NEW YORK
– It’s been years since NASA last heard from either of its two
Pioneer probes hurtling out of the solar system, but scientists are still
debating the source of an odd force pushing against the outbound spacecraft.
Dubbed the Pioneer
Anomaly, the unexplained force appears to be acting against NASA’s
identical Pioneer 10 and 11 probes, holding them back as they head away from
the Sun.
Whether that force stems from the probes themselves, something exotic
like dark
matter, or some new facet of physics or gravity,
remains in doubt.
But a
wealth of newly recovered data and telemetry, spanning decades of observations
by both Pioneer 10 and 11, may yield the final answer to whether conventional
physics or perhaps something new is at work on the two spacecraft. An answer could
arise from the new data after about a year of analysis by an international team
of researchers.
“I
would like to see this story reach its finality,” said Slava Turyshev, an astrophysicist
with NASA’s Jet Propulsion Laboratory (JPL) who has spent the last 14
years—some of it on his own time—studying the Pioneer Anomaly.
"So if it’s conventional physics, that’s fine and we can all
go about our daily business. But if it’s something else, there may be
another page.”
He and
other fellow devotees discussed the astrophysics oddity late Monday during the
Seventh Annual Asimov Debate here at the American Museum of Natural History.
Turyshev’s international team includes researchers from all Pioneer Anomaly camps,
with some learning towards a conventional physics explanation while others
trend toward the unknown fringe. Still other researchers have their own
opinions.
“If I
were a betting man, which I am not, I would bet a whole case of cranberry juice
that the Pioneer Anomaly will have an ordinary explanation that is within known
physics,” said Irwin Shapiro, an astrophysicist at Harvard University
unaffiliated with the Pioneer Anomaly research team, during the debate.
Shapiro
said that the number of actual instances in which
oddities like the Pioneer Anomaly have opened pathways to fundamentally new
physics are rare, and that ongoing studies may yet yield a conventional
explanation.
Perplexing
push
Launched in
1972 and 1973, Pioneer 10 and 11 are both billions of miles from Earth as they
zoom out of the solar system in opposite directions.
As of Feb.
6, Pioneer 10 was about 92.12 astronomical units (AU) from the Sun and headed
towards the constellation Taurus. One AU is the distance between the Earth and Sun, or about 93 million miles (150
million kilometers).
Researchers
first noticed the Pioneer Anomaly as a navigation discrepancy while bouncing
microwaves off each Pioneer probe as they moved farther from Earth. They found
an unexpected drift in each probe’s Doppler frequency, one so small that
the three-axis stabilized probes like NASA’s Voyager
spacecraft—also headed out of the solar system—may have drowned it
out with their in-flight activities.
The Doppler
effect is the shortening or lengthening of waves, such
as the pitch change of an ambulance as it approaches, races past, then heads
away from you.
“We
had a fitting model and it had all the effects in it that would influence the
spacecraft out in interstellar space, except that it didn’t work,”
said John Anderson, a retired JPL researcher who first discovered the Pioneer
Anomaly. “And all we had to do to make it work was to add a constant
acceleration towards the Sun.”
The
discrepancy found that Pioneer 10 and 11 were each about 240,000 miles (400,000
kilometers) closer to the Sun than they should be according to the current
understanding of gravity. Isaac Newton
described gravity as a force that weakens with distance, and the Pioneer probes
are speeding out of the solar system at about 30,000 miles (48,280 kilometers)
per hour.
“The
Pioneer spacecraft conducted the largest ever gravitational experiment that
humanity attempted to test Newton’s Law, and it failed,” Turyshev told SPACE.com.
“If we will identify an anomaly due to conventional physics thermal
mechanism or propulsion or a combination there off, that’s a major event.”
Finding a
physical source will not only prove Newton right, but also allow engineers to
cancel out the Pioneer Anomaly on future spacecraft to make them more stable,
added Turyshev, who said that he is striving to
remain unbiased to the anomaly’s cause.
Researchers
want to determine whether heat from Pioneer probes’ electronics or two
nuclear power sources—known as radioisotope thermal generators (RTGs)—could be emitting infrared photons that then
smack into the spacecraft’s dish-like main antenna, causing a recoil
effect that Turyshev likened to sunlight striking a
solar sail.
Analysis
and modeling of how the Pioneer 10 spacecraft emits heat from various sources,
including its RTG, found that they account for between 55 percent and 75
percent of Pioneer Anomaly, said Gary Kinsella, a
group supervisor for spacecraft thermal engineering and flight operations at
JPL.
“We’re
really encouraged by the preliminary results and we think we’re going
down the right track,” Kinsella said during the
Monday discussion.
Edward Belbruno, a former JPL researcher and gravitational trajectory expert at Princeton
University who also served the panel but is unconnected with the anomaly research, said
that another possible explanation for the Pioneer Anomaly rests in the mass of the Milky Way galaxy, which – when taken to account –
yielded the exact acceleration change for Pioneer 10 as that observed. While he
found that the technique did not yield a specific direction for the
acceleration, it may shed some insight into the anomaly and warrants further
study, Belbruno added.
Recovered
data
During
their first Pioneer
Anomaly analysis, researchers relied on data that spanned about 11.5 years
of Pioneer 10’s mission, though they only had about four years worth for
Pioneer 11.
After an
exhaustive search sponsored by the Planetary Society, Turyshev
and his team recovered complete telemetry data sets for both Pioneer probes, as
well as about 30 years of data for Pioneer 10 and a 20-year set for Pioneer 11.
Much of the
data sat inert, recorded on about 400 magnetic tapes in deep storage at JPL.
Altogether, it included almost 40 Gigabytes of Pioneer 10 and 11 mission data,
or about the equivalent of a half hour of high-definition television (HDTV)
programming from your local cable TV provider.
Transferring
the data from 9-track magnetic tapes to a modern digital format, and screening
it to reduce artifacts and other corrupted material, has proven time-consuming
for Pioneer Anomaly researchers. But Turyshev remains
confident that once the information is ready for analysis, the anomaly shed new
secrets.
He is also
keeping a close watch on NASA’s New
Horizons probe, which may one day show signs of the anomaly as it heads out
beyond Pluto’s orbit after 2015, but only if the mystery force is found
to be an actual effect.
“We
are truly in a unique situation now with the recovery of the new data assets,”
Turyshev said. “Once this data set is analyzed,
let’s talk then.”
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