Month: October 2024

Orionids Meteor Showers May Offer Treat for Stargazers

By Wayne Smith

From a total solar eclipse in April to a partial lunar eclipse of September’s Harvest Moon, 2024 has been sweet for skywatchers – professionals and amateurs alike. And just in time for Halloween, stargazers can anticipate another treat in the pre-dawn hours of Oct. 20 and 21 thanks to the Orionids Meteor Showers. That is unless the Moon or cloudy skies don’t provide too many tricks.

The Orionids, which peak during mid-October each year, are considered to be one of the most beautiful meteor showers of the year. Orionid meteors are known for their brightness and for their speed. Of course, the ability to see them will depend on clear evening skies. And a bright waning gibbous Moon – where it moves between full and last quarter phases – will outshine fainter meteors, greatly reducing the number of meteors visible to skywatchers.

A bright meteor streaks across a dark sky in front of a crescent Moon.
Orionid meteors appear every year when Earth travels through an area of space littered with debris from Halley’s Comet. Credit: NASA/JPL

Still, a few Orionids should hopefully be viewable in both the Northern and Southern Hemispheres during the hours after midnight through before dawn on the mornings of Sunday, Oct. 20, and Monday, Oct. 21.

The Orionids are also framed by some of the brightest stars in the night sky, which lend a spectacular backdrop for these showy meteors.

“Find an area well away from the city or street lights,” said Bill Cooke, who leads NASA’s Meteoroid Environment Office at the agency’s Marshall Space Flight Center in Huntsville, Alabama. “Come prepared with a blanket. Lie flat on your back and look up, taking in as much of the sky as possible. In less than 30 minutes in the dark, your eyes will adapt and you will begin to see meteors.”

Aside from potentially producing spectacular fireballs, the Orionids reflect quite a legacy. Their parent comet is the most famous one of them all – Halley’s Comet. Each time that Halley returns to the inner solar system its nucleus sheds ice and rocky dust into space. These dust grains eventually become the Orionids in October and the Eta Aquarids in May if they collide with Earth’s atmosphere.

Comet Halley takes about 76 years to orbit the Sun once. The last time comet Halley was seen by casual observers was in 1986 and it will not enter the inner solar system again until 2061. The comet is named for Edmond Halley, who discovered in 1705 that three previous comets seemed to return every 76 years or so and suggested that these sightings were in fact all the same comet. The comet returned as he predicted, and so it was named in Halley’s honor.

So, while it’ll take another 37 years to see Halley’s Comet, the Orionids offer a glimpse of its past.

The Orionids start winding down what has been an eventful calendar year for skywatching events. There was the total solar eclipse across most of North America on April 8 – the alignment of the Sun, Moon, and Earth – creating a total solar eclipse lasting 4 minutes and 12 seconds. The Perseids brought nightsky fireworks in August. And the partial lunar eclipse of a full supermoon – the Harvest Moon – welcomed fall and provided some spectacular images. Other skywatching events for 2024 include the Geminid and Ursid meteor showers in December.

Read more about meteors here.

Lane Figueroa
Marshall Space Flight Center, Huntsville, Ala.
256.544.0034
lane.e.figueroa@nasa.gov

Ancient Oort Cloud Comet to Make First Documented Pass By Earth in Mid-October

An ancient celestial traveler will make its first close pass by Earth in mid-October. Mark those calendars – because it might not be back.

The Oort Cloud comet, called C/2023 A3 Tsuchinshan-ATLAS, was discovered in 2023, approaching the inner solar system on its highly elliptical orbit for the first time in documented human history. It was identified by observers at China’s Tsuchinshan – or “Purple Mountain” – Observatory and an ATLAS (Asteroid Terrestrial-impact Last Alert System) telescope in South Africa. The comet was officially named in honor of both observatories.

NASA astronaut Matthew Dominick captured this timelapse photo of Comet C/2023 A3 (Tsuchinshan-ATLAS) from the International Space Station as it orbited 272 miles above the South Pacific Ocean southeast of New Zealand just before sunrise on Sept. 28, 2024. At the time, the comet was about 44 million miles away from Earth. (NASA/Matthew Dominick)

The comet successfully made its closest transit past the Sun on Sept. 27. Scientists surmised it might well break up during that pass, its volatile and icy composition unable to withstand the intense heat of our parent star, but it survived more or less intact – and is now on track to come within approximately 44 million miles of Earth on Oct. 12.

“Comets are more fragile than people may realize, thanks to the effects of passing close to the Sun on their internal water ice and volatiles such as carbon monoxide and carbon dioxide,” said NASA astronomer Bill Cooke, who leads the Meteoroid Environment Office at NASA’s Marshall Space Flight Center in Huntsville, Alabama. “Comet Ison, for example, reached the inner solar system in 2013, but failed to survive the Sun’s intense heat and gravity during perihelion in 2013.”

A bright green blur streaks across the black sky.
Comets traveling through the inner solar system aren’t uncommon, but many never survive a close pass by the Sun. Icy comet ISON, photographed here on Nov. 19, 2013, reached solar perihelion later that month – but couldn’t endure the punishing heat and gravity so close to Earth’s parent star and disintegrated. (NASA/MSFC/Aaron Kingery)

Though Comet Tsuchinshan-ATLAS will be ideally positioned to view from the Southern Hemisphere, spotters above the equator should have a good chance as well. Peak visibility will occur Oct. 9-10, once the half-moon begins to move away from the comet.

Choose a dark vantage point just after full nightfall, Cooke recommended. Looking to the southwest, roughly 10 degrees above the horizon, identify the constellations of Sagittarius and Scorpio. Tsuchinshan-ATLAS should be visible between them. By Oct. 14, the comet may remain visible at the midway point between the bright star Arcturus and the planet Venus.

“And savor the view,” Cooke advised – because by early November, the comet will be lost to the unaided eye, perhaps gone forever.

Comet light, comet bright

It’s highly unlikely Tsuchinshan-ATLAS will be visible in daylight hours, except perhaps at twilight, Cooke said. In the past 300 years of astronomical observation, only nine previous comets have been bright enough to spot during the day. The last were Comet West in 1976 and, under ideal conditions, Comet Hale-Bopp in 1997.

A bright green blur streaks across the black sky.
Comets with long, elliptical orbits around the Sun may reach perihelion – their closest point to our star – too rarely to observe more than once in a lifetime. This comet, Lovejoy (C/2014 Q2), reached perihelion in early February 2015, and isn’t expected to do so again until 2633. Comet Tsuchinshan-ATLAS, which is expected to come within approximately 44 million miles of Earth on Oct. 12, will not enter the inner solar system again for some 80,000 years. (NASA/Goddard/Damian Peach)

The brightness of comets is measured on the same scale we use for stars, one that has been in use since roughly 150 B.C., when it was devised by the ancient scholar Hipparchus and refined by the astronomer Ptolemy. Stellar magnitude is measured on a logarithmic scale, which makes a magnitude 1 star exactly 100 times brighter than a magnitude 6 star. The lower the number the brighter the object, making it more likely to be clearly seen, whether by telescope or the naked eye.

“Typically, a comet would have to reach a magnitude of –6 to –10 to be seen in daylight,” Cooke said. “That’s extremely rare.”

At peak visibility in the northern hemisphere, Tsuchinshan-ATLAS’s brightness is estimated at between 2 and 4. In comparison, the brightest visible star in the night sky, Sirius, has a magnitude of –1.46. At its brightest, solar reflection from Venus is a magnitude of –4. The International Space Station sometimes achieves a relative brightness of –6.

Comets are often hard to predict because they’re extended objects, Cooke noted, with their brightness spread out and often dimmer than their magnitude suggests. At the same time, they may benefit from a phenomenon called “forward scattering,” which causes sunlight to bounce more intensely off all the gas and debris in the comet’s tail and its coma – the glowing nebula that develops around it during close stellar orbit – and causing a more intense brightening effect for observers.

“If there is a lot of forward scattering, the comet could be as bright as magnitude –1,” Cooke said. That could make it “visible to the unaided eye or truly spectacular with binoculars or a small telescope.”

What will become of Comet Tsuchinshan-ATLAS? Cooke said it could be flung out of the solar system – like a stone from a sling – due to the gravitational influence of other worlds and a “jetting” effect caused by the comet’s offgassing during its solar transit.

But the hardy traveler likely still has miles to go yet. “I learned a long time ago not to gamble on comets,” Cooke said. “We’ll have to wait and see.”

Learn more about comets here.

Lane Figueroa
Marshall Space Flight Center, Huntsville, Alabama
256-544-0034
lane.e.figueroa@nasa.gov

Editor’s Note: The orbits of comets are continuously revised as new observational data becomes available. An earlier version of this article cited a period of 80,000 years for C/2023 A3 Tsuchinshan-ATLAS, which is no longer accurate based on newly available data. As of Oct. 14, the comet’s path may take it out of the solar system altogether.

Smith, an Aeyon employee, supports the Marshall Office of Communications.