Sagittarius. Pioneer 11 may pass near one of the stars in the constellation
in about 4 million years.

Frequently-asked Questions (FAQ's):

Question:
Who were the Pioneer Project managers?
Answer:
Charles Hall, the original Project Manager from 1962 to 1980, is the
manager most responsible for the
immense success of Pioneer. He was Project Manager from conception through
successful implementation of
the primary missions of Pioneer 6 through 13. Charlie retired after the
encounter of Pioneer 11 with Saturn in
1979. Richard Fimmel succeeded him to successfully manage Pioneer 6 through
12 through their extended
missions. Fred Wirth was the third Project Manager and is responsible for
the design of this Web document.
Larry Lasher, the present Project Manager, organized the 25th anniversary
celebration of Pioneer 10 and
presided on the retirement of the Pioneer Mission program. He instituted
the training program that allows us to
continue following Pioneer 10. Larry is the primary point of contact for
information about the Pioneer Missions.

Question:
How far will Pioneer travel and on what path?
Answer:
Pioneer 10 will be in galactic orbit for billions of years. It is moving in
a straight line away from the Sun at a
constant velocity of about 12 km/sec. Until Pioneer 10 reaches a distance
of about 1.5 parsec (309,000 AUs) -
about 126,000 years from now - it will be dominated by the gravitational
field of the Sun. After that Pioneer 10
will be on an orbital path in the Milky Way galaxy influenced by the field
of the stars that it passes.

Question:
Why does the RTG power decrease?
Answer:
Power for the Pioneer 10 is generated by the Radioisotope Thermoelectric
Generators (RTG's). Heat from the
decay of the plutonium 238 isotope is converted by thermoelectric couples
into electrical current. The electrical
output depends on the hot junction temperature, the thermal path to the
radiator fins, and the cold junction
temperature. It is the degradation of the thermoelectric junction that has
the major effect in decreasing the
power output of the RTG. In the 26-year time scale operation of Pioneer 10,
the 92 year half-life of the isotope
does not appreciably affect the RTG operation. The nuclear decay heat will
keep the hot junction temperature
hot for many years but unfortunately will not be able to be converted into
enough electricity to power the
transmitter for much longer.

Question:
How much has Pioneer been eroded?
Answer:
All the wear, pitting, and erosion that Pioneer 10 has sustained are
probably over now. The asteroid belt and
the severe conditions of Jupiter have already been experienced. Now,
Pioneer is in the vacuum of space
where the average spatial density of molecules is one trillionth the
density of the best vacuum we can draw on
Earth. We expect Pioneer to last an indeterminate period of time, probably
outlasting its home planet, the
Earth. In 5 billion years, the Sun will become a red giant, expand, envelop
the orbit of the Earth, and consume it.
Pioneer will still be out there in interstellar space. Erosional processes
in the interstellar environment are
largely unknown, but are very likely less efficient than erosion within the
solar system, where a characteristic
erosion rate, due largely to micrometeoritic pitting, is of the order of 1
Angstrom/yr. Thus a plate etched to a
depth ~ 0.01 cm should survive recognizable at least to as distance ~ 10
parsecs, and most probably to 100
parsecs. Accordingly, Pioneer 10 and any etched metal message aboard it are
likely to survive for much longer
periods than any of the works of Man on Earth.

Question:
What about Pioneer 1 to 5?
Answer:
Pioneers 1 through 5 were launched from 1958 through 1960 and made the
first thrusts into space toward the
Moon and into interplanetary orbit. Pioneer 1 was the first spacecraft
launched by NASA and provided data on
the extent of the Earth's radiation belts. Pioneer 2 suffered a launch
vehicle failure. Pioneer 3 discovered a
second radiation belt around Earth. Pioneer 4 was the first American
spacecraft to escape Earth's
gravitational pull as it passed within 58,983 km (36,650 miles) of the
moon. The spacecraft did return data on
the Moon radiation environment, although the desire to be the first
man-made vehicle to fly past the moon was
lost when the Soviet Union's Luna 1 passed by the Moon several weeks before
Pioneer 4. Pioneer 5 was
designed to provide the first map of the interplanetary magnetic field. The
vehicle functioned for a record 106
days and communicated with Earth from a record distance of 36.2 million km
(22.5 million miles). The early
Pioneers were exploratory missions that led to intriguing new questions
that required more advanced types of
spacecraft capable of exploring space to considerable distances within and
beyond Earth's orbit. This led to
the Pioneer 6 through 9 series that made the first detailed comprehensive
measurements of the solar wind,
solar magnetic field, and cosmic rays.

Question:
Why and how is Pioneer 10 being maneuvered?
Answer:
The Pioneer spacecraft is spin-stabilized, spinning at approximately 4.28
rpm (Revolutions Per Minute), with
the spin axis running through the center of the dish antenna. If a person
were to sit in the spacecraft, looking
through a hole in center of the dish antenna with a telescope, he would see
the Sun traveling very slowly to the
left. The Earth's path would describe a very narrow ellipse (the orbit is
seen nearly edge-on) around the Sun. In
July the Earth is near the right hand edge of the ellipse, and 6 months
later will be near the left hand edge of the
ellipse. The angle to the spacecraft between the left edge of the ellipse
and the right edge is less than 2
degrees. In order to communicate with the spacecraft, the Earth has to be
within 0.8 degrees of the boresight
of the spacecraft antenna. Since the Earth moves by almost 2 degrees, the
spacecraft has to be re-aimed at
the Earth about twice a year. This is done by a "CONSCAN (conical scan)
precession maneuver" executed by
the spacecraft.

The radio signal transmitted from an antenna on Earth is focused and
reflected by the spacecraft dish antenna
toward a small feed horn located on a tripod which is centered in front of
the spacecraft dish antenna, and then
conducted to a receiver in the spacecraft. During a CONSCAN maneuver, the
feed horn is physically moved by
8 inches to one side. A ground command turns on a heater in a bellows
filled with liquid Freon. The Freon boils,
the bellows expands, and moves a mechanical piston and cam attached to the
feed horn mounting plate
against a mechanical stop. A micro switch cycles the heater power on and
off to keep the feed in the offset
position.

With the feed in the offset position, the radio signal from the tracking
station is seen by the spacecraft receiver
as varying sinusoidally in amplitude (amplitude modulated). This error
signal contains amplitude and phase
information on the pointing angle between the spacecraft spin axis and the
Earth and the direction to the Earth
during the spin cycle. The minimum amplitude occurs during the spin cycle
when the antenna points to the
Earth, whereas the maximum occurs when the antenna dish points away from
the Earth. The frequency of the
modulation is equal to the spacecraft spin rate (4.28 rpm). The error
signal is processed on board the
spacecraft to calculate the timing requirements for firing the jets at the
appropriate instant in the spin cycle to
precess the spin axis towards the Earth.

The CONSCAN processor averages the modulation over two revolutions of the
spacecraft. On the third
revolution, it orders two hydrazine thrusters (mounted 180 degrees apart on
the rim of the dish antenna) to fire
a short pulse of 0.0312 seconds duration. This moves the spacecraft spin
axis a tiny amount toward the
minimum amplitude value, i.e., the Earth, reducing the amplitude of the
modulation by a small amount. This
process is repeated each three revolutions, each time reducing the pointing
angle error and the modulation
amplitude. When the pointing angle is within 0.3 degrees of boresight, the
processor terminates the maneuver
automatically. Typically, about 20 to 28 pulses are fired. A ground command
then executes to turn off the power
to the feed offset heater, the gaseous Freon recondenses to pull the
mechanism back to the normal centered
position, and the maneuver is completed.

Question:
If the spacecraft are leaving the Solar System, why does the distance from
Earth sometimes get shorter?
Answer:
It is a matter of a hyperbolic escape trajectory, geometry, and relative
velocity vectors. The distance from the
Sun is always increasing. However, since the Earth travels around the Sun
faster than the spacecraft moves
away from the Sun, the spacecraft-earth distance decreases for a few
months, and then rapidly increases
again.
                                                                     

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Project Manager: Dr. Lawrence Lasher (e-mail:llasher@mail.arc.nasa.gov)
Webmaster: Beverly Akins



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