archiv~1.txt: SETI Pioneer 10 Status Report - March 2, 1999

SETI Pioneer 10 Status Report - March 2, 1999

Larry Klaes ( lklaes@bbn.com )
Tue, 02 Mar 1999 15:34:37 -0500

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

STATUS UPDATED: 2 March 1999

Pioneer 10

(Launched 2 March 1972)

Distance from Sun (1 March 1999): 72.08 AU Speed relative to the Sun: 12.24
km/sec (27,380
mph) Distance from Earth: 10.81 billion kilometers (6.716 billion miles)
Round-trip Light
Time: 20 hours 01 minutes

CONGRATULATIONS PIONEER 10! Happy 27th Birthday. Two years into retirement,
Pioneer 10 is flying high and still calling home.

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 >6.7 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.

[ Pioneer Home Page | Ames Research Center | NASA Home Page ]

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