War of the Worlds : Attack on a harmless asteroid. And while Testing Earth's Defenses, the astronomers witnessed an unexpected show.

NASA'S DART spacecraft was not able to take pictures of the very moment it slammed into an asteroid last week at more than 14,000 per hour. Or of the aftermath.

But telescopes on Earth, seven million miles away, were watching. The images they recorded showed a spectacular outburst of debris rising from the asteroid after the collision.

The celestial show was a bonus, on top of the spacecraft's main objective of demonstrating a method for defending the planet from deadly space rocks in the future.

''I saw ground-based images in the minutes after the impact, and they were absolutely phenomenal,'' said Cristina Thomas, a professor of astronomy and planetary science at Northern Arizona University and the leader of the observations working group for the mission.

Take, for example, images that were captured with a 20-inch telescope in South Africa. It shows the asteroid Didymos, about half a mile wide, moving across the night sky. What cannot be seen is Dimorphos, the 500-foot-wide moon of Didymos - and the target of DART.

''Our telescope in South Africa - we simply pointed in the direction of the asteroid,'' said John Tonry, a professor of astronomy at the University of Hawaii. '' And we started taking images every 40 seconds.''

The sudden brightening comes from a cloud of debris tossed into space by the impact of the spacecraft into Dimorphos.

''We didn't really expect to see such a big plume of dust coming out,'' Dr. Tonry said. ''But, you know, discovery favors the prepared.''

Dr. Tonry is co-principal investigator of the Asteroid's Terrestrial-impact Last Alert Systems, or ATLAS, which uses the South African telescope and three others around the world to scan the sky for asteroids that might be on a collision course with Earth.

Even though the telescope was nearly half a world away from him, Dr. Tonry saw the pictures second  after they were taken. ''That's the amazing thing about the internet,'' he said. ''We looked at the pictures and said : ' Oh my God, look at that Wow.' ''

Dr. Tonry was surprised by how much debris was knocked off the asteroid and how fast it was moving. ''The stuff was screaming out at two kilometres a second, like, like 4,000 miles an hour,'' he said. ''And so within an hour, that cloud was as big as the Earth.''

Most of the debris was ejected from the point of impact, moving away from the side where DART struck. ''Which is exactly what you'd expect for a plume to be recoiling off the surface,'' Dr. Tonry said. But he said there also appeared to be a shell of debris rising from the opposite side, moving in the same direction as DART.

''It may be that DART created a wave that went right through Dimporphos and kind of blasted stuff off the far side,'' he said.

Right after the impact, the brightness jumped by a factor of 10 from sunlight bouncing off the debris. It has dimmed since then, but the dot is still four times as bright as it had been.

A cloud of slower moving-debris that remains in the vicinity of Didymos and Dimorphos is likely to fall to the surface of the two asteroids in the coming weeks.

A similar sequence of images was taken by another telescope in South Africa by Amanda Sickafoose, an astronomer who lives in South Africa but works for the Planetary Science Institute in Tucson, Ariz., and Nicolas Erasmus of the South African Astronomical Observatory. [South Africa was a prime location for viewing the impact.]

''Seeing the ejecta was phenomenal,'' Dr. Sickafoose said.'' I feel like I might never have the opportunity to something like that again in my life.''

Last week, the Italian Space Agency released images that had been taken by LICIACube, a shoebox size spacecraft that trailed DART to take before-and-after pictures of Dimporphos.

In images after the impact, ''the ejecta cloud is very complicated,'' said Angela Stickle, a planetary scientist at the John Hopkins Applied Physics Laboratory and the lead for the group of scientists who performed computer simulations of the impact.

''You get sort of things that look a little bit like streamers. You get a sort of asymmetric ejection. And so it is very definitely very complex.''

NASA'S two space telescopes, Hubble and James Webb, also managed to capture streams of debris kicked up by the impact.

That was a particular challenge for Webb, which was not designed to track fast-moving asteroids within the solar system. Scientists will make additional observations of the asteroid in coming months with one of its spectographs, which could help identify the chemical composition.

Hubble was unable to photograph Didymos at the time of impact, because Earth was in the way, but it did capture before-and-after photographs.

All of the plume data gives Dr. Stickle and her colleagues plenty to work with as they try to understand the structure and composition of Dimorphos. The large plume and the boulder-strewn surface of the DART saw on approaching the asteroid indicate a rubble pile that Dr. Stickle said was loosely held together.

The other key measurement is the orbital period of Dimporphos around Didymos; it was 11 hours 55 minutes before the smashup. But the head-on impact would have sapped some of Dimorphos's momentum, causing the moon to fall closer to Didymos.

The size of the change - expected to be about 1 percent - would give an indication of how much material was kicked of the surface.

'' Once we get the period change measurement from the telescope,'' Dr. Stickle said, ''then we can start matching that to our simulation.''

The World Students Society thanks author Kenneth Chang.


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