Why some meteor showers are so unpredictable

Why do comets and their meteoroid streams weave in and out of Earth’s orbit and their orbits disperse over time? In a paper published in the journal Icarus, two SETI Institute researchers show that this is not due to the random pull of the planets, but rather the kick they receive from a moving sun.

“Contrary to popular conception, everything in the solar system does not orbit the sun,” said lead author and SETI Institute scientist Stuart Pilorz. “Rather, the sun and planets all orbit their common center of mass, known to scientists as the solar system barycenter.”

The solar system barycenter is the proverbial point where the Greek god Atlas would keep his finger to balance the mass of the sun and planets. All planets circle this barycenter, but so does the sun.

“Usually when we build our numerical models,” said Pilorz, “we put the sun at the center out of convenience because it’s the most massive body in the solar system, and it simplifies the relativistic equations.”

The team found that this perspective may not be the best way to understand the physical processes underlying the orbital evolution of long-period comets. These move in orbits that take longer than 200 years to circle the sun.

“Long-period comets spend most of their lives so far away from the solar system that they feel the tug from the barycenter,” said Pilorz. “But every few hundred years they swoop inside Jupiter’s orbit and come under the sun’s influence.”

Close to the sun, the comets shed particles called meteoroids. These meteoroids follow along with the comet, but some travel a shorter orbit and return early, others late, creating a meteoroid stream. When they first form, these streams are extremely thin and the chances of hitting Earth are low.

“Back in 1995, our field was in its infancy and many thought that predicting when these streams would cause a meteor shower on Earth was as hard as predicting the weather,” said meteor astronomer and co-author Peter Jenniskens of the SETI Institute and NASA Ames Research Center.

Jenniskens noticed that the streams were weaving in and out of Earth’s orbit following the sun’s wobble around the solar system barycenter. He predicted that the shower would return when Jupiter and Saturn were back at certain positions along their orbit.

“We traveled to Spain in an attempt to record one of these showers and saw what was described in the past as ‘stars fall at midnight,'” said Jenniskens. “The whole shower lasted only 40 minutes, but there was a bright meteor every minute at the peak.”

That prediction had been based on how the sun’s wobble mostly mirrors the motion of the two most massive planets, Jupiter and Saturn, in their orbit around it. The wobble is small, barely outside the sun itself, but enough to move the position of the sun and its velocity over periods of 12 years (Jupiter’s orbit) and 30 years (Saturn’s orbit), roughly causing a 60-year pattern.

“We were previously able to show in computer models that these streams do wander in and out of Earth’s path and do follow the sun’s wobble,” said Jenniskens, “but we didn’t know why.”

In this newly published study, Jenniskens teamed up with Pilorz to investigate how the meteoroid streams of long-period comets disperse over time to learn how best to use that trail of crumbs to search for their parent comets.

“A principal result of this study,” said Pilorz, “was merely noticing that if we keep track of the fact that the sun is in motion about the barycenter, we see that most of what actually causes the comets and meteoroids to disperse is that they each pick up a gravitational boost or braking from the moving sun as they pass close to it—exactly in the same way that we use planetary encounters to speed or slow down spacecraft.”

The phenomenon of gravitational boost or braking is often compared to bouncing a tennis ball off the front or back of a moving train.

“But the train has to be moving for it to work,” Pilorz noted. “In our case, if we consider the sun fixed at the center, we don’t see that this is all that’s happening.”

The researchers noticed that inside the orbit of Jupiter, the meteoroid changed from moving around the barycenter to moving around the sun’s center.

“We found that the two jumps in the plane of motion, when the sun takes control as the comet approaches and then again when it hands control back to the barycenter as the comet heads away, kicked the inclination and node of the orbit by a small amount,” said Pilorz. “Again, if we consider the sun fixed at the center, the reason for this change is not obvious.”

Meteoroids at different locations in the stream encounter the sun at different times, so they get different kicks over time and the stream weaves and disperses. The randomness is primarily due to the sun’s place and velocity in its orbit around the barycenter when each meteoroid encounters it.

“This is where one’s point of view can be important,” added Pilorz. “We’re used to telling ourselves that a comet’s motion changes randomly due to a series of complex perturbations from the planets. That isn’t wrong, but if we recall that the sun also orbits the barycenter, the explanation becomes much simpler.”

To be fair, the planets determine the sun’s motion equally as much as it determines theirs. However, to know how quickly long-period comet streams tend to disperse, the details of this dance are not needed.

“It’s still necessary to account for the planetary forces to provide a systematic torque that causes precession,” said Pilorz. “This happens mostly when the meteoroids are between the orbits of Jupiter and Saturn.”

From the measured shower dispersions, the team calculated the ages of over 200 long-period comet meteoroid streams, which were published in Jenniskens’ most recent book “Atlas of Earth’s Meteor Showers.”