The European Hera mission will follow NASA's DART asteroid-deflecting spacecraft to the binary space rock Didymos and detail the aftermath of DART's collision with the smaller of the two asteroids, Dimorphos. It will even attempt to peek inside the asteroid duo in a scientific first.
According to the European Space Agency's (ESA) original plans, Hera would have witnessed DART's suicidal encounter with Didymos' moon Dimorphos in 2022 firsthand. But initial hesitation among ESA's member states led to funding delays. As a result, this investigator spacecraft will only arrive at the scene more than two years after the cataclysmic impact. The "dust" will have settled at that point, and astronomers will have known from Earth-based observations whether DART achieved its goal of altering Dimorphos' orbit around the larger Didymos.
What else will be there for Hera to learn? Surprisingly, quite a lot. Astronomers know very little about Didymos and its moon Dimorphos. And the information Hera will gather will help researchers finetune a possible future mission that would aim to deflect an asteroid on a collision course with Earth.
"Hera is currently on track to launch in October, 2024," Michael Kueppers, Hera project scientist at ESA, told Space.com. "It will arrive in late 2026 or early 2027. Although we originally wanted to observe the impact directly, there are certain advantages to arriving later. We will be able to see the final outcome, which may be the most relevant point from the planetary defence point of view."
Before it rams into Dimorphos, DART will photograph the two asteroids with its single instrument, the high-resolution DRACO (Didymos Reconnaissance and Asteroid Camera for Optical Navigation) camera. Ten days before its demise, the spacecraft will release a cubesat that will stand in for the delayed Hera and take basic images of the impact's immediate aftermath. Hera will then follow with a more advanced suite of instruments that will allow it to analyze in detail the outcome of the crash, as well as the structure of Didymos and Dimorphos and their chemical composition.
Rubble pile or solid block?
"Right now, we know quite well how the two asteroids orbit each other and how they together orbit the sun," said Kueppers. "We know that the larger Didymos is about 800 meters [2,600 feet] across and the smaller Dimorphos about 170 meters [560 feet] across. But we don't know their shapes, we don't know the mass of Dimorphos and we have no information about their composition and chemistry."
Astronomers think that the larger Didymos is not a single solid block of stone but rather what they call a "rubble pile," a conglomeration of boulders and pebbles loosely held together by gravity. The same may be true for Dimorphos. What happens during the impact depends to a large degree on these unknowns. A rubble pile will respond differently compared to a solid block of rock. It could fall apart into a number of fragments that might then fly away on their separate trajectories.
The strength and chemical composition of the material will determine how much of the energy delivered by DART the asteroid absorbs. Scientists, for example, have no idea how much material will be stirred up from the surface of Dimorphos by the DART impact, which might affect how much the impact changes the asteroid's orbit.
"The more detail we learn, the better we will be able to scale up the mission to achieve a desired outcome if it was ever needed one day to protect Earth," Kueppers said. "We would need to be better able to much better predict the outcome of such an impact if it's ever needed in a real case."
First look inside
Some of the most interesting measurements of the Hera mission might come not from the Hera spacecraft directly but from two cubesats that will travel to Didymos aboard Hera. One of these cubesats, called Juventas, will carry a novel radar instrument which will enable it to analyse the interior of the two asteroids. If successful, this would be a scientific first, said Kueppers.
"The cubesat carries a radar instrument that will send radio waves into the asteroids and measure the reflection," said Kueppers. "These waves will penetrate the asteroids and reveal the subsurface structure."
The second cubesat, called APEX (for Asteroid Prospection Explorer) will measure the crater created by the DART impact using optical and infrared imagers.
Both cubesats will orbit the two space rocks at a closer distance than the mothership and will attempt to land on Dimorphos at the end of their missions.
With the dust settled and Dimorphos recovered from the orbit-altering impact, Hera and its companions will have a much clearer view of the newly born crater than they would have in the direct aftermath of the collision.
Yet, the crater will be fresh. Much fresher than all of the other craters previously studied by astronomers, many of which were born in violent asteroid impacts millions of years ago.
"We have many craters on the moon and asteroids in the solar system," Kueppers said. "But this is a unique case where we can investigate a crater where we know exactly the properties of the impacting object. That will help us to significantly advance our understanding of the physics of cratering and the scaling of craters, which is a valuable piece of information for both, science and planetary defense."