One asteroid at a time? Nah.

Posted by Jake Parks
on Thursday, September 21, 2017

An ambitious new mission aims to use a fleet of nanosatellites to study hundreds of asteroids in one trip. Credit: Finnish Meteorological Institute.

Asteroids are some of the oldest and most undisturbed objects in our solar system. These miniaturized, rocky worlds have been—for the most part—peacefully orbiting the Sun since the formation of the solar system some 4.5 billion years ago.

It’s estimated that over a million of the floating boulders are crowded together in the main asteroid belt, located about halfway between Mars and Jupiter. And every once in a while, one of them gets bumped out of orbit. But, over the course of a few billion years, once in a while happens a lot.

Although an asteroid getting bumped out of its stable orbit can be a huge problem for us, it can also provide us with a great opportunity to learn.In a press release this week from the 2017 European Planetary Science Congress, Dr. Pekka Janhunen said, “Asteroids are very diverse and, to date, we’ve only seen a small number at close range. To understand them better, we need to study a large number in situ. The only way to do this affordably is by using small spacecraft.”

Enter the Asteroid Touring Nanosat Fleet.

According to a study from the Finnish Meteorological Institute, the Asteroid Touring Nanosat Fleet, consisting of 50 tiny spacecraft, could visit over 300 asteroids in just over three years. Each of the 50 nanosats would be equipped with a 4-centimeter telescope and an infrared spectrometer.

A 4-cm telescope may not sound like much, but while parked just around 1,000 kilometers away, it would be capable of resolving the asteroid’s surface down to the scale of about 100 meters. Furthermore, the spectrometer—which analyzes the asteroid’s light spectrum—would aid researchers in determining the asteroid’s mineralogy.

All in all, there is a lot we can learn from just these two tiny instruments. “The nanosats could gather a great deal of information about the asteroids they encounter during their tour,” Janhunen said, "including the overall size and shape, whether there are craters on the surface or dust, whether there are any moons, and whether the asteroids are primitive bodies or a rubble pile. They would also gather data on the chemical composition of surface features, such as whether the spectral signature of water is present.”

Additionally, each nanosat would use an electric solar sail (E-sail) for propulsion, allowing it to visit six or seven asteroids before heading back to Earth to beam down the data. But why do the nanosats need to come back to Earth? Because they are designed to be as small as possible, which means they cannot afford the added weight of a massive antenna capable of long-distance transmissions.

In this diagram of a single-tether E-sail spacecraft, the solar wind imparts momentum to the extremely long tether, rotating it around the systems center of mass. This rotational momentum generates an extremely small amount of thrust for the main spacecraft and provides the ability to change direction. Credit: Janhunen et al.

Like ship sails, E-sails depend on wind. However, this is not the type of wind that is made up of air particles and blows over the oceans. Instead, E-sails depend on solar wind—which is made up of electrically charged particles streaming from the Sun.

The E-sail itself would actually be an extremely long, conductive tether that maintains a positive charge using an electron gun—which, as you may have guessed, sheds negative charge by shooting away electrons. As the positively charged sail is bombarded by solar wind protons, the like charges repel each other, imparting a tiny bit of momentum into the sail.

With the spacecraft attached to one end of the tether, the momentum from the cosmic wind will slowly spin the other end, tracing out a wide cone with the nanosat near its apex. This slowly rotating tether—which completes a revolution about once every 50 minutes—will provide the teeniest bit of thrust to the nanosat. Then, by adjusting the sail’s position relative to the solar wind, the nanosat can adjust both its acceleration and direction.

By using the initial thrust from launch, and by making small corrections to direction and acceleration using E-sails, the nanosat fleet will be able to explore the asteroid belt and return to Earth in just 3.2 years, as shown in this orbital trajectory diagram. Credit: Janhunen et al.

Even with a 20-kilometer tether, a 5-kilogram nanosat would only accelerate at around 1 millimeter per second squared. For reference, Earth’s gravity accelerates a skydiver at 9,800 millimeters per second squared. Although each nanosat will have very little thrust, when you add in the initial boost from launch, it will still be able to cruise out to the asteroid belt and back in just 3.2 years.

“The cost of a conventional, state-of-the-art mission to visit this number of asteroids could run into billions,” said Janhunen. “This mission architecture, using a fleet of nanosats and innovative propulsion, would reduce the cost to just a few hundred thousand Euros per asteroid. Yet the value of the science gathered would be immense.”

Although there are likely millions of asteroids in our solar system, we have still only studied a few over the entire course of human history. So, the idea that in just three short years, we could learn detailed information about over 300 asteroids is truly astounding. The fact that it’s cheap, that’s just icing on the cake.

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