ESA’s Hera Mission (Updated)
by Sam Atkins
NOTE: Hover mouse or tap images for captions.
Back in September 2022, NASA's Double Asteroid Redirection Test (DART) mission intentionally crashed a spacecraft into the asteroid Dimorphos and successfully deflected its orbit. The operation was simply a test of planetary defense capabilities, and the asteroid was not posing a threat to Earth.
A new mission by the European Space Agency (ESA) called Hera has launched. This mission’s objective is to follow up on and assess the results of NASA’s DART mission. When the Hera spacecraft arrives at the Didymos binary asteroid system in December 2026, it will analyze how Dimorphos’ mass and orbit were affected by the collision.
This image was taken from a distance of 920 km (570 miles) just a few minutes before NASA’s DART spacecraft impacted Dimorphos. Didymos is the larger, closer asteroid pictured at the bottom left and Dimorphos is the smaller, further asteroid at the top right. Image credit: NASA/Johns Hopkins APL
Discovered in 1996, this binary pair of asteroids orbit each other while together orbiting the Sun. Their distance from the Sun ranges from 1 astronomical unit (the average distance between Earth and Sun which is about 150 million km or 93 million miles) and 2.3 AU. They can get as close as 6 million km to Earth in their orbit, though this isn’t expected to happen for about another century. Their orbit can also very occasionally take them close to Mars as well.
The larger of the two, Didymos, is named for the Greek word for “twin.” It is about 780 meters wide which is almost the height of the world’s tallest building, the Burj Khalifa in Dubai. The smaller asteroid, Dimorphos, is often referred to as a “rubble pile” and a moon of Didymos, orbiting in a near circle around it about every twelve hours. It is about 180 meters wide which is about the height of the Space Needle in Seattle, Washington. It was the smaller Dimorphos that was impacted by NASA’s DART spacecraft back on September 26, 2022. The impact reduced the asteroid’s orbit around its larger sibling by about half an hour.
Artist’s concept of the Hera spacecraft. Image credit: ESA-Science Office
Hera's trajectory to the Didymos binary asteroid system. Video credit: ESA
The ESA’s Hera spacecraft will launch from Cape Canaveral, Florida on October 7th. Now, if you haven’t heard, there are potential problems with approving SpaceX’s partially reusable Falcon 9 rocket for launch following a recent incident in which the rocket’s second stage veered off course following separation and re-entry. However, the crew is confident that they will receive a green light from the FAA.
Assuming things go off without a hitch, we won’t have to wait five and a half years for Hera to get to its destination like Europa Clipper. The journey to the asteroid belt will take less than half that time, starting familiarly enough with a trip to Mars. It should reach the red planet by March 2025 (the same month as the Europa Clipper will) which will afford it the opportunity to test out its science equipment by studying the surface of Mars’ smaller moon, Deimos. It is to then perform a gravity assist maneuver which involves passing very close to the planet and using the increased gravity to give itself a big boost in speed that it needs to travel out to the Didymos system in the asteroid belt. Hera is expected to reach its final destination by the tail end of 2026. This will begin a six-month investigation.
Once it is in orbit around Didymos, Hera will begin studying Dimorphos. Astronomers have been wondering what to expect from the aftermath of the DART mission a few years ago. How big is the debris field? Did the kinetic impactor leave a crater, or did it completely reshape the asteroid? How did the impact affect the asteroid’s rotation? The answers to these questions will be invaluable to those designing our future planetary defense plans. To obtain these answers, Hera has a plethora of science instruments at her disposal.
Artist’s concept of Hera and its two cubesats studying Dimorphos. Image credit: ESA
Hera will approach within 20-30 km (12-18 mi) of Dimorphos to get a look at its motions and physical characteristics (such as shape and density). Across twelve flybys, Hera will map the surface of Dimorphos. It will deploy two nano-satellites, called CubeSats, which look like shoebox-sized toasters with solar panels attached. These cute little guys will be operated independently by other teams on Earth and each have their own responsibilities. The first cubesat, Juventas, will use radar to probe the interior of the asteroid and then land on it to determine its gravity. The second cubesat, Milani, will use spectroscopic instruments to determine the composition of the dust and rock that make up the asteroid. It’s also possible that the Hera spacecraft itself may attempt to land on Dimorphos!
UPDATE: The Hera mission launched on October 7, 2024 from Cape Canaveral, Florida (just days before Hurricane Milton slammed the state) and is currently on its way to Mars. It is expected to arrive in March 2025 where it will perform a gravity assist maneuver that will fling it out to rendezvous with the double asteroid system. On October 11th, Hera’s asteroid deck which houses all its science instruments was turned around and snapped the image below from over a million kilometers away from Earth.
The Earth and Moon as seen from the Hera spacecraft at about 1.6 million km away. Image credit: ESA