Asteroid Bites the Dust

Astronomers of the Armagh Observatory and Planetarium (AOP) have been part of one of the
many teams around the world studying the outcome of the Double-Asteroid Redirection Test
(DART): a space mission designed to test the human capability of deflecting an asteroid that
may pose a threat to Earth.

Don’t worry – although there are over two thousand celestial bodies out there officially
classified as potentially hazardous, astronomers have yet to find any objects that pose
significant danger to Earth like Chicxulub, the asteroid that is believed to have caused the
extinction of the dinosaurs. Despite this, it’s in our best interest to be prepared in the case
that our exterminator is discovered. If such an object were found, scientists can use the
information gathered from the DART mission to develop techniques to deflect it.

The target of the DART mission was the double asteroid system, Didymos-Dimorphos. Before
the mission, Dimorphos orbited around Didymos approximately once every 12 hours. The goal
of the mission was to reduce this time, by a few minutes at least, by crashing a spacecraft into
its surface. This collision successfully took place on September 26th, 2022. You can watch the
livestream here.

Measurements taken over the next weeks showed the world that DART far exceeded all
expectations: the orbital period of Dimorphos was reduced by 33 minutes. For the first time
in history, humans have moved a celestial body! Additionally, the impact caused millions of
kilograms of asteroid material to be ejected into space. From Earth, this appeared as a huge
dust cloud that engulfed the entire asteroid system for several hours. Over time, the ejected
debris shifted and developed into a 10,000 km long dust-tail. We also assume that some of
the new dust settled on the surface of Didymos.

It’s this ejected material that the AOP astronomers are interested in. Dr Stefano Bagnulo and
PhD student Zuri Gray have used one of the world’s largest telescopes to observe the asteroid
system both before and after the impact. Using a method known as spectropolarimetry,
which combines spectroscopic and polarimetric observing methods, the research group
discovered a dramatic drop in the value of polarisation after the DART impact. This suggests
that the material excavated by the impact is different to the material on the outer surface of
the asteroid.

One potential explanation for the phenomena is ’space weather’, a form of wear and tear that occurs when bodies are exposed to the harsh conditions of space. Another possible explanation is that the ejected material is made of smaller fragments than those present on the surface before impact. Understanding the inner structure of asteroids is vital in the world of planetary defense.

The long-term effects of the DART mission on the Didymos-Dimorphos system will be further
studied by the Hera mission, due to launch in 2024. Together, the DART and Hera missions
are an important step towards understanding how we can mitigate the risks of asteroid
impacts and safeguard our planet. Stay tuned for more updates on these exciting missions!