Quirks & Quarks·Bob McDonald's blog

Easy does it: the tricky job of landing on an asteroid

A Japanese spacecraft is deploying landers on a tiny asteroid — it's harder than it looks.

A Japanese spacecraft is deploying landers on a tiny asteroid — it's harder than it looks

An artist's rendition of the Hayabusa2 spacecraft which is deploying landers on an asteroid. (Go Miyazaki, cc-by-sa-4.0)

A Japanese robotic spacecraft called Hayabusa2 released the first of three small landers onto an asteroid this week in an attempt to survey its surface. Just getting to an asteroid is difficult, and landing turns out to be an even more challenging task. It takes some very delicate manoeuvring to finesse a touchdown on an ultra-low gravity object like a small asteroid or comet.  

Hayabusa2's target is an asteroid called Ryugu. It's a cube-shaped boulder less than a kilometre across, and is classified as a "near-Earth object." It has an elliptical orbit that crosses Earth's orbit about twice a year. The entire asteroid could fit within a few city blocks.

Asteroid Ryugu images by the ONC-W1. Photograph was taken on June 24, 2018 at around 15:00 JST and shows a section of the camera's wide field of view. Credit*: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST (JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University)

An object that small has almost no gravitational field at all, so if you were to stand on the surface, you would weigh less than the ink in a ballpoint pen. Walking would be impossible because the first step would launch you several metres off the surface and you would float for 10 minutes or so before slowly settling back down.

It would be much easier to jump and fly around like a superhero. In fact, in the original Superman story in 1938, he could leap, not fly, because his body was adapted to the higher gravity of his home planet. You would effectively have the same kind of super-strength on an asteroid. You could easily lift huge boulders over your head and literally run into orbit.

Hollywood often forgets about low gravity. A perfect illustration is in the 1998 movie Armageddon. In it,  Bruce Willis led an intrepid team of miners to an asteroid on a collision course with Earth, intending to blow it up.

I laughed out loud in the theatre during the scene where their space shuttle crash-landed on the asteroid, bouncing and skidding across the surface at high speed, plowing through piles of rock before sliding to a stop. In reality, they would have hit the asteroid once and the wreckage of their ship would have bounced back into space, never to return.

Landing a spacecraft on a small celestial body must be done with an extremely delicate touch to prevent a bounce into oblivion. In 2014, a similar manoeuvre​ foiled the European Rosetta mission that sent the first lander onto the surface of a comet.

This artist's concept of the Rosetta mission's Philae lander on the surface of comet 67P/Churyumov-Gerasimenko, is from an animation showing the upcoming deployment of Philae and its subsequent science operations on the surface of the comet. (ESA)

The lander, named Philae, was equipped with grippers on it's landing legs and a harpoon in its belly. These were supposed to dig into the comet and hold the spacecraft down. But the harpoon failed to fire and Philae made several slow motion bounces off the surface, ending up wedged on its side between a boulder and the side of a ridge.

The two small landers released from Hayabusa2, called MINERVA II1  are only 18 centimetres wide, about the width of your hand with fingers and thumb spread out. They will gently float down to the surface and  make several bounces lasting many minutes each before settling down. Cameras and a suite of other instruments will study the surface.

This computer graphic image provided by the Japan Aerospace Exploration Agency (JAXA) shows two drum-shaped and solar-powered Minerva-II-1 rovers on an asteroid. Japanese unmanned spacecraft Hayabusa2 released two small Minerva-II-1 rovers on the asteroid Ryugu on Friday, Sept. 21, 2018, in a research effort that may provide clues to the origin of the solar system. JAXA said confirmation of the rovers' touchdown has to wait until it receives data from them on Saturday. (JAXA via Associated Press)

Then, when the lander is ready to move, a motor will spin a weight inside the device. This will provide enough momentum to flip the entire lander off the surface so it will "hop" to a new location about 15 metres away (50 ft.). This method of locomotion was chosen because wheels wouldn't get enough traction to work in low gravity.

Hayabusa2 left Earth in 2014 and arrived at the asteroid last June. It will drop a second, identical lander next year and then a third, different device called MASCOT after that, so the asteroid will be visited in three different locations. Finally, the mothership itself will slowly descend and briefly touch the surface with a device to collect a sample and return it to Earth by 2020. It is the most complex mission ever sent to an asteroid.

Meanwhile, an American mission called OSIRIS-Rex, launched in 2016, will rendezvous with an asteroid called Bennu in December. It's meant to return a sample of that asteroid to Earth by 2023. Scientists from both countries will share information.

An artist's conception of the OSIRIS-REx spacecraft at asteroid Bennu. (NASA Goddard Space Flight Centre)

Robots have visited every planet in our solar system, as well as two dwarf planets, Ceres and Pluto, but there are tens of thousands, if not millions of asteroids and comets swarming around the Sun. Even though they are small bodies, they are incredibly old and largely unchanged since the very beginning of our solar system. They are the ancient leftovers that did not become incorporated into the planets, the crumbs from a celestial cake.

By gently touching these small worlds, we are reaching into deep history and gaining an insight into our own origins.

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