Astronomers around the world will watch DART’s impact, and its effect will be closely followed to see if the mission passed the test.

Then, the European Space Agency’s Hera mission, named after the ancient Greek queen of the gods, will follow in its footsteps.

The Hera spacecraft is planned to launch in October 2024, aiming to arrive at Dimorphos in 2026 to measure the exact impact DART had on the asteroid.

But scientists are not only excited to see DART’s crater, but also to explore an object that is very much out of this world.

- ‘A new world’ -

Dimorphos, which orbits a larger asteroid Didymos as they hurtle together through space, provides not only a “perfect testing opportunity for a planetary defense experiment, but it is also a completely new environment,” the ESA’s Hera mission manager Ian Carnelli said.

Hera will be loaded up with cameras, spectrometers, radars and even toaster-sized nano-satellites to measure the asteroid’s shape, mass, chemical composition and more.

NASA’s Bhavya Lal said that it was critically important to understand the size and composition of such asteroids.

“If an asteroid is made up of, for example, loose gravel, approaches to disrupt it may be different than if it was metal or some other kind of rock,” she told the International Astronautical Congress in Paris this week.

So little is known about Dimorphos that scientists will discover “a new world” at the same time as the public on Monday, Hera mission principal investigator Patrick Michel said.

“Asteroids are not boring space rocks — they are super exciting because they have a great diversity” in size, shape and composition, Michel said.

And because they have low gravity compared to Earth, matter there could behave completely differently than expected.

“Unless you touch the surface, you cannot know the mechanical response,” he said.

- ‘Behaved almost like fluid’ -

For example, when a Japanese probe dropped a small explosive near the surface of the Ryugu asteroid in 2019, it was expected to make a crater of two or three metres. Instead, it blasted a 50-metre hole.

“There was no resistance,” Michel said.

“The surface behaved almost like a fluid,” rather than solid rock, he added. “How weird is that?”

One way the Hera mission will test Dimorphos will be to land a nano-satellite on its surface, in part to see how much it bounces.

Binary systems like Dimorphos and Didymos represent around 15 percent of known asteroids, but have not yet been explored.

With a diameter of just 160 metres — around the size of the Great Pyramid of Giza — Dimorphos will also be the smallest asteroid ever studied.

Learning about the impact of DART is not only important for planetary defense, Michel said, but also for understanding the history of our Solar System, where most cosmic bodies were formed through collisions and are now riddled with craters.

That’s where DART and Hera could shine a light not just on the future, but on the past.

I’m not sure the extent that Didymos has been analysed but the impact will teach us a lot whatever the composition of that asteroid is. Not just how a hard or soft or in-between asteroid acts on impact but how our targeting spacecraft system works. And of course this is only the first of such experiments in how to deter a wide range of celestial threats.

I would suggest that the resources devoted to this mission would be better employed in a general survey program of the asteroid belt. We need to know more about the asteroids themselves, how many of them are dangerous, which ones can be easily nudged out of orbit, than in how easy it is to move THIS particular body.

A general asteroid survey project would also yield strong scientific dividends, the Belt is a reservoir of primordial stuff, material left over from the formation of the solar system. What we learn there can tell us much about how planetary systems form, what is unique to ours and what it has in common with others.

I don’t know much about this particular mission, perhaps Dart will collide with Didymos AFTER conducting a thorough survey. In that case, then I agree with your assessment. It is an efficient use of resources to piggyback the crash test with other studies. I agree planetary defense is important, but I believe that cause can best be served by learning about the Belt as a whole, not just concentrating on one of its members.