July 16, 2023

Bakersfield sky July 19 & 20, at 8:30PM looking west

Saturday, July 16, 2023

A couple of days after I turned in my draft of the column for the July 2nd paper, my wife got a message from her aunt about her grandson’s latest astronomy discovery about the origin of the Geminid meteor shower that peaks in mid-December. I wrote about Wolf Cukier’s first discovery when he was a high school student in January 2020. That one was the first “Tatooine” planet (exoplanet orbiting two stars like the home world of Luke Skywalker in Star Wars) discovered in the TESS mission. His most recent discovery is about something inside our solar system and he’s now a third-year undergrad at Princeton. 

Most meteor showers are the result of Earth plowing through the dust trail left behind by a comet as the comet orbits the sun. Most of the time, a comet consists of a frozen dirty iceberg called a comet nucleus about the size of a small town or mountain. The nucleus is made of dust particles and rocks embedded in a matrix of volatile (easily vaporized at low temperatures) materials such as water ice, carbon dioxide ice, carbon monoxide ice, etc. The comet nucleus spews out gas and dust when it gets close to the sun and warms up, vaporizing the volatile material. Over time, the dust spreads out along the comet’s orbit. The dust grains are the size of a grain of sand or smaller. If the Earth passes through the comet's orbit, the dust grains can hit the Earth's atmosphere to make the spectacular displays called meteor showers. After many passages around the Sun, the comet nucleus has no more volatile material left in it and it becomes "dead." Some asteroids are former comets!

All of the major meteor showers of the year can be tied to a comet except the Geminids. They are dust trail left behind by an asteroid called 3200 Phaethon. Phaethon may be a now dead comet nucleus. Well, as they might say in Monty Python, it’s not totally dead as Phaethon seems to still form a very faint tail at its closest approach to the sun. In another twist on the usual meteor shower origin story, Cukier discovered that the Geminids probably formed as the result of a violent collision just a few thousand years ago. 

Cukier’s faculty advisor at Princeton, Jamey Szalay, used data from the Parker Solar Probe to find that a lot of the dust in the Geminids follows a path that is slightly outside Phaethon’s orbit and that gap is more noticeable at perihelion (closest point to the sun). Phaethon has a very elliptical orbit with a perihelion point well inside Mercury’s orbit (the other end of the orbit, aphelion, is well beyond Mars’ orbit). The Parker Solar Probe’s flight path takes it very close to the sun and therefore, it can give us the best direct look at the dust grains close to the sun.

Cukier created computer simulations of possible formation scenarios for the Phaethon dust stream that is responsible for the Geminids. The usual evaporating comet model was the worst fit. The best fit to the data is a violent collision model (“catastrophic formation scenario”) that produced a cloud of debris in a very short time interval. Phaethon is either the parent body that was hit or it is the largest chunk of an even larger parent body that broke apart from the impact.   When Japan’s DESTINY+ spacecraft flies by Phaethon near the end of this decade, we may be able to figure out which of those scenarios is correct. 

Tomorrow the moon will be at new phase, so the following evenings this week will have some beautiful sights of a thin sliver waxing crescent moon low in the west after sunset. On the evening of July 19, the thin sliver will be to the upper right of brilliant Venus. Down and slightly right of the moon, you may be able to spot Mercury, which is now climbing upward from the sun over the next few weeks. The following night, July 20, a slightly fatter crescent moon will be above the midpoint of the line between Mars and Venus. On the evening of next Monday (July 24), an almost first quarter moon will be right next to the brightest star of Virgo, Spica, just a thumb width at arm’s length apart.

Venus is now dropping quickly toward the sun as it moves in between us and the sun in its inner orbit. By the end of July Venus will be lost in the glare of the sun and Mercury will be the evening star. Venus is at “inferior conjunction” between us and the sun on August 13. 

Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com