Juno Isn’t Exactly Where it’s Supposed To Be. The Flyby Anomaly is Back, But Why Does it Happen?

In the early 1960s, scientists developed the gravity-assist method, where a spacecraft would conduct a flyby of a major body in order to increase its speed. Many notable missions have used this technique, including the Pioneer, Voyager, Galileo, Cassini, and New Horizons missions. In the course of many of these flybys, scientists have noted an anomaly where the increase in the spacecraft’s speed did not accord with orbital models.

This has come to be known as the “flyby anomaly”, which has endured despite decades of study and resisted all previous attempts at explanation. To address this, a team of researchers from the University Institute of Multidisciplinary Mathematics at the Universitat Politecnica de Valencia have developed a new orbital model based on the maneuvers conducted by the Juno probe.

The study, which recently appeared online under the title “A Possible Flyby Anomaly for Juno at Jupiter“, was conducted by Luis Acedo, Pedro Piqueras and Jose A. Morano. Together, they examined the possible causes of the so-called “flyby anomaly” using the perijove orbit of the Juno probe. Based on Juno’s many pole-to-pole orbits, they not only determined that it too experienced an anomaly, but offered a possible explanation for this.

Artist’s impression of the Pioneer 10 probe, launched in 1972 and now making its way out towards the star Aldebaran. Credit: NASA

To break it down, the speed of a spacecraft is determined by measuring the Doppler shift of radio signals from the spacecraft to the antennas on the Deep Space Network (DSN). During the 1970s when the Pioneer 10 and 11 probes were launched, visiting Jupiter and Saturn before heading off towards the edge of the Solar System, these probes both experienced something strange as they passed between 20 to 70 AU (Uranus to the Kuiper Belt) from the Sun.

Basically, the probes were both 386,000 km (240,000 mi) farther from where existing models predicted they would be. This came to be known as the “Pioneer anomaly“, which became common lore within the space physics community. While the Pioneer anomaly was resolved, the same phenomena has occurred many times since then with subsequent missions. As Dr. Acebo told Universe Today via email:

“The “flyby anomaly” is a problem in astrodynamics discovered by a JPL’s team of researchers lead by John Anderson in the early 90s. When they tried to fit the whole trajectory of the Galileo spacecraft as it approached the Earth on December, 8th, 1990, they found that this only can be done by considering that the ingoing and outgoing pieces of the trajectory correspond to asymptotic velocities that differ in 3.92 mm/s from what is expected in theory.

“The effect appears both in the Doppler data and in the ranging data, so it is not a consequence of the measurement technique. Later on, it has also been found in several flybys performed by Galileo again in 1992, the NEAR [Near Earth Asteroid Rendezvous mission] in 1998, Cassini in 1999 or Rosetta and Messenger in 2005. The largest discrepancy was found for the NEAR (around 13 mm/s) and this is attributed to the very close distance of 532 Km to the surface of the Earth at the perigee.”

NASA’s Juno spacecraft launched on August 6, 2011 and should arrive at Jupiter on July 4, 2016. Credit: NASA / JPL

Another mystery is that while in some cases the anomaly was clear, in others it was on the threshold of detectability or simply absent – as was the case with Juno‘s flyby of Earth in October of 2013. The absence of any convincing explanation has led to a number of explanations, ranging from the influence or dark matter and tidal effects to extensions of General Relativity and the existence of new physics.

However, none of these have produced a substantive explanation that could account for flyby anomalies. To address this, Acedo and his colleagues sought to create a model that was optimized for the Juno mission while at perijove – i.e. the point in the probe’s orbit where it is closest to Jupiter’s center. As Acedo explained:

After the arrival of Juno at Jupiter on July, 4th, 2016, we had the idea of developing our independent orbital model to compare with the fitted trajectories that were being calculated by the JPL team at NASA. After all, Juno is performing very close flybys of Jupiter because the altitude over the top clouds (around 4000 km) is a small fraction of the planet’s radius. So, we expected to find the anomaly here.  This would be an interesting addition to our knowledge of this effect because it would prove that it is not only a particular problem with Earth flybys but that it is universal.”

Their model took into account the tidal forces exerted by the Sun and by Jupiter’s larger satellites – Io, Europa, Ganymede and Callisto – and also the contributions of the known zonal harmonics. They also accounted for Jupiter’s multipolar fields, which are the result of the planet oblate shape, since these play a far more important role than tidal forces as Juno reaches perijove.

Illustration of NASA’s Juno spacecraft firing its main engine to slow down and go into orbit around Jupiter. Lockheed Martin built the Juno spacecraft for NASA’s Jet Propulsion Laboratory. Credit: NASA/Lockheed Martin

In the end, they determined that an anomaly could also be present during the Juno flybys of Jupiter. They also noted a significant radial component in this anomaly, one which decayed the farther the probe got from the center of Jupiter. As Acebo explained:

“Our conclusion is that an anomalous acceleration is also acting upon the Juno spacecraft in the vicinity of the perijove (in this case, the asymptotic velocity is not a useful concept because the trajectory is closed). This acceleration is almost one hundred times larger than the typical anomalous accelerations responsible for the anomaly in the case of the Earth flybys. This was already expected in connection with Anderson et al.’s initial intuition that the effect increases with the angular rotational velocity of the planet (a period of 9.8 hours for Jupiter vs the 24 hours of the Earth), the radius of the planet and probably its mass.”

They also determined that this anomaly appears to be dependent on the ratio between the spacecraft’s radial velocity and the speed of light, and that this decreases very fast as the craft’s altitude over Jupiter’s clouds changes. These issues were not predicted by General Relativity, so there is a chance that flyby anomalies are the result of novel gravitational phenomena – or perhaps, a more conventional effect that has been overlooked.

In the end, the model that resulted from their calculations accorded closely with telemetry data provided by the Juno mission, though questions remain. Further research is necessary because the pattern of the anomaly seems very complex and a single orbit (or a sequence of similar orbits as in the case of Juno) cannot map the whole field,” said Acebo. “A dedicated mission is required but financial cuts and limited interest in experimental gravity may prevent us to see this mission in the near future.”

It is a testament to the complexities of physics that even after sixty years of space exploration – and one hundred years since General Relativity was first proposed – that we are still refining our models. Perhaps someday we will find there are no mysteries left to solve, and the Universe will make perfect sense to us. What a terrible day that will be!

Further Reading: Earth and Planetary Astrophysics

Case Closed on the Pioneer Anomaly

Caption: An artist’s view of a Pioneer spacecraft heading into interstellar space. Both Pioneer 10 and 11 are on trajectories that will eventually take them out of our solar system. Image credit: NASA

The case of the Pioneer Anomaly has intrigued and perplexed scientists, engineers and the space-savvy public since 1980, when analysis of tracking data from the twin Pioneer spacecraft showed a small, unexplained slowing of the duo. The answer to this puzzle — now firmly found — lies not in weird physics or mysterious dark matter, but simply the effect of heat pushing back on the spacecraft – heat from the spacecraft itself, emanating from electrical current flowing through instruments and the thermoelectric power supply.

If you’re thinking, “hasn’t this mystery been solved before?” – you’d be right.

Slava Turyshev from the Jet Propulsion Laboratory has laboriously worked on the project since 2004, recovering files from back corners of NASA closets and boxes that were on their way to the trash, converting 1970s punch card data to today’s digital format, and poring over all the data that the spacecraft have beamed back to Earth from billions of miles away.

Along the way, Turyshev has published a couple of papers on his work (here’s one from 2011), and in April of this year, The Planetary Society – who was supporting in part Turyshev’s research – claimed victory that the Pioneer Anomaly was solved.

But now, Turyshev has officially published his findings in the journal Physical Review Letters, and JPL saw fit to put out a press release.

However, over the years other scientists figured out that the culprit might be the heat coming from the spacecraft’s components. In 2001, for example, a scientist named Louis K. Sheffer published a paper, “Conventional Forces can Explain the Anomalous Acceleration of Pioneer 10” and with some good number crunching, determined that “non-isotropic radiation of spacecraft heat” could account for the slowing and “that the entire effect can be explained without the need for new physics.”

Why Sheffer’s paper wasn’t considered more seriously is uncertain, but perhaps at that time the “new physics” idea – that we may have to revise our understanding of gravitational physics — was more intriguing than a mundane effect like heat from the spacecraft’s systems.

But nonetheless, it appears everyone is satisfied with the explanation dutifully resolved by Turyshev and his team of mostly volunteer helpers. And Turyshev’s description of the effect is beautiful in its simplicity:

“The effect is something like when you’re driving a car and the photons from your headlights are pushing you backward,” he said. “It is very subtle.”

Launched in 1972 and 1973 respectively, Pioneer 10 and 11 are still heading on an outward trajectory from our Sun. In the early 1980s, navigators saw a deceleration on the two spacecraft, in the direction back toward the Sun, as the spacecraft were approaching Saturn. They dismissed it as the effect of small amounts of leftover propellant still in the fuel lines. But by 1998, as the spacecraft kept traveling on their journey and were over 13 billion kilometers (8 billion miles) away from the Sun, a group of scientists led by John Anderson of JPL realized there was an actual deceleration of about 300 inches per day squared (0.9 nanometers per second squared). They were the ones who raised the possibility that this could be some new type of physics that contradicted Einstein’s general theory of relativity.

After that, all sorts of theories surfaced, some fairly wacky, some more serious.

In 2004, Turyshev decided to really dig into the matter and started gathering records stored all over the country to analyze the data to see if he could definitively figure out the source of the deceleration. In part, according to JPL, Turyshev and his colleagues were contemplating a deep space physics mission to investigate the anomaly, and he wanted to be sure there was one before asking NASA for a spacecraft.

And so they went searching for Doppler data, telemetry data, and anything they could find about the spacecraft, including picking the brains of navigators who worked with the spacecraft over the years.

They collected more than 43 gigabytes of data, which may not seem like a lot now, but is quite a lot of data for the 1970s. He also managed to save a vintage tape machine that was about to be discarded, so he could play the magnetic tapes. Viktor Toth from Canada, heard about the effort and helped create a program that could read the telemetry tapes and clean up the old data.

They saw that what was happening to Pioneer wasn’t happening to other spacecraft, mostly because of the way the spacecraft were built. For example, the Voyager spacecraft are less sensitive to the effect seen on Pioneer, because its thrusters align it along three axes, whereas the Pioneer spacecraft rely on spinning to stay stable.

Turyshev and his colleagues were able to calculate the heat put out by the electrical subsystems and the decay of plutonium in the Pioneer power sources, which matched the anomalous acceleration seen on both Pioneers.

“The story is finding its conclusion because it turns out that standard physics prevail,” Turyshev said. “While of course it would’ve been exciting to discover a new kind of physics, we did solve a mystery.”

Turyshev’s paper: Finding the Origin of the Pioneer Anomaly.

Source: JPL

Pioneer Anomaly

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Named after the Pioneer 10 and 11 space probes, the Pioneer anomaly refers to the fact that they seem to be moving a teensy bit different from how we think they should be moving (or, more technically, the spacecraft seem to be subject to an unmodeled acceleration whose direction is towards the Sun).

The anomaly was first noticed, by John Anderson, in 1980, when analysis of tracking data from the spacecraft showed a small, unexplained acceleration towards the Sun (this was first published in 1995, with the main paper appearing in 1998). Since then it has been studied continuously, by quite a few scientists.

The Pioneer anomaly is one of the (very few!) true mysteries in contemporary physics, and is a great example of how science is done.

The first step – which Anderson and colleagues took – was to work out where the spacecraft were, and how fast they were traveling (and in what direction), at as many times as they could. Then they estimated the effects of gravity, from all known solar system objects (from the Sun to tiny asteroids and comets). Then they estimated the effects of things like radiation pressure, and possible outgassing. Then … They also checked whether other spacecraft seemed to have experienced a similar anomalous acceleration (the net: not possible to get an unambiguous answer, because all others have known – but unmodelable – effects much bigger than the Pioneer anomaly). Several independent investigations have been conducted, using different approaches, etc.

In the last few years, much effort has gone into trying to find all the raw tracking data (this has been tough, many tapes have been misplaced, for example), and into extracting clean signals from this (also tough … the data were never intended to be analyzed this way, meta-data is sorely lacking, and so on).

And yet, the anomaly remains …

… there’s an unmodeled acceleration of approximately 9 x 10-10 m/s2, towards the Sun.

The Planetary Society has been funding research into the Pioneer anomaly, and has a great summary here! And you can be a fly on the wall at a meeting of a team of scientists investigating the Pioneer anomaly, by checking out this Pioneer Explorer Collaboration webpage.

Universe Today has several stories on the Pioneer anomaly, for example The Pioneer Anomaly: A Deviation from Einstein Gravity?, Is the Kuiper Belt Slowing the Pioneer Spacecraft?, and Ten Mysteries of the Solar System.

Astronomy Cast has two episodes covering the Pioneer anomaly, The End of Our Tour Through the Solar System, and the November 18th, 2008 Questions Show.

Source:
The Planetary Society