Hopping on an asteroid

Ken Tapping, October 23, 2018

In the sky this week…

  • Mars is still conspicuous in the southern sky.
  • Saturn lies low in the south and Jupiter very low in the southwest after sunset.
  • The Moon will reach Last Quarter on the 2nd and be New on the 8th.

Last summer a spacecraft arrived at the 900 m diameter asteroid Ryugu, and deployed landers on its surface. The gravity of that small body is far too weak for the landers to run on wheels because there would be no traction, so they hop. This is giving us our best close-up encounter with an asteroid that we have had so far.

There are millions of asteroids orbiting the Sun. The largest are around 1000 km in diameter; we have no firm lower size limit. Most of them orbit the Sun neatly between the orbits of Mars and Jupiter, forming what we refer to as the “asteroid belt”. However, a good number have highly elliptical orbits taking them between the inner and outer Solar System, crossing the paths of some planets, which raises the issue of collisions. Some cross the Earth’s orbit, and during our planet’s 4.5 billion-year history, we have been hit a number of times; one impact helped end the age of the dinosaurs.

When we look at our world, the Moon, or other planets such as Mars and Venus, we see bodies that have changed and evolved over time, making it hard to deduce how things got started. What we need to look at is the basic building material from which the Solar System formed, and also stuff involved in various stages of its development.

The Solar System, like the thousands or more of other star and planet systems we have found so far, formed from the collapse of a cloud of cosmic gas and dust. In the outer reaches of the Solar System, where it is dark and very cold, there remains a lot of the unused construction material. There is one spacecraft out there looking at some of it, but an easier route is to let that material come to us. On occasion, a collision or some other sort of interaction puts a lump of it into an orbit that brings it into the inner Solar System. These lumps become comets. That is why we have dedicated such efforts on getting close looks at comets, getting spacecraft to orbit them and have dropped landers on their surfaces so we can study them really close up.

During the growth of the Solar System, those small lumps of basic building material coagulated into bigger and bigger lumps, ultimately forming the Sun and planets. However, between Mars and Jupiter, where we would expect there to be another planet, there is instead the asteroid belt.

of the lumps of material, stopping their coagulation into a single body. They are a frozen recording of what was happening around the midpoint in planetary formation, where the big lumps would be about to form one bigger body.

There are many interesting questions we have about the formation of our Earth and other planets that the asteroids should help us answer. One of them is about the processes leading to the beginning of life on our world, and possibly on some other bodies in the Solar System. For example, we know that the chemical precursors to our form of life are present in the raw material in the cosmic gas and dust clouds. We also know that planet formation involves high-energy impacts and high temperatures. Young worlds are mostly big balls of lava. Many of those chemical precursors don’t like high temperatures. The asteroids have already undergone a fair amount of that, so are there still any of those chemicals on them? If they survived that far into the process, then the likelihood they could survive to the completion of a new planet is higher. Asteroids are Solar System time capsules with much to tell us about our origins, and we are all ears.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory.

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