“It’s normal to make mistakes in science”: NASA did not expect to deflect the asteroid Dimorphos so much

DART crashed into an asteroid to change its orbit. NASA predicted a change of just 1 minute in the time the object takes to revolve around its parent body. The result of the test is different: why?

For the first time in history, humanity has succeeded in deliberately deflecting the trajectory of an asteroid in space. On October 11, 2022, we learned that the DART mission had fulfilled its main objective: to disrupt the orbit of the asteroid Dimorphos, on which the probe crashed at the end of September. All this is an exercise: the asteroid does not represent any proven danger.

Dimorphos orbits a larger asteroid, Didymos. After the DART impact, scientists scanned the sky using powerful telescopes. Their conclusion: the orbit of Dimorphos around Didymos has been reduced by 32 minutes. In other words, the smaller of the two asteroids takes less time to go around the larger one. However, this is much more than what NASA had anticipated: the space agency predicted a change of barely 1 minute and 13 seconds in the orbit of Dimorphos. So why such a deviation from forecasts?

Scientists Studying These Asteroids Made 2 Mistakes

There are two reasons for this, explains to Numerama Stéphanie Lizy-Destrez, teacher-researcher at ISAE-SUPAERO: “ uncertainties about the mass of the main body, Didymos, and an error about the composition of Dimorphos — the second reason being the bigger “mistake” of the two.

To better understand these asteroids, scientists are working with models. Here it is a binary system, with one asteroid orbiting another. The models used are comparable to those used to study the movement of the Moon, which revolves around the Earth. ” We are thus able to approach the distance, the period (the number of laps made in a given time), thanks to physical properties. In particular, the mass of each object “, continues Stéphanie Lizy-Destrez.

Representation of Didymos and Dimorphos.  // Source: YouTube ESA screenshot
Representation of Didymos and Dimorphos. // Source: YouTube ESA screenshot

Nevertheless, as long as an exploration mission is not launched towards an asteroid, to see it a little closer, it is complex to determine its mass. ” Information comes to us from light. We can estimate the mass [d’un astéroïde] depending on the light received, which gives an intuition of its composition. Since these calculations remain estimates, error is possible. This is what happened for the larger asteroid of this pair. ” Didymos’ mace wasn’t quite right. However, this plays on the mass of Dimorphos. It is as if we changed the mass of the earth, the Moon would have another movement. This is the first uncertainty that we had. »

“It’s normal to make mistakes in science”

What happened with the other parameter, the composition of Dimorphos? ” It was thought to be quite sturdy, but footage revealed it’s made of a lot of dustsummarizes the scientist with Numerama. This means that it had a large volume, but a small mass.. When the DART crash occurred on Dimorphos, a huge amount of mass was ejected from it. ” What was left [de Dimorphos] was therefore smaller. And, since this nucleus was smaller than we had estimated, the deviation was larger than we had expected.. »

These errors in the forecasts do not call into question the success of the DART mission. ” It’s normal to make mistakes in science, that’s how we learnsays Stéphanie Lizy-Destrez. It’s even almost reassuring: the overall principle, of deflecting an asteroid by a probe that hits it, works. We have seen that it works. Then, it is in the precision of the result obtained that we can improve. But, the fact that we manage to touch an object so far from the Earth, it is extraordinary. »

For further

Dimorphos after DART impact.  // Source: OBS E. Guido, M. Rocchetto, G. Savini, S. Fossey (cropped image)

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