Imagine walking on an asteroid: Hera mission to reveal what it's like

"Jump too fast and you might never come down again, because you could exceed the local escape velocity. Plus it would be easy to trigger an avalanche of rocks.”
Amal Jos Chacko
An illustration of Hera, her CubeSats, and their rocky target destination.
An illustration of Hera, her CubeSats, and their rocky target destination.

ESA/Science Office 

The European Space Agency’s (ESA) Hera mission team caught a glimpse of the rocky surface of Dimorphos, a moonlet orbiting the colossal asteroid Didymos. 

Images returned by NASA's DART mission, which successfully impacted Dimorphos and altered its orbit, have provided valuable insights for Hera's upcoming CubeSat landings, according to a press release

"The boulders covering the surface of Dimorphos are much bigger than they might look," revealed Naomi Murdoch, a planetary scientist at ISAE-Supaero, France. Some of these rocks reach sizes as large as houses, measuring around 5-7 meters across. 

These colossal boulders hint at the Dimorphos moonlet's formation possibly involving its parent asteroid Didymos spinning fast enough to eject material. Didymos’ shape resembling a spinning top supports this theory.

Naomi emphasized the difficulty of traversing this rugged terrain, stating, "Moving across these boulders would likely involve much more climbing and jumping than walking. Be careful though- jump too fast and you might never come down again, because you could exceed the escape velocity."

Navigating this low-gravity environment makes it easy to generate significant ground motion, posing a risk of triggering rock avalanches. 

Imagine walking on an asteroid: Hera mission to reveal what it's like
Lockheed Martin's hypothetical 'Plymouth Rock' mission imagined combining two Orion spacecraft for a two-person expedition to a small asteroid.

Sink or Soar: Landing Challenges

Patrick Michel, Hera's Principal Investigator and Director of de Research at Observatoire de la Côte d'Azur highlighted the significance of Dimorphos' surface properties in determining the behavior of landing astronauts

"A lot depends on whether its material is hard or soft, which would determine how high an astronaut might bounce, or else sink," explained Michel. 

Drawing comparisons to NASA's OSIRIS-REx mission on the asteroid Bennu, Michel pointed out that a hard surface could lead to astronauts sinking upon impact. On a harder body, even a mere 6 cm per second of upward motion could propel them into orbit.

Dimorphos— approximately the size of the Great Pyramid of Giza at ~525 feet (160m)— orbits the massive Didymos asteroid measuring about 780 meters across. 

DART's impact with Dimorphos successfully shifted its orbit and blasted an estimated 1,000 tonnes of debris, enough to fill 60 train carriages. 

ESA's Hera mission, scheduled for October 2024, aims to collect close-up data on Dimorphos, including the size of the impact crater and the mineral composition of the asteroid.

Designing for the Ultra-Low Gravity Environment

Hera will deploy two CubeSats, Juventas and Milani. While the former will probe the asteroid’s interior and make gravity field measurements, Milani will carry out mineral prospecting with its hyperspectral imager.

The release velocity of the pair of CubeSats will be carefully controlled to prevent them from escaping the feeble gravity of the asteroid and getting lost in space. In 2005, the MINERVA lander of Japan’s Hayabusa mission was lost when it was deployed in the wrong direction while attempting to land on the Itokawa asteroid.

Human astronauts will use spikes, crampons, and thruster units to glide over the surface, similar to a scuba diver exploring a coral reef.

Naomi warned about the potential dangers while gliding over the surface, cautioning against contact with sharp rocks that could snag spacesuits. “Your weight would shift by about 10-20% depending on where you are on the surface, because of tidal forces from the Didymos parent asteroid,” she explained.

Furthermore, the ever-shifting rotation of Dimorphos around Didymos would cause the local sky to change continuously, posing a potential disorientation risk for astronauts, noted Patrick. 

Juventas and Milani will embark on their primary missions of maneuvering around Dimorphos using cold gas thrusters before landing on the asteroid's surface. These CubeSats are equipped with instruments to gather data independently of their orientation upon landing, providing valuable insights into the weak gravity field of Dimorphos.

As the Hera mission sets its sights on Didymos and Dimorphos, scientists eagerly await results via its inter-satellite links— a tantalizing preview of what human explorers might encounter on their ventures into the rocky landscape of the cosmos.

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