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Danger near Pluto

Brazilians are involved in assessing risk for the space probe New Horizons

Artist's rendering of the New Horizons probe, which should arrive near Pluto in 2015

Illustration NASAArtist’s rendering of the New Horizons probe, which should arrive near Pluto in 2015Illustration NASA

The group led by researcher Silvia Giuliatti Winter at the Guaratinguetá campus of the Universidade Estadual Paulista (Unesp) has been using computational simulations to explore with increasing detail the possibility of debris—from dust grains to boulders—accumulating in certain regions of space in the vicinity of Pluto and its moons. The New Horizons space probe, designed to study the far reaches of the solar system, is expected to pass through the area in 2015. The work of the Brazilian physicists was the first to call attention to the risk to New Horizons, launched in 2006 by NASA. The spacecraft travels at a speed of 14 kilometers per second and its instruments can be damaged or even destroyed by a collision with a grain of sand.

“The work of Brazilians has been extremely important,” affirms astronomer Harold Weaver, of the Applied Physics Laboratory at Johns Hopkins University, one of the New Horizons project leaders. “We have followed their publications closely.”

Since 2010, the Unesp group has published its findings in a series of articles in the journal Monthly Notices of the Royal Astronomical Society (MNRAS). The latest results, also submitted to MNRAS, were presented at the Pluto Conference in the United States in July, organized by the New Horizons team.

Illustration NASA / Infographic: Ana Paula CamposEverything seemed under control in January, 2006, when NASA launched the space probe on its nine-year journey towards Pluto, reclassified that same year as a dwarf planet. In January 2015, the probe will turn on her eight scientific instruments, including an interplanetary dust detector and an ultrasensitive telescope appropriate for the darkness that reigns in this region of space, 40 times farther from the Sun than the Earth. On July 14, 2015, New Horizons will be at its nearest to Pluto. It will pass between the dwarf planet and its largest moon, Charon, recording images of the unknown surfaces of both celestial bodies with a resolution of up to 100 meters. At least, that was the original plan.

Stable orbits
Things began to become complicated when the Deputy Project Scientist for the New Horizons mission, astronomer Leslie Young of the Southwest Research Institute in Colorado, learned of the Unesp team’s work, presented in 2009 at the annual meeting of the International Astronomical Union, held in Rio de Janeiro. The study showed, for the first time, that what planetary dynamics experts call stable orbit regions exist between Pluto and Charon. These are regions of space where smaller celestial bodies orbiting larger bodies may remain indefinitely. Stable regions tend to accumulate material, sometimes even moons and rings.

And there is no lack of interplanetary material in the Kuiper Belt, the region of the solar system where Pluto and possibly thousands of other dwarf planets and other smaller bodies are located. The Kuiper Belt is home to the remains of the first blocks of rock and ice formed around the Sun billions of years ago.

Winter and her colleagues at Unesp are experts in determining the movement of celestial bodies interacting simultaneously under the force of gravity. They can predict the trajectories of these bodies through computer simulations that take weeks to run. The Pluto-Charon pair was a unique challenge. The dwarf planet and its largest moon are similar in size: Pluto’s diameter is 2,300 kilometers, while Charon’s is 1,200 km. Because their dimensions are similar, they behave differently than other pairs in the Solar System, such as the Earth and the Moon.

057-059_Plutao_210_2Illustration NASA / Infographic: Ana Paula CamposThe search for stable regions is done by determining the trajectory of hypothetical particles with a small mass compared to the masses of Charon and Pluto, configured with different initial conditions for position and velocity. “We gained a sense of where the stable regions are located, and their volume, by observing the orbits of these particles,” explains Winter.

In November 2011, she and researcher Othon Winter, her husband, were invited to attend a special New Horizons team event in Boulder, Colorado. It was a workshop on the risk of the spacecraft colliding with objects near Pluto. The mission’s chief scientist, Alan Stern, then asked them to study the stable regions even further. In the first article, in 2010, the Brazilians had sought regions with stable orbits in the plane formed by the orbits of Pluto and Charon. In the following article, which came out this year, they also analyzed orbits outside that plane and got a better idea of their shape and location.

The stable orbits are concentrated in some bands near Pluto, and others near Charon, and also in a region between the two objects named the sailboat region, due to its shape similar to that of a sailboat, through which New Horizons could pass. Initial estimates suggest that the risk of collision is not negligible, but it needs to be quantified.

New moons
In addition to the theoretical studies of orbital dynamics, the concern with what might be in the path of New Horizons has intensified observations of Pluto, Charon and its smaller, more distant moons Nix and Hydra, each with a diameter of 150 km, discovered in 2005 with the Hubble Space Telescope. In 2011, Astronomer Mark Showalter, of the SETI Institute in the United States, a specialist in working at the resolution limit of images taken by space probes and telescopes, led a campaign to make observations with the Hubble telescope to search for rings around Pluto. Analysis of the images showed no signs of rings, but led to the discovery of two more moons, christened this year: Kerberus and Styx.

The absence of rings corroborates the results of a study published by Silvia Winter’s group this year in MNRAS. Her doctoral student, Pryscilla dos Santos, simulated the formation of rings around Pluto, which would be made of rock and ice grains ejected from Nix and Hydra during collisions with meteorites. They found that despite the fact that Pluto and its moons are far from the sun, solar radiation pressure would be enough to disperse the grains, practically preventing the formation of rings—if they do exist, they are so diffuse that they cannot be seen. Winter points out, however, that other mechanisms not studied, such as the existence of other natural satellites around Pluto, could form rings too faint to be seen with the Hubble, but not by New Horizons.

The discovery of Kerberos was an even more pleasant surprise. That moon, with an estimated diameter of between 5 and 15 km, orbits Pluto in one of the stability regions that the Unesp team predicted existed for bodies of that size, between the orbits of Nix and Hydra. Presented in 2011, this result suggested that more moons could be discovered between Nix and Hydra. But the moon most recently discovered, the fifth, is not there. Its orbit is smaller, near Charon. The orbit of tiny Styx, which is 4-12 km in diameter, is still a mystery. According to Silvia Winter, there is great uncertainty about the mass of the moon, which is needed to calculate its motion. In any case, the discovery of Styx indicates problems for New Horizons. Collisions between interplanetary objects and Styx or smaller, undiscovered moons could spread debris between Pluto and Charon.

In the coming months, it might be necessary to review the damage mitigation plan for New Horizons, which was approved in June. Weaver explains that, after evaluating all relevant information, he and his colleagues concluded that the probability of an impact able to terminate the mission on its original trajectory is less than 0.3%. This is because the probe must pass through an unstable region near Charon. If no new evidence of danger arises, the probe will follow the path set before the risk of collisions was raised by the group at Unesp. Regardless, the NASA team has two emergency plans. One is to reorient the spacecraft to use its communications antenna as a shield against debris. The other is to have the probe approach Pluto at a smaller distance, 2,200 km from its surface, in order to use the atmosphere of the dwarf planet as protection against particles. “The course was originally designed to optimize scientific gains and any change will result in losses,” says Weaver. “Even if there are losses, the mission will revolutionize our understanding of Pluto and the Kuiper Belt.”

1. Orbital dynamics of small bodies (No. 2011/08171-3); Grant mechanism Thematic Project; Coord. Othon Cabo Winter / Unesp; Investment R$560,886.80 (FAPESP).
2. Pluto-Charon binary system dynamics (No. 2009/18262-6); Grant mechanism Doctoral research grant; Recipient Pryscilla Pires dos Santos/Unesp; Investment R$121,831.32 (FAPESP).

Scientific articles
PIRES DOS SANTOS, P.M. et al. Small particles in Pluto’s environment: effects of the solar radiation pressure. Monthly Notices of the Royal Astronomical Society. v. 430. Apr. 2013.
GIULIATTI WINTER, S. M. et al. Stable regions around Pluto. Monthly Notices of the Royal Astronomical Society. v. 430. Apr. 2013.