We are about to visit a small planet in the asteroid belt for the first time

As the world awaits Pluto’s first visit in July, another space mission will reach a strange world much closer to home. And months earlier.

A little robot named Dawn is nearing the end of its seven-year, 3.1 billion-mile journey into the depths of the asteroid belt. Pulling ions from behind and stealing some Martian orbital energy in 2007, Dawn is advancing at around 450 mph toward the largest unexplored object between the Sun and Pluto.

In March, we will arrive in Ceres. It’s the largest thing in the asteroid belt, with a diameter of 590 miles, an area four times the size of Texas, and a mass about a fourteenth that of Pluto. Although it is technically an asteroid and dwarf planet, some scientists consider Ceres to be a real planet, the smallest in our solar system.

Regardless of its classification, we know surprisingly little about it. Powerful and sophisticated telescopes like Hubble can only glean clues as to what Dawn will see when she orbiting Ceres. A brown, cratered and clayey surface is likely. But it’s possible that Dawn will spot more exciting things from above, like an atmosphere or “cryovolcanoes” spitting out frozen water.

“Ceres is going to be a really cool item when we get there,” predicts Mark Sykes, CEO of the Planetary Science Institute and scientist of the Dawn mission. Because Ceres contains a lot of water ice near its surface, periodic defrosting may have softened the characteristics of the planetoid, erasing its oldest craters. “Maybe we’ll have a Salvador Dali planet, where everything is kind of melted,” says Sykes.

Other members of the mission are trying to keep an open mind on the larger question mark between Mars and Jupiter, and possibly the entire solar system.

“What is the nature of this alien world? What does it look like, what is it made of, how does it work? Asks Marc Rayman, Dawn’s chief engineer. “Whatever we find, there will be unexpected phenomena out there. There will be structures or features that people haven’t thought of.

A planet in the asteroid belt?


This image, taken by the Hubble Telescope in 2003 and 2004, is one of the sharpest images we have of Ceres. When the Dawn spacecraft gets there this spring, we’ll finally know what the planetoid really looks like.

In the late 1700s, astronomers wondered about the great gap between Mars and Jupiter. Bode’s law predicted that a planet should be there, and in 1801, scientists finally found one… sort of. For half a century, Ceres has been classified as a planet. Then, as telescopes improved, scientists found a number of smaller objects in a similar orbit. Collectively, these objects became known as the asteroid belt, and Ceres was reclassified as an asteroid.

The identity of Ceres was again called into question in 2006, when scientists debated the status of Pluto and the definition of a “planet”. Planetary scientists like Sykes have argued for the name to apply to any object that has enough gravity to be round. The International Astronomical Union, however, has defined a planet as something with enough gravity to wipe everything out of orbit. Pluto failed this test. And Ceres too.

“That would have been cool, because if Pluto was a planet, then Ceres should be too,” says Sara seager, planetologist at the Massachusetts Institute of Technology.

Ceres is, in a sense, a big time machine, round and muddy.

Researchers are still wondering how to define a planet, but most agree that Ceres is probably a protoplanet, that is, an embryonic planet that froze early in its development. By visiting Ceres, the Dawn mission hopes to gain a better understanding of how planets form and what conditions were like in the early days of the solar system. It’s, in a sense, a big time machine, round and muddy.

The Dawn mission team can’t wait to see how Ceres compares to Vesta, another large asteroid / small protoplanet, which Dawn visited in 2011. Vesta is dry and rocky, like the inner planets of the solar system, while Ceres is more like icy objects. of the outer solar system. Studying the two could reveal the processes that determine the fate of an infantile planet.

Closeup of Vesta

The Dawn mission visited asteroid Vesta in 2011 and 2012, returning a lot of data and stunning images.

The search for life

Beneath its surface, Ceres can harbor a liquid ocean. This is exciting for biologists who seek life on other worlds, because on Earth at least, wherever there is water, there is life.

Astronomers working on the Herschel Space Telescope in January 2014 reported water vapor detection in a few plots around the mid-latitudes of Ceres. Something sprayed water into the air in very small but detectable amounts. No one knows what the source is yet – it could be annoying, like ice sublimating into water vapor. But the universe is full of surprises, if we are to believe the steam jets of Enceladus and the sulfur-spitting volcanoes of Io. Ceres could have cryovolcanoes spouting out of heat-molten water from radioactive decay deep below the planetoid’s surface.

Dawn is currently the best and the only way to find out more.

“It certainly makes Ceres interesting from an astrobiological point of view,” says NASA planetologist Chris McKay, which is not affiliated with the Dawn mission.

Ceres could also harbor many organic molecules. Combined with water, radioactive heat, and other potential energy sources, including the Sun, Ceres is about as attractive a destination as an astrobiologist might hope. These three elements – organic molecules, water and energy – are considered the crucial ingredients necessary for the evolution of life.

“Small planets like Ceres and Pluto can be essential in understanding where life comes from and where life might be in the universe.”

The chances that life already exists on Ceres are probably very, very slim. Even though Ceres is a lifeless rock, the lessons to be learned would be invaluable, says Rayman. On a world with all the ingredients for life, for example, why could life not to evolve? And what does that say about the chances of life elsewhere in the universe?

While microbial beings live in the inner ocean of Ceres, Dawn is not equipped to detect them. But the spacecraft’s observations could influence NASA to select future missions capable of detecting life.

“If there’s water below the surface and it’s communicating with the surface through these cryovolcanoes,” said Sykes, “maybe we could send another mission to pick up water and ask: “Are there any dead insects here?” “”

Small and frozen worlds like Ceres are far more numerous than Earth. In fact, these are the most common types of planet-like bodies in our solar system and possibly the universe. “If the ice melts, would life evolve in this region? Sykes asks. “Could they be incubators of life in the universe? I think these little planets, like Ceres and Pluto, can be essential in understanding where life comes from and where life might be in the universe.

What awaits us

Are we there already?

An illustration of the Dawn spacecraft arriving in Ceres.

The Dawn spacecraft was launched on September 27, 2007. Since then, it has moved in a spiral path out of the solar system. He reached the protoplanet Vesta in July 2011 and studied it for fourteen months. Now the spaceship is about to reach its ultimate destination.

Here is the plan once arrived:

  • March 6, 2015: Dawn arrives in orbit around Ceres. (Note: Scientists don’t know the exact nature of Ceres’ gravity, and since gravity will affect the spacecraft’s travel time, all subsequent dates are estimated.)

  • April 23, 2015: The spacecraft initially orbits 8,400 miles above Ceres. Here he begins to take photos, analyze the planetoid’s surface, and search for evidence of water vapor.

  • June 10, 2015: Dawn descends to an altitude of 2,730 miles for further examination.

  • August 15, 2015: From an elevation of 910 miles, Dawn extensively maps the surface of Ceres. It takes photos from different angles, allowing researchers on Earth to gradually build a 3D view of the landscape.

  • December 20, 2015: Dawn is moving to its closest orbit, just 230 miles above the surface, almost 40 times closer than its first orbit. “It will be like looking at a soccer ball seven inches from your eyes,” says Rayman. From there, Dawn makes more detailed observations. It maps Ceres’ gravity field, which should reveal how its mass is distributed within it (and possibly reveal an underground ocean). The spacecraft will also use gamma-ray and neutron spectrometers, which measure signature emissions from different elements in the crust and interior of Ceres. This will tell us what Ceres is made of.

  • June 2016: The mission ends. While Dawn’s fuel and solar panels could keep it alive for a few more years, the spacecraft could continue to orbit Ceres for hundreds of years.

Popular science covers the latest news from Dawn and Ceres throughout the mission. Follow our full coverage at popsci.com/ceres

Correction, January 23, 2015: A previous version of this article listed future chronological dates as 2014. As it is already 2015, that would be impossible. We regret the error.

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