From the April 2019 issue

What’s that odd meteor?

These aren’t your ordinary shooting stars.
By | Published: April 2, 2019
On the evening of July 17, 2018, something fortuitous happened. I was out taking long exposures of the Milky Way and Mars near opposition when a large and diffuse meteorlike object passed between them. It wasn’t the first time I’ve seen one of these ghostly streaks, which, like sprites, can make you shake your head in wonder. But this time I nailed an image of one — though, admittedly, it almost escaped my notice.

Weird and wonderful phantasms

Nebulous, or diffuse, meteors have caused a stir since at least the turn of the 20th century. That’s when French astronomer Fernand Baldet published in a 1909 Bulletin of the Société Astronomique de France his systematic study of what he called a pretty remarkable phenomenon — “that of nebulous meteors.” He first noticed one the night of March 18, 1903, from Paris, when a “slightly luminous nebulosity, similar to a shred of Laced Way” appeared suddenly above Lambda Orionis and proceeded toward Cassiopeia.

The author almost failed to notice the dim, diffuse streak of a nebulous meteor in one of his long-exposure images of Mars (the bright “star” at left) and the Milky Way. It lies between the planet and our galaxy’s edge, and shows up better in the negative (black and white) view at right.
All images: stephen james o’meara
During the two seconds it remained visible, the object changed shape. It began as an oval cloud 2° wide (with no nucleus), condensed into a nebulous star, and then resumed its large and nebulous nature. Baldet reported that he and others had witnessed six more events over the next five years, with some objects appearing as small as 5′ in diameter.

The most striking features of these phantom streaks are the lack of luminosity and, as Baldet pointed out, lack of a definable core. These mysterious objects appear suddenly like a meteor, move swiftly like a meteor, last just as long as a meteor, but look like a persistent meteor train. How weird… and wonderful!

The straight and not so narrow

Unlike Baldet, I have never seen a diffuse meteor change its shape. Most I have witnessed over the past 20-odd years have been faint — around magnitude 4. They seem to slip through the night like mini-comets penetrating the atmosphere. I have also witnessed a few diffuse meteors through binoculars. These differed from naked-eye events in that the brief diffuseness visible seems to “energize” before vanishing from view. It’s like what we see on TV’s Star Trek, when crew from the Enterprise get beamed up or down somewhere, although what happens to the meteor lasts only a fraction of a second.

French astronomer Fernand Baldet’s drawing shows a nebulous meteor’s changing shape as it traversed the sky in two seconds the night of March 18, 1903. When the nebulous meteor appeared as an oval cloud (the far-left sketch), it measured about 2° across, according to Baldet. The Moon is at lower right for comparison.
The meteor I saw July 17 from Botswana was definitely the largest I had ever witnessed. What I found most fascinating was that the trail near its widest point, near Theta1 and Theta2 (θ1 and θ2) Sagittarii, spanned about 2°, which, as Baldet says about his own sighting, was rare. Again, most of the diffuse meteors I’ve seen are more around the several-arcminute size. In a 1992 Journal of the Association of Lunar and Planetary Observers article, John S. Gallagher says these events may be caused by some sort of “cosmic dust bunny” from vapor-cloud remnants of very small comets — a theory proposed by Louis A. Frank from the University of Iowa.

This May might be a perfect time to search for your own cosmic dust bunnies because this is when the Eta Aquariid meteor shower, whose stream is associated with Comet 1P/Halley, occurs. According to the International Meteor Organization, modeling of the stream by Mikhail Maslov yields two trail encounters (when Earth passes through a stream of particles) for 2019: May 4, between 4h and 10h UT; and May 6, between 12h and 20h UT. Either encounter could boost the maximum hourly rates to nearly 80. And the New Moon falling on May 4 creates excellent viewing conditions.

As always, send reports of anything weird and wonderful to