Timing is extremely important in many aspects of astronomy. If an astronomer or his instrument is looking the wrong way at the wrong time, he could be missing out on something spectacular. Alternatively, there are moments when our instruments capture something unexpected in regions of space where we were looking for something else. This is exactly what happened recently when a team of scientists led by Rohini Giles of the Southwest Research Institute saw a picture of a meteor likely to affect Jupiter’s atmosphere.
The team is collecting data from the UVS, one of the instruments on Juno, NASA’s mission to examine the largest planet in the solar system at close range. UVS is Juno’s ultraviolet spectrograph that collects data in the ultraviolet spectra from 68 to 210 nm. Your main job is to study Jupiter’s atmosphere and watch out for its stunning aurors.
Jupiter has a spectacular aurora, like this view captured by the Hubble Space Telescope. Aurors are created when charged particles in space surrounding the planet are accelerated to high energies along the planet’s magnetic field.
Photo credit: NASA, ESA, and J. Nichols (University of Leicester)
Recently, one of Dr. Giles saw a large increase in brightness in an area outside of the normal auroral zone while reviewing a series of images received from the sensor. As with many other scientific discoveries, this discovery began with someone finding interesting data that they had not expected.
The team’s first thought was to eliminate other sources that might have been causing the spike. First, they eliminated the aurora they were looking for as part of their normal research. This area of the planet on which the tip appeared was outside the normal limits of the Aurors they were studying.
Illustration of what a TLE (Sprite or Elf) would possibly look like on Jupiter.
Photo credit: NASA / JPL-Caltech / SwRI
Next, they wanted to understand whether it was possibly a “transient light event” (TLE) that had previously appeared in their data. These TLEs, commonly known by the quirky names “elves” or “sprites,” are believed to be lightning bolts in Jupiter’s upper atmosphere. While seen in the same general area of the event, TLE’s Aurors are very similar in terms of their spectral profile, and one has never been seen anywhere near the size or extent of the event that UVS captured this time.
One final check required an understanding of whether the data was an artifact from the instrumentation itself. However, there were so many photons clumped together in a given spatial area that it was highly unlikely that it was an artifact. If the signal was indeed caused by an instrumentation error, it is much more likely that it is accidental than that it is spatially as concentrated as it was.
Example of a type of instrument artifact when observing Sirius A.
Photo credit: NASA / ESA / H-Anleihe (STSci) / M Barstow (University of Leicester)
Through this process of elimination and Occam’s razor, the team appears to have encountered a sighting of a meteor that struck Jupiter’s atmosphere. This is not the first time astronomers have noticed such an event – the best known is Comet Shoemaker-Levy 9, which struck Jupiter in 1994. However, this is Juno’s first discovery to be in orbit around the planet since 2016.
One advantage of Juno over previous observational efforts is that it can spot much smaller impactors because of its proximity. The scientists estimate that the observed object weighed between 250 and 5,000 kg. They also estimate that there are approximately 24,000 similar sized impacts on Jupiter each year.
Scott Manley’s description of how Juno’s camera works.
Photo credit: Scott Manley YouTube Channel
24,000 impacts seem like a lot if Juno has been in orbit for nearly four and a half years and only found one. During all this time in orbit, however, the observation time in each individual area of the planet is shorter than you might think. Orbital mechanics and spacecraft stabilization techniques have a huge impact on the time that UVS can collect data.
Juno is in an elliptical orbit around Jupiter and only passes the planet once every 53 days at its nearest point (known as the “Perijove”). During each perijove, the UVS can only collect data for about 10 hours. To make things even more complicated, radiation wrecks destroy the sensor. So if the spaceship happens to pass through a particularly high area of radiation, it cannot collect any useful data.
UT video about the idea of how Jupiter could protect us from even more meteor strikes.
But that’s not all – Juno itself is actually spinning to stabilize the spacecraft’s orbit. It rotates approximately every thirty seconds, and since the UVS is on one side of the spacecraft, it can only collect data for approximately 7 seconds each time the spacecraft rotates when Juno is the closest point of approach.
All of this rotation, orbital, and radiation navigation results in very little coverage during the 4 year mission. With this small period of observation, the spaceship still managed to capture this spectacular image of a re-entry. And with a simple statistic, the team calculated that if Juno or another spaceship or telescope happens to be looking in the right direction, thousands more can be seen each year.
Image from Junos UVS showing the impactor indicated and where it appears on the planet.
Photo credit: Giles et al
The detection of another such event would give the theory that what was seen this time was actually a “Bolide” (the technical name for these impactors). Additionally, the team could better calculate the total number of proposed impacts and therefore make a rough estimate of the total amount of material added to Jupiter’s mass each year.
No matter how many random impacts it captures, the UVS will continue to search for the aurora and provide great data on this spectacular light show. If there is another influence as well, it will be another great example of random timing that goes into great science.
arXiv: Detection of Bolid in Jupiter’s atmosphere with Juno UVS
UT: Asteroids hit Jupiter more often than astronomers thought
UT: Solar storms ignite aurora on Jupiter