Tale of La Sagra: how a common asteroid became an oddball comet

Comet P/2012 NJ (La Sagra) photographed on July 23 from France. Check out that pointy tail. Click image to see more photos. Thanks and credit to Jean-Francois Soulier

Ever wish you were somebody you’re not? No? Good, but for anyone who’s fantasized about being a rock star or rocket scientist, you’re sure to find company in the little comet called La Sagra. It’s full name is P/2012 NJ (La Sagra) and it began life as an asteroid when first discovered by the La Sagra Sky Survey Asteroids appear star-like in nearly all telescopes because they’re almost all tiny and relatively far away. La Sagra was no exception.

Comet NJ La Sagra has a steeply inclined orbit. It’s currently moving up and away from Earth and the sun. Credit: NASA

The survey, based in the mountains of southern Spain, and dedicated to finding small solar system bodies, employed a 17.7-inch (.45 meter) telescope to scoop up the asteroid on July 13. For a brief time it went by the name 2012 NJ, but followup observations by German astronomer Gerhard Hahn showed that the new-found rock sported a very faint needle-thin tail pointing to the southwest. Tails don’t grow from rocky asteroids. You need something like ice that vaporizes in sunlight to form a tail, and that’s exactly what a comet does.

Another tip-off was La Sagra’s  cigar-shaped orbit tilted nearly perpendicular to the solar system. Highly inclined, elongated orbits are more typical of comets than asteroids. Three days after discovery, thanks to additional observations, astronomers discovered their first conclusion had been too hasty, and La Sagra was re-branded a comet.

Comet La Sagra on July 19 taken by Stefano Mottola with a 1.23-meter telescope at Calar Alto Observatory in southern Spain.

La Sagra passed closest to the sun back in June at the rather large distance of 120 million miles and returns to Earth’s vicinity every 24.3 years.

I took at look at it three nights ago, and the comet was indistinguishable from a 14th magnitude star – no fuzziness, no tail. Just a pinpoint of light zipping along at a good clip in the constellation Pegasus the Flying Horse. In less than 10 minutes I could see it move against the background of stars.

Faraway comets often look no different from faint stars, but a pinpoint one in Earth’s neighborhood this close to the sun is out of the ordinary. Nearly all comets have a coma or atmosphere of dust and gas boiled off by sunlight when near the sun. That’s what gives them their characteristic fuzzy appearance. Either La Sagra is coma-less or its coma is very tenuous.

Currently closest to Earth at some 56 million miles, the comet is expected to fade as it heads back to the far end of its orbit in the months and years ahead. You can hunt for it now in 10-inch and larger scopes as it tracks to the northwest in Pegasus at nightfall. Click HERE to go the Horizons site where you’ll find La Sagra’s orbital elements. Input those in your sky charting software and you’re good to go.

Comet 96P/Machholz photographed July 23 by German amateur astronomer Waldemar Skorupa. Like most comets near the sun, Machholz displays a bright, fuzzy head called a coma. It also has a short tail and is visible this month in evening twilight. Click image to see more of Waldemar’s comet photos.

The origin of the comet’s unusual tail – already fading –  isn’t known. Is La Sagra a dormant comet that came back to life when a burst of fresh ice and dust erupted from a new crack in its icy surface? Or is more like asteroid 596 Scheila, which was struck in late 2010 by a tiny asteroid? The impact released a cloud of dust forming a brief-lived coma and tail.

I’m hoping astronomers continue their studies of this very interesting object while it’s still close to home.

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