How the sun’s motion affects meteor showers

Here’s a little spoiler for next month’s Cosmic Calendar: early May will see the return of the Eta Aquarid meteor shower. The shower will be visible for two weeks or so, peaking on the evening of May 5.

In fairness, this doesn’t really constitute a spoiler for veteran stargazers, because the Eta Aquarids pretty much always peak in early May. As far as meteor showers go, this one is as reliable and predictable as they get. Others, however, are less predictable. A paper published April 13 in the journal Icarus sheds some light on why that is. 

Of meteors and comets 

While meteor showers are generally named for the constellation from which they appear to originate, their actual origin has nothing to do with distant stars. They actually come from trails of dust left by relatively small celestial objects–mostly comets, but occasionally asteroids, too. In the case of the Eta Aquarids, the comet in question is Halley’s Comet, which readers of a certain age may have been lucky enough to see when it passed close to Earth in 1986.

At first glance, this may seem surprising. Most comets follow highly elliptical orbits that can take many years to complete, while meteor showers happen around the same time every year. So, how are the two connected?

[ Related: An ‘oddball’ asteroid-comet hybrid dates back 4.6 billion years.]

The answer lies with the process that produces a comet’s signature diaphanous tail. When a comet gets close enough to the sun, it begins to shed material. Frozen compounds on its surface begin to melt, and the solar wind blasts away tiny particles of dust and rock. The result is a trail of dust and debris along the trajectory of the comet’s orbit, like a line of chalk left in space.

In the case of comets linked to meteor showers, the Earth passes close enough to this trail for its gravity to capture some of the lost material. Every year, innumerable tiny chunks of ice and rock are dragged out of the trail and pulled toward the Earth, burning up as they plummet into our atmosphere. We see the resultant shower in the same part of the sky every year because that’s where the interaction happens every year.

The trouble with forecasting showers

In theory, meteor showers should be perfectly predictable. The Earth’s orbit doesn’t really change from year to year, and the comet’s trail is already there, waiting for us. The interaction between the two should, happen at the same time every year, right? 

If the only celestial objects involved in the process were the Earth, the sun, and the comet, this would be true. The comet would follow a perfectly predictable path, shedding material that would remain where it was left until the Earth’s annual arrival to claim its meteoric dues. In reality, things are a lot more complicated. The solar system isn’t just the Earth and the sun–it’s the Earth, the sun, the moon, the other planets and their moons, and any number of other objects. In turn, all of these bodies exert gravitational influences on one another.

Within the midst of this whole mess, there’s the comet itself, wobbling along its elliptical orbit, leaving its forlorn trail of dust and ice in the void. To predict a meteor shower accurately, we need to calculate where the debris is, which means calculating the comet’s trajectory. To do this with perfect accuracy, we’d need to make a calculation that factored in the gravitational effects of a huge number of celestial objects. 

The actual three-body problem

Unfortunately, gravitational calculations involving more than two bodies are a problem. This means that we have to simplify such calculations, modeling them as far less complex systems than they actually are.

Part of the reason that gravitational calculations are so difficult is that no gravitational interaction is one-way–any massive object exerts its own gravitational influence on every other massive object. For example, think of the Earth and the moon. We tend to assume that the latter orbits the former, but that’s not actually true. The Earth pulls at the moon, but the moon also pulls at the Earth, and the result is that both objects orbit a point somewhere between them. This point is known as the barycenter. 

[ Related: Scientists finally solve the mystery of why comets glow green. ]

The solar system also has a barycenter, a point around which everything–including the sun–orbits. In most cases, we assume that the solar system’s barycenter is the sun, for the simple reason that the sun is many orders of magnitude more massive than anything else in the solar system. This means that if we’re calculating an object’s solar orbit, it makes no more sense to factor in the reciprocal gravitational influence that object exerts on the sun than it does to factor in the gravitational influence a coin dropped from a building exerts on the Earth.

The subtle motion of the sun

However, the new study shows that making this assumption in the case of comets presents two subtle problems. The first is that the more distant an object is from the sun, the more its orbit tends to revolve around the solar system’s barycenter. If an object is close enough to the sun, it really does orbit the sun alone. More distant objects, however, orbit the barycenter. 

Most objects generally remain the same distance from the sun. Comets, by contrast, follow highly eccentric orbits that take them way out into the furthest reaches of the solar system. Halley’s Comet, for instance, passes just 54.7 million miles from the sun at its closest approach. Its aphelion–the point at which it is furthest from the sun–is 3.3 billion miles away, somewhere out near Pluto. These eccentric orbits take comets across the transition point–and the Icarus paper shows that the transition causes a sort of jolt to the comet’s trajectory. 

The second problem is that assuming the sun to be the solar system’s centre of gravity also means assuming the sun itself to be stationary. This means that the sun’s own motion can’t be factored into these calculations. This doesn’t really matter in the case of objects in regular orbits, because those orbits simply move with the sun. Again, the picture is different for comets: the sun’s motion provides a gravitational boost to comets as they hurtle past, and this again causes a measurable shift in their trajectory. 

The new research shows that incorporating both these factors into a model of a comet’s orbit provides a more accurate result–and, therefore, a more accurate prediction of when a meteor shower will happen. These are still simplified models, but they provide an insight into when we will next see a cascade of shooting stars in the sky–and also into the intricate dance of gravity that governs our motion through the cosmos.

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