Astronomers using NASA's Hubble Space Telescope captured images of Comet 252P/LINEAR just after a close encounter with Earth on March 21. The close proximity to the comet offered scientists new insights on the body's nucleus. "Because comets are usually only a few kilometers in size, and probably less than 1 kilometer for this comet, reliable measurement of size is best done when they are close to us.

That's why the close approach to Earth of this comet offered us a great opportunity to study it," said PSI Senior Scientist Jian-Yang Li, who led this project to observe 252P with HST during its close approach to Earth.

The visit was one of the closest encounters between a comet and our planet. The comet traveled within 3.3 million miles of Earth, or about 14 times the distance between our planet and the Moon.

"With the small distance of this comet to us, and the high spatial resolution of HST, we reached 1.6 kilometers per pixel resolution, which is the highest for HST ever on a comet," Li said.

"For comparison, ground-based observations of this comet have more than 10 times lower resolution than HST."

The images reveal a narrow, well-defined jet of dust ejected by the comet's icy, fragile nucleus. These observations also represent the closest celestial object Hubble has observed, other than the Moon. The comet will return to the inner solar system again in 2021.

"Comet 252P is one of the smallest comets we know of. Our main goal is to determine its size and study how comets become smaller and smaller as they pass around the Sun.

"rom this we will infer the properties of building blocks of planets at the start of the solar system," Li said. "In addition, we will also study other dynamic properties of the comet, such as its rotation and how it releases dust under the heating of the Sun."

PSI Senior Scientist Nalin Samarasinha also worked on the project, which was funded by a grant to PSI from NASA through Space Telescope Science Institute.