Hera launch — to verify DART
On 7 October 2024 ESA's asteroid probe Hera left Earth on a Falcon 9 rocket. It is on its way to study Dimorphos, the small asteroid whose orbit NASA's DART changed by crashing into it in 2022, and is due to arrive in November 2026.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 7 October 2024 the European Space Agency's Hera spacecraft left Earth on a Falcon 9 rocket from Cape Canaveral in the United States. Its destination is a pair of asteroids, Didymos and Dimorphos. The smaller one, Dimorphos, travels around the larger Didymos much as the Moon travels around Earth. Hera is going to check the results of an experiment carried out on this pair two years earlier.
That experiment was done by NASA's DART spacecraft on 26 September 2022. DART deliberately crashed into Dimorphos at about 22,000 km/h. As a result, the time Dimorphos takes to go once around Didymos dropped from 11 hours 55 minutes to 11 hours 23 minutes, a change of 32 minutes. It was the first time people had deliberately changed the orbit of an object in space. These asteroids pose no danger to Earth; they were chosen as a safe test target.
Hera's job is to measure closely and precisely what the crash left behind. It needs to find the size and shape of the crater DART made, how much rock and dust was thrown off, and how heavy Dimorphos is and what it is like inside. Only with these numbers can scientists work out how hard and how early an asteroid would have to be pushed if one were ever heading for Earth.
On 12 March 2025 Hera flew past Mars at a distance of about 5,000 km, which changed its speed and direction. It is due to reach the asteroids in November 2026, a month earlier than first planned. Two small satellites carried on board, Milani and Juventas, will help with the survey. The orbit view reproduces Hera's two-year journey from Earth, past Mars and on to the asteroids.
Mission goals
DART and Hera are two halves of one experiment. DART showed that an asteroid can be pushed, and Hera measures how far it was pushed. Telescopes on Earth could only tell that Dimorphos now takes 32 minutes less to go around. How big the crater is and how heavy Dimorphos is can only be found by going there. Those values are needed to calculate how efficiently the impact pushed the asteroid.
A small asteroid like Dimorphos may not be one solid rock; it may be a loose pile of stones and gravel held together weakly. How such an asteroid responds to a crash was not well known. After DART hit, a large amount of debris was seen streaming away like a long tail, which led scientists to think Dimorphos is probably a loose rubble pile. Hera's close-up observations should help confirm this idea.
The mission is also practice in protecting Earth together. NASA did the pushing and ESA checks the result, with each agency taking one role. If a truly dangerous asteroid were ever found, this kind of international teamwork would be essential. Hera's results will serve as a reference if the same method is ever used on another asteroid.
Mission chronology and background
Hera grew out of ESA's Asteroid Impact Mission (AIM), planned in the early 2010s. NASA and ESA originally planned to send an impactor and an observer together. At the end of 2016, however, AIM was cancelled for lack of funding, and DART flew first on its own. ESA then built Hera as AIM's replacement. The name comes from Hera, a goddess in Greek mythology.
Hera carries two small satellites. Milani will study what the surfaces of the two asteroids are made of, using a camera that splits light into many colours. Juventas will look inside the asteroid with radar and measure its gravity to learn about its internal structure. Milani is named after the Italian asteroid scientist Andrea Milani, and Juventas is the Roman goddess of youth, who corresponds to a daughter of Hera.
Asteroid research will continue after Hera. When the asteroid Apophis passes very close to Earth in 2029, ESA's Ramses and NASA's OSIRIS-APEX plan to study it. The values Hera measures will become an important reference for working out how to deflect any asteroid that heads toward Earth in the future.
Other milestones of this mission
- Hera arrives at Dimorphos(planned)
Where to see it — sky and simulator
Hera was launched at 10:52 a.m. US Eastern time on 7 October 2024 from Space Launch Complex 40 at Cape Canaveral. The spacecraft went straight onto a path leaving Earth and now travels around the Sun. Once in deep space it cannot be seen with the eye or with a telescope, so news of its progress comes from ESA's mission announcements.
The orbit view reproduces the journey in time order, starting from Earth at launch, passing Mars in March 2025 and arriving at Didymos in November 2026. When the view follows the spacecraft, Hera's path can be seen curving outward beyond the orbit of Mars. The change in speed and direction during the Mars flyby can also be seen in the trajectory.
Didymos is a small, dark asteroid about 780 m across, too faint to see with a small telescope. When it came relatively close to Earth in 2022, large telescopes watched the debris spreading out after DART's impact. Hera's own results are expected to arrive step by step over its roughly six months of work after arrival.
See this for yourself — replay the moment in the orbit view
FAQ
When does Hera reach the asteroid?
It is due to arrive in November 2026, a month earlier than first planned, after passing Mars in March 2025 to change its speed and direction.
How much did DART change the orbit?
It shortened the time Dimorphos takes to circle Didymos by 32 minutes, from 11 hours 55 minutes to 11 hours 23 minutes.
What are the key things Hera measures?
The size of the impact crater and the mass and inner structure of Dimorphos. From these, scientists calculate how efficiently the impact pushed the asteroid.
Are these asteroids dangerous to Earth?
No. They have no chance of hitting Earth, which is why they were chosen as a safe target for the test.
Related
- DART impact — the day an orbit was changed
- Hera arrives at Didymos
- The 2029 Apophis flyby
- Psyche — asteroid mission
- Hera — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.