PHYSICS & ASTRONOMY_Asteroids The oddballs of the solar system Small bodies orbiting the Sun are either comets or asteroids – for many years, this was the official line in textbooks. At the Max Planck Institute for Solar System Research in Goettingen, Jessica Agarwal is studying “active asteroids”, small solar system bodies that don’t quite fit into the traditional categories. TEXT THORSTEN DAMBECK hose who cast their gaze to to a distance of 0.14 astronomical units dust that comets release into space is the night sky in the Advent (AU), or around 20 million kilometers. swept away by sublimating ice and is the season between December 4 At this point, the temperature on the ori gin of the periodic meteor streams. and 14 can hope for a special surface of the body, which is just five At any rate, that is what has long been gift: shooting stars. If you kilometers wide, rises to over 700 de- taught in textbooks. extendT the trails of light backwards, grees centigrade, and the radiation Phaethon is different: although it the meteors appear to be coming from pressure from the intensive sunlight too emits dust, it does so in the absence the constellation Gemini, and they can catapult tiny dust particles off the of ice. Asteroids have only been known are therefore known as the Geminids. surface and out into space. about for around 200 years. The first These shooting stars are made up of example, Ceres, was their largest rep- tiny grains of cosmic dust, and Earth is MOST ASTEROIDS ORBIT resentative for a long time but is now in precisely the right position every Ad- IN THE MAIN BELT considered a dwarf planet (see box on vent for this dust to burn up in its at- page 51). These objects have since been mosphere. When it does, it produces “It’s highly unusual for an asteroid cataloged in their hundreds of thou- the fleeting bursts of light that will to be the source of a meteor stream,” sands, with most of them orbiting the make a secret wish come true, as popu- says Jessica Agarwal of the Max Planck Sun in what astronomers call the main lar belief would have it. Institute for Solar System Research. belt. This lies between the two large Taking a more rational view, astron- “Normally, meteor streams come from planets Mars and Jupiter. > omers have long known the origin of comets.” The latter have been known the dust. It comes from Phaethon, an about since ancient times and can An optical illusion! This sequence of images asteroid identified in 1983 that orbits sometimes produce impressive tails of from the Hubble Space Telescope shows the Sun on an elongated elliptical path. gas and dust when they are close to the that the asteroid 288P consists of two parts orbiting one another. In addition, the celes- In the course of each orbit, its distance Sun. Then, the spectacular goings-on in tial body exhibits the key characteristics of from the Sun varies considerably – every the night sky even become a talking a comet – a coma, which envelops the two 1.4 years, Phaethon approaches the Sun point among the general public. The nuclei, and a dust tail. Photos: NASA, ESA, and J. Agarwal (Max Planck Institute for Solar System Research) 48 MaxPlanckResearch 4 | 18 August 22, 2016 September 1, 2016 September 9, 2016 September 20, 2016 Photos: NASA, ESA, and J. Agarwal (Max Planck Institute for Solar System Research) September 29, 2016 Cosmic odd and ends: the dwarf planet Ceres (left) is an asteroid, whereas Hale Bopp belongs to the family of comets. Until a few years ago, astronomers drew a clear distinction between the two classes because of their different features, but the boundaries are becoming increasingly blurred. Asteroids and comets – Jessica Agarwal and which the researchers dated back ed the spot of light to be a large frag- knows her way around both fields, as to the year 2009. The following year, ment from an asteroid collision. An well as the boundary region between they observed the developing tail of X-shaped structure in the dust tail, di- them: “We conduct research at the in- P/2010 A2 for five months using the rectly behind the fragment, makes this terface between astronomy and geo- Hubble Space Telescope. It turned out asteroid a unique specimen. physics,” says the Goettingen-based that it was different from a normal Some of the active asteroids are also astronomer, whose work examines comet’s tail because, since comets emit referred to as main-belt comets. In their physical processes that can have a pro- gas and dust almost continuously while case, the astronomers suspect that the found effect on these small celestial they are near the Sun, they usually have dust emission is driven by water ice or bodies. One focus of the scientist’s work a fan-shaped tail. other volatile substances that subli- is an even more elusive subgroup of the mate and sweep near-surface dust par- minor planets: the so-called active aster- A UNIQUE X-SHAPED ticles away in the process, as happens oids. Phaethon belongs to this particu- STRUCTURE IN THE DUST TAIL in comets. One example of this group lar group of oddballs, whose common of objects is 288P, which follows an el- feature is that they can emit dust. A pa- In contrast, the linear tail of P/2010 A2 liptical path in the outer main belt and per published in 2015 already included suggested that it was formed by a sin- takes 5.3 years to complete a full orbit 18 such specimens, most of which are gle, brief event. On the first detailed of the Sun. 3.0 only a few kilometers in size, with the Hubble photos, the researchers noticed This minor planet has only been notable examples of Scheila and Ceres, a spot of light and estimated the diam- known about since 2006, after the which have estimated diameters of 113 eter of the corresponding object to be Spacewatch telescope in Kitt Peak Na- and 975 kilometers respectively. around 120 meters. “The nucleus al- tional Observatory, Arizona, discovered Asteroids become smaller and smaller most seemed to be detached from the a small, weak spot of light; this tele- due to collisions with other small bodies dust cloud,” says Agarwal, who at that scope is specially designed for small so- in what Agarwal compares to a grind- time was still analyzing the images at lar system bodies. Just five years later, ing process. One example is the colli- the ESA’s research center in Noordwijk, the object was observed during several sion that struck the object P/2010 A2 the Netherlands. The experts interpret- months of activity close the Sun. The Photos: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA (left), E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria (http://www.sternwarte.at)/CC-BY-SA 50 MaxPlanckResearch 4 | 18 PHYSICS & ASTRONOMY_Asteroids The Main Mars Asteroid Belt Sun photos clearly showed a short dust tail ter of gravity; both have a diameter of Earth pointing away from the Sun and an ap- around 1.8 kilometers. The striking Jupiter proximately six-times-longer trail run- thing is their relatively large separation ning along its direction of motion. of approximately 100 kilometers. Even at that time, the long period of ac- The active binary asteroid is a good tivity pointed to the sublimation of ice example of how these bodies can turn as the driving force for dust emission. out to be fascinating objects. They al- September 2016 offered especially low researchers to study – live, so to Rich source: most asteroids orbit the Sun favorable conditions for observing speak – the processes of change they are on trajectories between the planets Mars and 288P, which came as close as 1.45 AU undergoing. 288P probably originates Jupiter, in what is known as the main belt. to Earth, providing a great opportunity from a collision that long ago destroyed for Jessica Agarwal and her colleagues a precursor body with a diameter of to unravel further secrets about their around 10 kilometers. That instant, ing, it is a very young family. A second object of study. High-resolution photos some 7.5 million years ago, marked the collision at a later stage might then taken by the Hubble Space Telescope birth of not only 288P but also a whole have split 288P into two fragments. Or now clarified what previous images had family of asteroids. At least eleven it might already have been a binary ob- only hinted at: 288P is binary! In other members of this group have already ject when it emerged from its violent words, it consists of two separate com- been identified, with their similar or- birth. “Neither option can be ruled ponents of roughly the same size, bits around the Sun betraying their out,” says Jessica Agarwal. “We don’t which orbit about their common cen- common origin. Astronomically speak- know exactly what happened.” > AN ACTIVE GIANT On the scale of the entire solar system, Ceres is only a dwarf, asymmetric hilltop rising from its center – this is the scene of with an average diameter of 975 kilometers and just 0.28% of past cryovolcanic activity. Scientists from the Max Planck the mass of the lightest planet, Mercury. However, it is a gi- Institute for Solar System Research have discovered that salt ant among the active asteroids, most of which are just a few solutions escaped from this location until well into recent kilometers in diameter. Since June 2018, the space probe Dawn geological history.