Meteoroids, especially the tiny particles called micrometeoroids, are extremely common throughout the solar system. Credit: modified from Jacek Zmarz.Meteoroids are lumps of rockВ or ironВ that orbitВ the sun, just as planets, asteroids, and comets do.

There, photons of light turned the neutral particles into charged particles (ions), which one of MESSENGERвЂ™s instruments could detect. Particles (neutral atoms) ejected by the meteoroid skyrocketed over 3,000 miles above MercuryвЂ™s surface, outside the bow shock of MercuryвЂ™s magnetosphere. Lead image caption: ArtistвЂ™s illustration depicting how MESSENGER observed the first meteoroid impact on another planetвЂ™s surface. Regoli noted that researchers will need to hone their models before using BepiColombo to make new observations, but the opportunity to see another Mercurian impact would be invaluable, he said.

Perhaps the European Space AgencyвЂ™s BepiColombo mission, which launched for Mercury in 2018 and will approach the planet in late 2025, will be able to capture more meteoroid impacts during its mission. вЂњThis was a special observation, and really cool to see the story come together.вЂќ вЂњIt just shows how rare it is to have the spacecraft at the right place and time to be able to measure something like this,вЂќ said study co-author Leonardo Regoli, from Johns Hopkins Applied Physics Laboratory in Maryland - where MESSENGER was built and operated. Artist view of the spacecraft orbiting the innermost planet Mercury. But none were seen in images during the mission, which lasted from 2011 to 2015.Д«ut in sifting through the old spectrometer data, the anomaly stood out. Interestingly enough, before the MESSENGER mission, scientists expected the spacecraft would capture some impacts on Mercury - perhaps up to two impacts per year during its four years in orbit. But computer models suggest something that size would create a plume with a height and density closely matching what FIPS detected. They estimate the meteoroid was likely just a little over three feet long, which is relatively small. Using the particlesвЂ™ speed and direction, the researchers вЂњrewound the clock, tracking the particlesвЂ™ motion back to their source.вЂќ They found the particles clustered in a dense plume, one that had erupted from MercuryвЂ™s surface and extended nearly 3,300 miles into space. Oddly, these particles were traveling in a tight beam, nearly all in the same direction, and at the same speed. The archival data revealed a strange anomaly: on December 21, 2013, MESSENGERвЂ™s Fast Imaging Plasma Spectrometer (FIPS) saw an unusually large number of sodium and silicon ions blowing in the SunвЂ™s solar wind, the powerful charged gases that spew from the Sun. Scientists think meteoroid impacts, in part, are responsible for putting such material into the exosphere. The exosphere forms on MercuryвЂ™s Sun-facing side from material originally on the planetвЂ™s surface. MercuryвЂ™s tiny atmosphere, called an exosphere, has a pressure thatвЂ™s one-quadrillionth of that felt at sea level on Earth. Credit: NASA/Johns Hopkins APL/Carnegie Institution of Washington

MESSENGER very likely witnessed one of the most recent of these crater-forming collisions in 2013. MercuryвЂ™s surface, as captured here in 2013 by the APL-operated MESSENGER spacecraft, is pockmarked with craters that have formed from millions of years of meteoroid impacts.

Meteoroid that strikes the surface