It was December 8, 1990, and NASA’s Galileo spacecraft was hurtling toward Earth after more than a year in space. Galileo could not stay long; the mission was on its way to Jupiter and was only stopping by to use the gravity of its home world as a boost to speed it up on its way to the outer solar system.
As the spacecraft swung around our planet, its speed and distance was monitored by mission operators here on the surface. They noticed something odd: Galileo appeared to travel about four millimetres per second faster than its predicted trajectory.
This seemingly small variation was the first instance of an intractable problem known as the “flyby anomaly,” a discrepancy between expected and recorded motion during planetary flybys that remains unexplained to this day.
In the decades since Galileo’s first pass by Earth, the anomaly continued to crop up as other spacecraft performed gravitational assists. The Near Earth Asteroid Rendezvous (NEAR) spacecraft had the biggest change, a speed boost of 13 mm/s logged in 1998, but the Cassini–Huygens, MESSENGER, and Rosetta missions also experienced unexpected shifts in velocity when they conducted Earth flybys.
Sometimes the missions traveled slightly faster than predicted, but when Galileo made its second Earth flyby in 1992, it moved slightly slower. Complicating matters further, the flyby anomaly is not consistently observed, and the recorded speeds of many spacecraft flybys neatly line up with predictions.
Plenty of ink has been spilled to try to account for the isolated occurrences of this odd spaceflight disconnect. Researchers across a vast range of STEM disciplines have speculated that the anomaly could be the result of special effects of general relativity, a halo of mysterious matter around Earth, a new type of force, or simple instrumental error, among numerous other hypotheses.
It’s still an open question whether the cause of the anomaly will ever be resolved. But here’s some ideas about what it might be.
Dark matter, or a glitch?
Stephen Adler, a particle physicist and professor emeritus at the Institute for Advanced Study in Princeton, got thinking about the flyby anomaly more than a decade ago after reading an article about it. Adler’s fascination with the problem happened to coincide with his budding interest in dark matter, an enigmatic, non-luminous substance that makes up most of the mass of the universe.
As a result of these twin interests, Adler wondered whether dark matter in orbit around Earth might be influencing spacecraft that perform flybys, thereby causing the strange anomaly.
“I wrote a number of papers on analyzing whether dark matter could produce the flyby anomaly,” Adler said in a call. “I was interested in the idea that maybe dark matter should give things an energy kick through a certain decay mode.”
In 2013, Adler published a model of Earth surrounded by two shells of dark matter in the International Journal of Modern Physics A. The hypothesis suggested that the anomalous acceleration and deceleration was caused by elastic and inelastic scattering of particles in each respective shell.
Based on this idea, he predicted that NASA’s Juno spacecraft, due for an Earth flyby in October 2013 would show a “large anomaly of 11.6 millimeters/second,” according to the study.
However, when Juno did pass by Earth as part of its voyage to Jupiter, it traveled at the expected speed. Adler decided that the nominal Juno flyby ruled out his hypothesis that dark matter had been influencing the trajectories of interplanetary spacecraft. Now, he suspects that past flyby anomalies may be an artefact, meaning they could be simple instrumental errors.
“I’m not convinced there’s real physics in it at this point,” said Adler of the flyby anomaly, noting that Galileo’s first flyby of Earth had a similar orbit to Juno. “If the effect was real, Juno should have shown it.”
“Maybe something changed, in doing the experiment much later,” he added. “You change your equipment, and if something was going wrong in the previous equipment, you’re not going to know if it’s a somewhat different setup.”
The idea of a dark matter halo messing with the velocities of spacecraft is mind-boggling, so it’s a bit sad that Juno cast doubt on the idea. However, as so often happens in science, pursuing one problem ended up shedding light on another unexplored idea. Adler used his experience studying the flyby anomaly around Earth to estimate the upper limit of dark matter around Earth, which is a useful parameter for future research.
A ‘missing fourth element of force’
Juno’s perfectly normal flyby is puzzling in the context of past anomalies, but many scientists are not ready to attribute the problem to artefacts or errors just yet.
Mario J. Pinheiro, a professor of physics at the University of Lisbon in Portugal, thinks that the anomaly may be the result of an undiscovered “missing link between linear and angular motions“ according to his 2014 paper, published in Physical Letters A.
Linear motion, or momentum, is the result of an object's mass multiplied by its velocity; angular motion is the rotational version of the same concept. Pinheiro predicts that an undiscovered force he calls the topological torsion current, or TTC, can directly convert angular motion to linear momentum in ways that are not predicted by standard physics.
“The TTC points to the existence of a relationship between momentum and angular motion through the agency of a vector potential,” Pinheiro said in an email. “It can be a driving force increasing the rotational energy of a system, such as the orbital-energy change observed on the so-called flyby anomaly.”
Pinheiro was inspired to develop the concept of TTC by his students’ questions about energy and entropy in mechanical systems. He has since published follow-up research that delves into the “antisymmetric behavior” of the hypothetical current.
In the context of the flyby anomaly, this means that the TTC is expressed differently if a spacecraft has a prograde motion, meaning that it is traveling in the same direction as a planet’s rotation, compared to a retrograde motion, in which the spacecraft is moving in the opposite direction of a planet’s spin. Only retrograde approaches should produce the flyby anomaly, which may explain why Juno’s prograde approach did not show any unexpected speed changes.
“It is a very small driving force, with an antisymmetric structure, that depends on the satellite orbital vector,” Pinheiro said. “I do believe it exerts effects on satellites when approaching big astronomical bodies.”
Pinheiro hopes that applying his TTC formula to orbital data from space missions will help to unravel the cause of the flyby anomaly.
Weird extensions of general relativity
If the flyby anomaly is real, scientists would expect to see odd shifts in spacecraft speed around planets other than Earth, which might hint at the discrepancy’s underlying cause. That’s why a team led by Luis Acedo, a professor of mathematics at University of Extremadura in Spain, kept tabs on Juno once it reached Jupiter in 2016.
When Acedo and his colleagues compared their models to the orbital data of Juno at Jupiter, they found that “some unexplained” accelerations had occurred that were “consistent with the flyby anomalies found for the Earth,” according to the team’s 2018 study in Advances in Space Research.
Acedo had previously proposed that a hypothetical gravitomagnetic field may have been the cause of the anomaly in a 2014 paper, though he said that this was “a purely phenomenological proposal” that he does not think it is the correct solution.
“I am currently working on some extensions of general relativity with spacetime torsion that predict similar forces and that could put all this under a solid theoretical framework,” Acedo noted in an email.
In other words, the trippy nuances embedded inside of Einstein’s famous theory could yield insights into the persistent discrepancy. There may even be a way to test out general relativity’s potential link to the flyby anomaly using satellites with eccentric elliptical orbits.
The European Space Agency proposed just such a mission, which was called the Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST), but it was eventually passed over in favor of an exoplanet-hunting space telescope. That said, future missions similar to STE-QUEST may constrain some of the intricacies of general relativity, including its possible role in the flyby anomaly.
Acedo also thinks that the unexpected acceleration of ‘Oumuamua, the first interstellar object ever spotted in the solar system, could be related to the anomaly.
“In 2018, the analysis of the orbit of the interstellar object ‘Oumuamua revealed an excess velocity as it passed close to the Sun,” he explained. “This was attributed to outgassing but no cometary tail was observed. Perhaps here is another case of a flyby anomaly but further study is required.”
Solving the flyby anomaly
Scientists tend to have mixed feelings when their predictions don’t match observed data. The downside is that it means we must be missing something important, which is annoying when you are trying to understand the vagaries of the universe.
That said, gaps between what we expect and what we actually see often lead to major discoveries that reshape our fundamental assumptions.
“I’ve seen a lot of possible discrepancies come and go,” Adler said, adding that a lot of them “have gone away when the experiments or theory have been refined.”
“On the other hand, every now and then, an anomaly persists and turns out to be a harbinger of real new physics,” he continued. “So one has to take them somewhat seriously, and it’s fun looking into them, but one also has to take them with a certain grain of salt.”
It may be that we never find out what causes the anomaly, why it popped up in such different ways, and why it sometimes didn’t occur when scientists expected that it might.
“The flyby anomalies are a purely phenomenological riddle for the moment, so its absence in a particular flyby does not justify the dismissal of the rest of cases in the other flybys,” Acedo said. “If It does not rain today, it does not mean we are living in the desert.”
“If the anomalies are caused by a data analysis error or an overlooked conventional effect they will be solved in the near future,” he predicted, citing the resolution of a similar problem called the Pioneer anomaly. “[I]f they are the consequence of new physics, it would take far more time to reach a conclusion because we will need more quality data and a theoretical framework in which we can understand what is happening.”
Ultimately, the flyby anomaly is a great example of how one unsolved mystery can generate mountains of speculation about the nature of our interactions with our cosmos, with interdisciplinary experts from around the world all weighing in from their unique perspectives.
“Controversy in science is normal and desirable,” Pinheiro said. “Scientists work out mysteries with simple concepts they imagined about the world using mathematics to quantify what is, frequently, unspeakable.”