NASA's STEREO satellite captured the first images ever of a collision between a solar "hurricane", called a coronal mass ejection (CME), and a comet. The collision caused the complete detachment of the comet's plasma tail.

Comets are icy leftovers from the solar system's formation billions of years ago. They usually hang out in the cold, distant regions of the solar system, but occasionally a gravitational tug from a planet, another comet, or even a nearby star sends them into the inner solar system. Once there, the sun's heat and radiation vaporizes gas and dust from the comet, forming its tail. Comets typically have two tails, one made of dust and a fainter one made of electrically conducting gas, called plasma.



CMEs are large clouds of magnetized gas ejected into space by the sun. They are violent eruptions with masses upwards of a few billion tons traveling anywhere from 100 to 3,000 kilometers per second (62 to 1,864 miles/second). They have been compared to hurricanes because of the widespread disruption they can bring when directed at Earth; CMEs are known to cause geomagnetic storms that can present hazards for satellites, radio communications, and power systems.



Comet Encke was traveling within the orbit of Mercury when a CME scrunched the tail and eventually tore it off the comet. Scientists at the Naval Research Laboratory (NRL) made the observations using the Heliospheric Imager (HI) in NRL's Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) telescope suite aboard NASA's Solar Terrestrial Relations Observatory (STEREO)-A spacecraft.



Scientists have been aware of this spectacular phenomenon, the disconnection of the entire plasma tail of a comet, for some time. However, the conditions that lead to these events remained a mystery.



Scientists suspected that CMEs could be responsible for some of the disconnection events, but the interaction between a CME and a comet had never been directly observed. Because the HI instrument can take many images rapidly, and the images are very detailed, scientists were able to obtain a series of images of the comet and tail disconnection as the event occurred.



The researchers combined the images into a movie. This never-before-seen movie was recorded on April 20, 2007, when a CME encountered comet Encke. The observations reveal the brightening of the comet tail as the CME swept by and its subsequent disconnection and transport by the CME front.

This is a still taken from an animation showing Comet Encke and the coronal mass ejection erupting from the surface of the Sun. Credit: NASA

+ View animation This is a still taken from an animation showing Comet Encke and the coronal mass ejection erupting from the surface of the Sun.

A preliminary analysis suggests that the tail was ripped away when magnetic fields bumped together in an explosive process called "magnetic reconnection." Oppositely directed magnetic fields around the comet "bumped into each" by the magnetic fields in the CME. Suddenly, these fields linked together-they "reconnected"-releasing a burst of energy that rent the comet's tail. A similar process takes place in Earth's magnetosphere during geomagnetic storms fueling, among other things, the Northern Lights.



"The comet had its own space weather event," said Angelos Vourlidas, Lead author and Researcher with the Naval Research Laboratory, Washington, DC. "We think it experienced a magnetic reconnection event very similar to what Earth experiences when CMEs impact our own protective magnetosphere."



The results are published online today in the Astrophysical Journal Letters Rapid Release website and in the October 10 print issue of the journal.

