Swarming Satellites Could Autonomous Characterize an Asteroid

An asteroid’s size, shape, and rotational speed are clues to its internal properties and potential resources for mining operations. However, of the more than 20,000 near-Earth asteroids currently known, only a tiny fraction have been sufficiently characterized to estimate those three properties accurately. That is essentially a resource constraint – there aren’t enough dedicated telescopes on Earth to keep track of all the asteroids for long enough to characterize them, and deep space resources, such as the Deep Space Network required for communications outside Earth’s orbit, are already overutilized by other missions. Enter the Autonomous Nanosatellite Swarming (ANS) mission concept, developed by Dr. Simone D’Amico and his colleagues at Stanford’s Space Rendezvous Laboratory. 

The concept behind ANS is relatively simple. A primary “mothership” spacecraft travels to an asteroid, where it deploys several smaller, autonomous nanosatellites upon arrival. These nanosatellites take up positions surrounding the asteroid and, using relatively inexpensive sensor and communications technology, map the asteroid’s features. After observing for some time, they send data back to the mothership, where an algorithm pieces together information similar to a stereo vision system on Earth and calculates the asteroid’s shape, size, and rotational speed.

There are several deeper layers to unpack in the mission, though. Communication is the first one. In ANS, only the mothership communicates back to Earth using a high-gain antenna. The smaller swarming robots all communicate with each other – partly to estimate distances between the different swarming satellites but also to coordinate observations.

Each nanosatellite utilizes only a few relatively inexpensive pieces of hardware, including a star tracker for overall positioning, short range camera (as compared to more expensive lidar systems typically used in asteroid characterization missions), atomic clocks to synchronize timing, and radio frequency communication modules. These components allow for relatively independent operation of each nanosatellite and lower the burden of communication back with Earth – freeing up those deep space communications resources for other critical work.

But the critical component of ANS isn’t so much the hardware—it’s the software, particularly the control and estimation algorithm. Dr. D’Amico and his team describe a technical tool known as an unscented Kalman filter, which allows them to estimate asteroid shape, size, and rotation based on landmarks noticed by each swarming nanosatellite and run through this algorithm.

To demonstrate the effectiveness of that algorithm, the team tested it using a relatively well-characterized asteroid: 443 Eros. That asteroid had the distinction of being both the first near-Earth object ever found, back in 1898, and the first ever visited by a mission – the NEAR mission 100 years later. The NEAR Shoemaker craft that visited 433 Eros also successfully landed on it, another first for humanity. Even with the comparatively simple sensing technology of a quarter century ago, Eros is still one of the most characterized asteroids in the solar system.

The demonstration results clearly showed that the ANS algorithm does its job well. It can coordinate the positioning of the nanosatellites surrounding the asteroid and coalesce their disparate data sets into a coherent picture of the asteroid they are monitoring. And it can do so remotely, with very minimal input from Earth.

For now, that is how far the algorithm has gotten. Several missions, some of which we’ll cover in the near future, further explore the idea of nanosatellite swarms. But ANS itself hasn’t yet been adopted into a formal mission architecture. One day, though, thousands of satellites might be swarming the tens of thousands of small bodies surrounding our home, leading to the first stages of a genuinely off-Earth economy.

Learn More:
NASA – Autonomous Nanosatellite Swarming (ANS) Using Radio Frequency and Optical Navigation
Stacey, Dennison, & D’Amico – Autonomous Asteroid Characterization through Nanosatellite Swarming
UT – What Are Asteroids Made Of?
UT – Water Found on the Surface of an Asteroid

Lead Image:
Artist’ depiction of an ANS mission to Eros.
Credit – Stacey, Dennison, & D’Amico