The European Space Agency’s (ESA) Hera mission, which will travel to asteroid Didymos and its moon Dimorphos as a follow-up to NASA’s Double Asteroid Redirection Test (DART) mission, is set to launch atop a SpaceX Falcon 9 rocket. This flight, the 95th Falcon flight of 2024, will also be the first Falcon 9 mission to fly after the second stage deorbit burn anomaly that occurred during the Crew-9 mission on Sept. 28.
Hera is currently scheduled to launch on Monday, Oct. 7, at 10:52 AM EDT (14:52 UTC) from Space Launch Complex 40 (SLC-40) at the Cape Canaveral Space Force Station (CCSFS) in Florida. With the formation of Hurricane Milton in the Gulf of Mexico, however, the current weather forecast for launch is 15 percent favorable.
ESA’s Hera spacecraft mounted atop its Falcon 9 before being encapsulated in its fairing. (Credit: ESA)
Following the Crew-9 second stage anomaly, the Federal Aviation Administration (FAA) began a mishap investigation into the anomaly with the second stage. Initially, it was believed Hera would be unable to launch until the investigation was completed, but the FAA recently granted SpaceX the license needed to only launch the Hera mission. The license was granted since the Falcon 9 second stage being flown with Hera will not perform a deorbit burn.
CCSFS and the nearby Patrick Space Force Base are currently under HURCON 5 status, which means that surface winds greater than 50 knots could reach the area within 96 hours. Hurricane Milton, currently located in the western Gulf of Mexico, is expected to strengthen significantly before making landfall on the west coast of Florida later this week and subsequently moving across the rest of the Florida peninsula, which would bring the storm near or directly over the Cape.
The booster, B1061-23, will be expended due to Hera’s mission performance requirements. Hera will be deployed at a velocity of 5.9 km/s, the fastest escape velocity mission flown by a Falcon family rocket to date. This record is likely to be brief, though, as the launch of the Europa Clipper mission atop a Falcon Heavy is expected to be launched later this month. Europa Clipper was initially expected to launch next week on Oct. 10 but has since been delayed to late October due to Hurricane Milton.
Hera’s launch window lasts from Oct. 7 to Oct. 27, with one instantaneous window available each day. NASA and ESA have agreed to stand down from launching Hera for 48 hours before any Europa Clipper launch attempt. If the Hera mission does not fly by Oct. 27, it will need to wait two more years before Earth and the Didymos system are in the ideal orbital positions for another launch.
The 1,081 kg Hera spacecraft and its two accompanying cubesats were to originally fly aboard an Ariane 6, but due to delays with the Ariane 6 program, ESA was forced to switch the mission to a Falcon 9. After its launch atop a Falcon 9, Hera will coast through interplanetary space before entering into orbit around the Didymos and, subsequently, Dimorphos in December 2026.
Once Hera is launched on its deep space trajectory, it will deploy its two five-meter-long solar panels starting immediately after spacecraft separation. Given that Hera will be further out from the Sun than Earth for most of its mission, these solar panels need to be large and capable of collecting adequate amounts of sunlight.
Once safely commissioned and en route to Didymos, a course-correction maneuver will be conducted in November to ensure Hera is on the correct orbital trajectory around the Sun. Before arriving at Didymos, Hera will perform a flyby of Mars in March 2025. During the flyby, Hera will study the Martian moons Phobos and Deimos, coming within 1,000 km of Deimos’ surface, with some of its instruments, and use the planet’s gravity to direct it on a path to the Didymos system.
In February 2026, another course-correction maneuver will place the probe on its final trajectory to Didymos. Hera is scheduled to rendezvous with the Didymos system in October 2026 and will be maneuvered into an orbit around the asteroid by December. Within four weeks of being in orbit, the spacecraft will release the Juventas and Milani cubesats for their missions around Dimorphos.
Hubble Space Telescope and James Webb Space Telescope images of the ejecta from DART’s impact on Dimorphos. (Credit: NASA)
Once Hera arrives at Didymos, the spacecraft’s 11 science instruments will study Didymos and Dimorphos in great detail, while the two cubesats conduct their own observations of Dimorphos and DART’s impact location. DART impacted Dimorphos on Sept. 26, 2022, and reduced its orbital period around Didymos from 11 hours and 55 minutes to around 11 hours and 22 minutes.
DART’s impact also made Dimorphos’ orbit non-circular and released a large cloud of debris from the asteroid that extended 10,000 km into space. The impact and debris cloud was observed by the Italian LICIACube satellite, which was carried to Dimorphos with DART, and observatories on Earth and in space.
One of the primary objectives of the Hera mission is to observe and investigate DART’s impact location and determine the extent of the spacecraft’s “damage” to the asteroid. Hera is also designed to quantify the efficiency of the momentum transfer from DART’s impact and remove uncertainty — which is currently estimated to be around 10 percent — from the measurements of Dimorphos’ post-impact orbit around Didymos.
The Hera mission is also tasked with measuring the exact mass of Dimorphos as well as its internal structure. The data gathered will be used to validate computer models of DART’s impact, allowing for greater confidence in using DART’s deflection method to divert the course of a dangerous asteroid headed for Earth.
Image of the Didymos system from DART. Dimorphos is to the lower right of Didymos. (Credit: NASA)
What’s more, Hera will gather similar data for Dimorphos’ 780-meter diameter parent asteroid Didymos, and there is a possibility of Hera performing a landing on one of Didymos’ poles at the end of its mission.
Didymos takes just two hours and 15 minutes to complete one rotation around its axis. This fast spin rate is thought to be caused by differential warming across the asteroid’s surface, which causes material to leave the surface and impart small amounts of thrust in the process — a phenomenon known as the Yarkovsky-O’Keefe-Radzievskii-Paddack effect.
Hera and the two cubesats incorporate a number of advanced technologies. All three spacecraft will be capable of autonomous navigation using visible-light cameras and star trackers. Additionally, Hera will have a laser altimeter, while the two cubesats will incorporate LIDAR into their autonomous navigation abilities. Hera is designed to safely maneuver up to 200 m from the surface of Dimorphos and can image the surface of the asteroid with a resolution of two centimeters.
The Juventas and Milani cubesats will use radio-based inter-satellite links to communicate with Hera, which will send their data to Earth. Juventas is equipped with a synthetic aperture radar — the smallest radar ever flown on a spacecraft — which will map the interiors of Didymos and Dimorphos.
Artist’s impression of the Juventas CubeSat near Dimorphos. (Credit: ESA)
The Milani cubesat is equipped with a spectrographic imager and will be able to image both Didymos and Dimorphos in wavelengths invisible to the human eye. Both cubesats will land on Dimorphos at the end of their missions, and it is hoped Juventas will be able to operate for one terrestrial day, or roughly two orbits of Dimorphos around Didymos, to gather data on the local gravity field around the asteroids.
Didymos is the fastest spinning object to ever be visited by a spacecraft, while Dimorphos is the smallest asteroid to ever be visited. The Didymos system is not a current hazard to Earth, but there are an estimated 30,000 asteroids that are similar in size to Dimorphos that orbit in close proximity to Earth. A majority of these have not yet been discovered, and these asteroids are large enough to destroy entire cities.
The Hera mission is one of several planned by ESA to study small bodies in the Solar System in the coming years. ESA has a storied history of flying spacecraft to small bodies, starting with the Giotto mission to observe Halley’s Comet, which launched in 1985. The Rosetta mission, which launched in 2004, orbited the comet Churyumov-Gerasimenko, with its Philae lander touching down on the comet’s surface.
The lessons learned from these missions, including orbital techniques and technology derived from the Rosetta mission, have been incorporated into the Hera spacecraft. The Hera mission also features contributions from many European countries, Japan, and NASA. Japan is contributing a thermal imager, with NASA selecting 12 scientists to participate in the mission.
The Hera mission will be the first repeat visit to an asteroid by a different spacecraft. Future missions to the potentially hazardous asteroid 99942 Apophis, already set to be visited by the repurposed OSIRIS-APEX spacecraft, and other bodies are in the works.
(Lead image: The Hera spacecraft being encapsulated inside Falcon 9’s payload fairing. Credit: SpaceX)
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