STEREO Watches Comet ATLAS as Solar Orbiter Crosses Its Tail

NASA’s Solar Terrestrial Relations Observatory, or STEREO-A spacecraft, captured these images of comet ATLAS as it swooped by the Sun from May 25 – June 1.
During the observations and outside STEREO’s field of view, ESA/NASA’s Solar Orbiter spacecraft crossed one of the comet’s two tails.

In the animated image, ATLAS emerges from the top of the frame and approaches the Sun — off camera to left — against gusts of solar wind. Its dust tail, which reflects sunlight, appears white. Mercury is also visible as a bright dot emerging from the left against the stationary starfield. The vertical streaks in the image are artifacts created by saturation from bright background stars.

While STEREO recorded this footage, Solar Orbiter crossed one of comet ATLAS’s tails. Launched in February 2020, the spacecraft wasn’t scheduled to enter full science operations until June 15, but engineers adjusted Solar Orbiter’s testing schedule and turned on its four most relevant instruments for the encounter. It’s the first time a comet tail crossing by a spacecraft not designed to chase them was predicted in advance.

As material sheds from a comet’s nucleus, it leaves behind two tails: a thin ion tail, made of charged particles, and a more diffuse dust tail that reflects visible light. The ion tail always points away from the Sun regardless of the comet’s trajectory; the dust tail more closely follows the comet’s path. Solar Orbiter crossed the ion tail on May 31, some 27 million miles downstream and outside STEREO’s field of view. The team is still awaiting those results. It will fly through the remnants of the dust tail on June 6.

Comet ATLAS was discovered on Dec. 28, 2019 in images captured by the Asteroid Terrestrial-impact Last Alert System, or ATLAS robotic astronomical survey system in Hawaii. Comets are traditionally named after the instruments or person that discovered them. The comet follows an orbit that takes it past the Sun approximately every 6,000 years, though observations suggest the comet is currently disintegrating and is unlikely to return. It likely originated in the Oort cloud, a spherical cloud of ice and rocks surrounding our solar system. The Oort cloud begins about 185 billion miles away, some 67 times farther than Neptune.