Astro Bob: Comet Leonard loses its head!
2021's brightest comet, which passed by Earth last December, now appears to be falling apart.
Remember Comet Leonard? It became a fine binocular object late last year through early January, especially for southern hemisphere skywatchers. When brightest, it reached third magnitude and grew a long, beautiful tail. Tails form when the sun vaporizes a comet's dust-laden ice; sunlight then pushes the dust away from the comet's head to form a tail. Although Leonard hovered near the horizon at dusk for northern viewers, it was briefly visible in binoculars in late December if you knew where to look.
During its best appearance, Leonard repeatedly brightened and faded every 3-5 days, a sign of rapid changes occurring in its nucleus, the solid, frozen core of the comet. Surface cracks and collapsing cliffs expose fresh ice, which rapidly vaporizes in sunlight to produce clouds of reflective dust. The comet came within 2.5 million miles (4 million km) of Venus on Dec. 18 and then passed closest to the sun on Jan. 3 at a distance of 57 million miles (92 million km). As it departed, Leonard gradually faded as expected.
But then something strange happened. At least one amateur astronomer noticed that the comet's head appeared fainter than expected in late January. The head is centered on the nucleus and normally the brightest part of a comet as fresh dust and gas liberated by solar heating concentrate here before departing to grow the tail. On Feb. 23, amateur Martin Mašek
photographed the comet and discovered that the head had all but disappeared. This was confirmed in photos taken by Thomas Lehmann (featured here) and a clear sign that Comet Leonard had begun disintegrating.
Seen up close with a spacecraft, comets look as solid as ice, but in truth they're fluffy. A typical comet is a small, irregularly-shaped body about 6 miles (10 km) across composed of water ice, dust, rock and frozen gases that include carbon dioxide, carbon monoxide and ammonia. They come in a variety of shapes from bowling pins to potatoes to dumbbells. Although firm and icy on the outside, if you could crack one open, the interior would appear fragile, resembling frozen cotton candy.
Here on Earth we're used to water ice in its crystalline form, where molecules of H2O line up in lockstep in a rigid order. Water ice in comets formed far from the sun where the temperature is more than 400 degrees below zero. Under these conditions water is "flash-frozen" along with other molecules and gases into porous, amorphous ice . This is the most common form of ice in deep space, where bitter cold temperatures are the rule.
As a comet approaches the sun, its heat transforms amorphous ice on the surface to the crystalline form. In the process, other molecules including the carbon-based, organic variety are squeezed out and form a dark residue on the surface ice. The sun also warms the comet's interior along with the more exposed water ice lining ridges and cliffs on its surface, converting the material directly to gas. Dust and ice grains swept up by the gas are shot into space in spectacular, geyser-like jets.
A comet is a study in dramatic contrasts — its hard, blackened surface encloses a lacy interior, the whole resembling chocolate-dipped honeycomb candy. Heat from the sun nibbles away at the object every time it dips into the inner solar system for a visit.
Comets don't last forever. They often meet their end by breaking apart. And that's what appears to have happened with Leonard. There are a couple of ways this can occur. Depending on the direction the jets point, they can act like a rocket engine and speed up a comet's rotation. A faster spin can stress the fragile nucleus, causing cracks to form that ultimately fracture it to pieces.
When ice in a comet's interior vaporizes, gas can pummel the its delicate innards and break it apart. Either way the pieces are fresh meat for the sun. With lots of surface area exposed to sunlight, the fragments quickly vaporize as they spread apart. In short order the head is no more, and a once-grand sight fades and disperses.
A third way a comet can crumble is through gravitational stress. When a small body passes close to a big one, as Leonard did to Venus last December, the gravitational tug on the side closest to the planet is stronger than on the opposite side. This difference could have stressed and weakened the nucleus, ultimately leading to the breakup. There's no direct proof this happened, but it remains a possibility.
A sad end? Maybe. Consider that some of that dust may end up one day as streaking meteors or become part of the zodiacal light , so prominent at dusk this time of year. Nature wastes nothing, repurposing everything.
"Astro" Bob King is a freelance writer for the Duluth News Tribune.