Artist's conception of an asteroid orbiting the Sun. (Credit: NASA/JPL) Artist's conception of an asteroid orbiting the Sun. (Credit: NASA/JPL)

Asteroids are celestial bodies much smaller than planets. They have irregular shapes and very weak gravity fields. Space missions around an asteroid can be challenging due mainly to the large disturbing forces – such as those coming from the solar radiation pressure and from the asteroid non-spherical gravity field – actuating in the surroundings of the small body. In addition, a close proximity operation around an asteroid requires a high accuracy navigation in order to avoid the spacecraft to impact with the asteroid or to escape to the deep space. Therefore, to put and keep a spacecraft orbiting an asteroid can be an extremely difficult task. But, what makes asteroids so attractive?

Since the discovery of the first asteroid (Ceres) in 1801 by Giuseppe Piazzi, many of other similar objects have been found. In fact, the main asteroid belt (located roughly between the orbits of the planets Mars and Jupiter) contains millions of small bodies. When one of these objects has an orbit less than 7.5 million kilometers, and a diameter higher than 150 m, they are classified as a Potentially Hazardous Asteroid (PHA). However, the risk of collision with the Earth is not the only reason for exploration. The mining process has become an increasingly interesting activity for space agencies and private companies, since valuable minerals, scarce on Earth, can be found in such objects. Furthermore, there is the scientific motivation as well. It is believed that most asteroids still preserve information about the origin of the solar system.

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So far, many spacecraft have visited asteroids and brought meaningful information about those complex objects. However, no one has explored a triple system of asteroids such as that one discovered in 2008 by scientists from the radio astronomy station of Arecibo, in Puerto Rico, and named 2001SN263. Such system is composed of a central body (Alpha), with 1.25 km of radius, and two smaller bodies (Beta and Gamma), with a radius of 0.39 and 0.22 km, respectively. Beta has an orbital period of 6.23 days while Gamma orbits Alpha with a period of 0.69 days. It is notorious that the investigation, from close distances, of such systems can be of high scientific importance. In this sense, a partnership between Brazilian and Russian space institutes has proposed the ASTER mission with the objective of exploring the asteroid 2001SN263.

Several studies involving space maneuvers around asteroids systems have been developed by the astrodynamics group from the Brazilian institute INPE (National Institute for Space Research). In particular, a quite recently published research has shown how to identify the best strategy, in terms of long-term stability and fuel consumption, to execute a close proximity operation around the triple asteroid system 2001SN263. The work has been based on the very recent proposed rotating mass dipole model which considers oblateness effects on the bodies of the asteroid. Some of the fundamental questions answered in that research are: How to avoid a collision with the asteroid or an escape to the deep space using bi-impulsive maneuvers? How the solar radiation pressure affects the dynamics of the spacecraft?

An orbital transfer can be defined as a change in the spacecraft orbital velocity through the application of a velocity increment. In practice, that change is achieved with spacecraft thrusters and is called propulsive maneuver. However, in theoretical analysis, it is usually assumed that the velocity increment is added instantaneously such that the resulting transfer is called impulsive maneuver. In the research developed at INPE, it was considered a bi-impulsive transfer strategy, i.e., a first impulse is applied at a selected point of the natural trajectory around the asteroid system, in order to avoid a non-desired occurrence (as a collision or an escape), and then a second impulse is applied to place the spacecraft back to its initial position. Natural trajectories and bi-impulsive maneuvers for both collision and escape cases. In addition, grid map plots provided in our research has supported the mission designer to estimate the total propellant mass required to achieve a desired orbital lifetime.

The results have also shown that low values of fuel consumption can be achieved considering vehicles with small values of area-to-mass ratio and applying the impulsive maneuvers with the solar radiation force coming from specific directions. Our research has provided valuable support to the team of conceptual studies of the ASTER Brazilian space mission by increasing the understanding of the triple asteroid system 2001SN263.

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These findings are described in the article entitled Analysis of impulsive maneuvers to keep orbits around the asteroid 2001SN263, published in the journal Astrophysics and Space Science. This work was led by Willer G. Santos and their colleagues from the National Institute for Space Research at São José dos Campos, Brazil.