Unintentional detections of Juno spacecraft destroy concepts concerning the origin of the zodiac gentle – watts with that?
If you look up at the night sky just before sunrise or after dark, you may see a faint pillar of light stretching out from the horizon. This glowing glow is the zodiac light, or sunlight, reflected back to earth by a cloud of tiny dust particles orbiting the sun. Astronomers have long thought that the dust is brought into the inner solar system by some of the asteroid and comet families that venture from afar.
This photo shows the zodiac lights as they appeared in Skull Valley, Utah on March 1, 2021. The star cluster of the Pleiades is visible above in the pillar of light. Mars is just below that. Credits: Credits: NASA / Bill Dunford
But now a team of Juno scientists are arguing that Mars could be the culprit. They first published their results online on November 11, 2020 in the Journal of Geophysical Research: Planets.
An instrument on board the Juno spacecraft accidentally discovered dust particles that entered the spacecraft on its journey from Earth to Jupiter. The effects provided important clues as to the origin and orbit development of the dust and resolved some mysterious variations in the zodiac light. Although their discovery has had a major impact, the scientists who studied cosmic debris for years did not set out to do so. “I never thought we were looking for interplanetary dust,” said John Leif Jørgensen, professor at the Technical University of Denmark.
Jørgensen designed the four-star trackers that are part of Juno’s magnetometer research. These on-board cameras take photos of the sky every quarter of a second to determine Juno’s orientation in space by recognizing star patterns in their images – a technical task that is essential to the accuracy of the magnetometer.
But Jørgensen hoped that his cameras could also see an undiscovered asteroid. So he programmed a camera to report things that appeared in several consecutive images but were not in the catalog of known celestial objects.
He didn’t expect much: almost all objects in the sky are listed in the star catalog. When the camera began to project thousands of images of unidentifiable objects – stripes that appeared and then mysteriously disappeared – Jørgensen and his colleagues were amazed. “We looked at the pictures and said, ‘What could this be?'” He said.
Jørgensen and his team looked at many plausible and some implausible causes. There was a disturbing possibility that the star camera had picked up a leaking fuel tank on Juno. “We thought ‘something is really wrong,'” said Jørgensen. “The pictures looked like someone was shaking a dusty tablecloth out the window.”
It was only when the researchers calculated the apparent size and speed of the objects in the images that something finally became clear to them: Grains of dust had hit Juno at a speed of around 16,000 kilometers per hour and chipped pieces of submillimeter. “Even though they are tiny objects, they have a tremendous breakdown,” said Jack Connerney, Juno’s chief investigator for magnetometers and the mission’s assistant chief investigator, stationed at NASA’s Goddard Space Flight Center in Greenbelt. Maryland.
As it turns out, the spray was coming from Juno’s expansive solar panels – the largest and most sensitive accidental dust detector ever built.
“Every piece of debris we tracked records the effects of an interplanetary dust particle so we can compose a dust distribution along Juno’s path,” Connerney said. Juno launched in 2011. After a space maneuver in the asteroid belt in 2012, it returned to the inner solar system in 2013 for Earth gravity support that catapulted the spaceship towards Jupiter.
This visualization shows NASA’s Juno spacecraft on its five-year flight path to Jupiter, beginning with its launch from NASA’s Kennedy Space Center in Florida in 2011. Credits: NASA’s Goddard Space Flight Center; Trajectory animation courtesy of NASA / JPL-Caltech / SwRI
Connerney and Jørgensen found that most of the dust impacts were recorded between the Earth and the asteroid belt, with gaps in distribution being due to the influence of Jupiter’s gravity. According to the scientists, this was a radical revelation. So far it has not been possible for scientists to measure the distribution of these dust particles in space. Special dust detectors had limited collection areas and thus limited sensitivity to a sparse dust population. They mostly count the more frequent and much smaller dust particles from interstellar space. In comparison, Juno’s expansive solar panels have a collecting area 1000 times larger than most dust detectors.
Juno scientists found that the dust cloud ends on Earth because Earth’s gravity soaks up all of the dust that approaches it. “This is the dust that we see as the zodiac light,” said Jørgensen.
The outer edge, about 2 astronomical units (AU) from the sun (1 AU is the distance between the earth and the sun), ends directly behind Mars. At this point, the scientists report, the influence of Jupiter’s gravity acts as a barrier, preventing dust particles from entering space from the inner solar system. The same phenomenon known as orbital resonance also works the other way, where it prevents dust from entering the inner solar system from space.
The profound influence of the gravity barrier suggests that the dust particles are in a near-circular orbit around the sun, said Jørgensen. “And the only object we know of in an almost circular orbit around 2 AU is Mars. So the natural thought is that Mars is a source of this dust,” he said.
A team of Juno scientists argues that Mars may be responsible for the interplanetary dust behind the zodiac light, a faint pillar of light that extends from the horizon. An instrument on board the Juno spacecraft accidentally discovered dust particles that entered the spacecraft on its journey from Earth to Jupiter. The effects provided important clues as to the origin and orbit development of the dust. Credits: NASA’s Goddard Space Flight Center Download videos and animations here
“The distribution of dust, which we measure better, is consistent with the observed variation in zodiac light,” said Connerney. The researchers developed a computer model to predict the light reflected from the dust cloud, which is scattered by gravitational interaction with Jupiter and scatters the dust into a thicker disk. The scattering depends on only two quantities: the tendency to dust towards the ecliptic and its eccentricity of the orbit. When the researchers plugged in the orbital elements of Mars, the distribution accurately predicted the tell-tale signature of the variation in zodiac light near the ecliptic. “In my opinion, this is confirmation that we know exactly how these particles orbit our solar system,” said Connerney, “and where they come from.”
While there is now good evidence that Mars, the dustiest planet we know, is the source of zodiac light, Jørgensen and his colleagues are still unable to explain how the dust could have escaped the grip of Martian gravity. They hope other scientists will help them.
In the meantime, researchers find that determining the actual distribution and density of dust particles in the solar system will help engineers develop materials for spacecraft that are better able to withstand dust bumps. Knowing the exact distribution of the dust can also guide the design of flight paths for future spacecraft to avoid the highest concentration of particles. Tiny particles moving at such high speeds can stick out of a spaceship 1000 times their mass.
The solar systems from Juno were not damaged, as the solar cells are well protected against impacts on the back or the dark side of the system by the supporting structure.
Banner image: An image shows NASA’s Juno spacecraft as it entered Jupiter’s orbit on July 4, 2016, after traveling nearly five years and more than 1.7 billion miles. Credits: NASA / JPL / SwRI. Download picture here.
By Lonnie Shekhtman
NASA Goddard Space Flight Center, Greenbelt, Md.