On January 26, 2015, the news featured a headline that repeats almost every year: “Asteroid passes near Earth.” That day, a rocky object named 2004 BL86, measuring 325 m long, came within 1.2 million kilometers of Earth, and then moved away to follow its orbit around the Sun. Astronomers had never seen an asteroid that size get so close, even though the distance was three times larger than that separating the Earth and the Moon. It seems like a safe distance, but it is not. All asteroids over 150 meters long coming within 7.5 million km of the Earth’s orbit are cataloged by the International Astronomical Union as “potentially dangerous.” This sounds alarming, but there is no reason to panic. It just means that there is some risk, however small, that these objects might collide with the Earth in the next few hundred years. A team of astronomers from São Paulo State University (Unesp) has been performing calculations and making unprecedented observations in order to discover what these asteroids are made of and how long they stay in Earth’s vicinity.
The impact of an asteroid the size of 2004 BL86 would release energy equivalent to dozens of atomic bombs. It would not be enough to cause the extinction of life on Earth—to do so, an asteroid would need to be a few kilometers long, similar to that which fell 65 million years ago where the Gulf of Mexico is now and is believed to have made the dinosaurs extinct. Depending on where it strikes, however, an asteroid could destroy entire cities. Even a much smaller celestial object could cause serious damage. In February 2013, an asteroid only 20 meters in diameter exploded 29 km above the city of Chelyabinsk, Russia. No one died during the event, but the shock wave generated by the explosion injured about 1,500 people and damaged thousands of buildings. Some researchers estimate that impacts such as occurred over Chelyabinsk occur one to three times every century, but the fact is that we still know very little about small objects (measuring tens to hundreds of meters), preventing precise calculations.
Looking out into space
Over the last two decades, the United States and some European countries have been investing in programs to observe potentially dangerous asteroids. According to a report published by NASA in September 2014, 867 of the estimated 1,000 asteroids longer than 1 km that pass near Earth have been discovered. Fortunately, none of them is expected to pose a risk to the planet in the coming centuries. The problem, according to the report, is the smaller asteroids, measuring less than 150 meters. Theories and astronomical observations suggest that there are millions of these rocky objects near Earth, but only 10% have been discovered.
Monitoring projects like those funded by NASA are not enough, unfortunately. They scan the entire sky several times a night in order to identify new, potentially dangerous celestial objects, but cannot accurately determine their trajectories. The orbits of these asteroids can only be determined rigorously though observations over longer periods of time, such as those performed by the Impacton project (Near-Earth Asteroid Mapping and Research Initiative) at the Brazilian National Observatory. Impacton researchers designed and built an observatory in Itacuruba, in the scrublands of Pernambuco (see Pesquisa FAPESP Issue No. 156). With a mirror measuring 1 meter in diameter, the Impacton telescope has been operating since 2011, helping to determine the orbit, size and form of asteroids near Earth.
While the Impacton team performs observations, the specialty of Othon Winter and his colleagues at the Unesp Guaratinguetá School of Engineering is theoretical studies based on computer simulations. Winter wants to understand the dynamics of binary and triple asteroids dangerously near Earth. Recently, he and astronomer Rosana de Araújo, a post-doctoral researcher with the Unesp group, calculated how long these systems could roam the Earth’s neighborhood without fragmenting—eventually colliding with the planet in some cases.
International groups that had studied the orbits of the asteroids that circulate between Mercury, Venus, Earth and Mars, the innermost planets in the solar system, had estimated that no small bodies would be able to maintain a stable orbit in this region for more than 10 million years. These small objects include asteroids—which are denser and form closer to the Sun—and comets—which are less dense, and contain more water and volatile chemical elements. Their orbits become destabilized at some point when they near any of these planets. As a result, the small celestial body might collide with the Sun, with the planets or their moons, or be ejected towards the outer solar system.
Questions arose from this finding. If asteroids are expected to remain near Earth for only a few million years, why have most of them not disappeared, given that the solar system has an estimated age of 4.5 billion years? A possible explanation for the existence of so many asteroids near Earth is that some process is constantly re-populating the region.
Most astronomers believe that the majority of asteroids circulating in this region of the solar system came from what is known as the asteroid belt, a huge band between Mars and Jupiter containing half a million asteroids over 500 meters long and countless smaller asteroids. But not just any asteroid from the belt can migrate to Earth’s vicinity. Only the asteroids pertaining to certain groups or families of asteroids with similar trajectories and rocky compositions are launched into the innermost region of the solar system—their orbits feel the effect of the perturbations from the planets Jupiter and Saturn more strongly. These perturbations, together with the radiation of heat these celestial bodies absorb from the sun, cause them to migrate closer to Mars and the influence of the Red Planet can then, in turn, toss them out of the solar system or into Earth’s vicinity.
These objects can be grouped into families based on their surface mineral composition. The surface of 2004 BL86, for example, contains basalt, a rock rich in iron and magnesium and typical of asteroids pertaining to the family of Vesta, the second largest asteroid in the belt. Its composition, however, does not guarantee that 2004 BL86 is a fragment of Vesta. It could have been part of another larger asteroid that broke up.
While the life of a lone asteroid in Earth’s neighborhood is around 10 million years, Araújo and Winter’s study, published in 2014 in the journal Astronomy & Astrophysics, suggests that systems formed by pairs of celestial objects last even less time. Depending on how its orbit relates to Earth’s, a binary asteroid would not exist for more than 2.2 million years.
These calculations are helping to date the origin of one of the strangest celestial bodies ever seen near Earth: the triple asteroid known as 2001 SN263. Involving three bodies, it is believed to have drawn near Earth much more recently.
A special trio
The story of 2001 SN263 brings to mind the alien spacecraft in Arthur Clarke’s science fiction novel Rendezvous with Rama. What appears to be just a new bright spot in the heavens at the start of the book is revealed to be increasingly intriguing as it approaches Earth. When 2001 SN263 was discovered in 2001 by NASA’s Linear monitoring project, it seemed to be just another asteroid over 1 km long close enough to Earth to be classified as almost dangerous. The surprise came in 2008, when the asteroid came closer and pictures from the Arecibo radio telescope in Puerto Rico revealed that it was a system of three related objects: the two smaller bodies—Beta, 1.1 km long, and Gama, 400 m long—orbit the larger object, Alpha, which is 2.8 km long. There is only one other triple asteroid near Earth, 1994CC. But 2001 SN263 is the most peculiar. “Unlike the half dozen known triple asteroid systems in the solar system, 2001 SN263 is the only one in which the three bodies have similar sizes,” explains Winter. “In the other cases, one body in the system is much larger than the other two.”
The origin of most triple systems is a collision. It chips pieces off a larger asteroid and the smaller pieces then orbit the larger piece like two small moons. “There is no simple theory to explain how a triple system with comparable-sized asteroids could be formed,” says Winter. “The impact must have been significant if a large asteroid was broken into three large pieces, but the pieces should not have remained close to one another.”
One possible explanation is that the three asteroids making up 2001 SN263 are the pieces of a larger asteroid that suffered an effect similar to what happened to the Shoemaker-Levy 9 comet. In 1992, the comet’s nucleus of ice and dust passed too closely to the planet Jupiter and the planet’s gravitational pull caused the comet’s body to fall apart into nearly two dozen fragments. “It’s a possibility, but it could only have occurred if the structure of the asteroid that originated the 2001 SN263 system had been fragile and porous, like a comet,” explains Winter. “It’s what we call the ‘rubble pile’ type of asteroid, a cluster of loose rocks with lots of empty space between them.”
2001 SN263 is the target chosen by Winter for a first interplanetary space mission fully planned in Brazil. The idea behind the Aster Mission, as the project became known, was born of an informal challenge that Winter made to his colleague Elbert Macau, an aerospace engineer at the National Institute for Space Research (INPE), after the discovery that 2001 SN263 was a triple system. The project involves about 50 researchers from Brazilian institutions, including USP, Unesp, Unicamp, UFABC, INPE and the Mauá School of Engineering, in addition to collaboration with researchers from the Russian Space Research Institute. Its objective is to send a small space probe carrying equipment for two additional experiments—one in space geophysics and the other in astrobiology—all with Brazilian technology, to the asteroids in the 2001 SN263 system.
The probe will carry three instruments to investigate the surface of the asteroids: a high-resolution camera, an infrared spectrograph to analyze the composition of its minerals, and a laser to measure distances. Then, if all goes well, the probe will approach the asteroid Alpha, falling onto its surface and making further observations. “It is a concrete project, totally doable,” says Winter. Despite this, the mission still lacks funding. The cost is estimated at $40 million, a small amount compared to European or United States space missions. The Aster Mission team hopes to obtain these funds soon in order to take advantage of 2001 SN263’s closest approach to Earth, in 2022. After that, they would have to wait another three years, until the asteroid completes another orbit around the Sun and nears Earth again.
Even if the mission does not take place, Winter has already made some important findings in preparation for it. In 2010 he hosted Italian astronomer Davide Perna, from the Paris Observatory, at Unesp Guaratinguetá. Perna is a specialist in near-Earth asteroids and one of the researchers involved in the NEOShield project, financed by the European Commission, whose goal is to assess humanity’s options in deflecting an asteroid on a collision course with Earth. Perna led the work of a group of researchers from European institutions, the Brazilian National Observatory and Unesp that observed 2001 SN263 with the VLT telescope at the European Southern Observatory in Cerro Paranal, Chile. The team obtained VLT time on an emergency basis because the asteroids will only be visible again using that telescope many years in the future.
And 2001 SN263 surprised everyone once again. The analysis of the observations, published in 2014 in Astronomy & Astrophysics, found that the surface of the asteroids in this system are the darkest blue color so far observed in a small body in the solar system. The researchers suggest that this color is a strong indication that these asteroids are rich in organic matter and minerals containing water. “2001 SN263 appears to be made of very old material remaining from the formation of the first rocky bodies in the solar system 4 billion years ago,” says Winter.
This work also suggests that the asteroids in the 2001 SN263 system are made of grains of varying size and composition, which supports the idea that they are “rubble piles.” Studies from the NEOShield project indicate that asteroids of this type would be among the most dangerous, because even a rocket with a nuclear warhead would not be able to deflect it if it were on a collision course with Earth. Less dense, the asteroid would contain gases capable of absorbing the energy from the explosion without breaking apart or changing its route. “The success of methods to divert asteroids on a collision course with Earth depends on knowing what they are made of, their rigidity and porosity,” explains Winter. Hence the importance of missions such as Aster, or the NASA Osiris-ReX mission, to be launched in 2016 to visit a potentially dangerous asteroid—Bennu—whose composition appears to be similar to that of 2001 SN263. “If 2001 SN263 is indeed three fragments of a larger object that broke up, the Aster Mission could obtain unique information,” says Winter. “It would be like observing the inside of a larger asteroid, something which has never been done.”
Orbital dynamics of minor bodies (No. 11/08171-3); Grant Mechanism Thematic Project; Principal investigator Othon Cabo Winter (Unesp/Guaratinguetá); Investment R$560,886.80 (FAPESP).
PERNA, D. et al. The triple near-Earth asteroid (153591) 2001 SN263: an ultra-blue, primitive target for the Aster space mission. Astronomy & Astrophysics. V. 568. Aug. 15, 2014.
ARAÚJO, R. A. N. and WINTER, O. C. Near-Earth asteroid binaries in close encounters with the Earth. Astronomy & Astrophysics. V. 566. June 2, 2014.