After a few weeks of observation, the trajectory of the interstellar comet Borisov is now well established. It is estimated that it should pass close to the sun on December 8, 2019. This passage should be at a distance of 2 astronomical units, that is to say twice the distance between the sun and the Earth. This interstellar object obviously interests many astronomers. It has been closely watched.
The interstellar object Borisov was photographed by Hubble
On October 2, 2019, the Hubble Space Telescope photographed the comet Borisov. This photograph makes it possible to see the halo of dust which surrounds the comet. This is a significant difference from Oumuamua, the first interstellar object that was discovered in 2017. These two examples of interstellar objects are very different, which is rather exciting for future detections of this type of objects. It seems that we must expect everything.
The comet Borisov also teach us that what happens in the solar system seems to be the norm in the galaxy. The comets of our neighbors are similar to ours. Only the trajectory of Borisov allows to determine that it is an interstellar object. Of course, some researchers have tried to locate its origin.
Borisov would originate from the Kruger 60 binary star system
A Polish team tried to trace the trajectory of the comet back in time, to see where it could come from. They think that the interstellar object Borisov comes from a binary star called Kruger 60. Their simulations show that a million years ago, Borisov approached the Kruger 60’s system. In fact, it would have come to 5.7 light years away from this system.
But the speeds involved are more interesting to determine its origin. During this passage near Kruger 60, Borisov would have had a low relative speed compared to the two stars, which perhaps indicates that it is its home system. The binary stars could have very large Oort clouds, maybe several light-years. So even at 5 light-years away, the hypothesis formulated by the Polish team is plausible.
Hard to know exactly where an interstellar object comes from
Trying to determine the origin of an interstellar object is, however, very difficult. The stars are constantly moving relative to each other. Therefore, knowing where the interstellar object Borisov and the star Kruger 60 were a million years ago is only possible with margins of error. We may be able to refine these models a little more as the trajectory of the interstellar comet is better identified.
Meanwhile, Borisov continues to approach the sun. Unless it disintegrates as it passes close to our star, it should remain observable until at least September 2020. After that date, it will move to a new destination.
The interstellar object C/2019 Q4 Borisov was discovered at the end of August
– News of September 17, 2019 –
On November 20, 2017, a team of astronomers published an article announcing the discovery of Oumuamua, an extraordinary object for more than one reason. It had a very elongated shape and a reddish color and had just passed near the Earth. But what really distinguished it was its trajectory. After two weeks of observation, it was verified that its trajectory was very hyperbolic. This means that Umuamua was an interstellar object, the first observed in the solar system. This discovery has led to many speculations about the nature of Oumuamua and the frequency with which the solar system is visited by objects that have not been formed there.
Almost two years later, a new interstellar object was observed, comet C/2019 Q4 Borisov. This object has a very high speed compared to the sun, and its trajectory seems hyperbolic. If its interstellar origin is confirmed, C/2019 Q4 Borisov will be a very interesting object of study. It is still in the approach phase of the sun, which means that we will be able to observe it getting a little closer to the Earth. Its trajectory should bring it a little beyond the orbit of the planet Mars before moving away forever.
C/2019 Q4 Borisov is an active comet, which distinguishes it from Oumuamua. Although C/2019 Q4 Borisov is not yet close to the sun, it has already started degassing. One of the telescopes at the Gemini Observatory in the United States managed to take a picture of C/2019 Q4 Borisov surrounded by its tail. Oumuamua had passed much closer to the sun without any cometary tail, but it had been noticed that it accelerated slightly away from the sun, a phenomenon probably caused by a degassing phenomenon similar to that of a comet.
C/2019 Q4 Borisov is very difficult to observe because the interstellar object has for the moment a position in the sky close to the sun. Its interstellar nature should, however, be confirmed in the days or weeks ahead. Continuing observations should also be able to determine its chemical composition. Comets are usually made up of the raw material of a system, in other words it is a superb opportunity to study the primitive chemistry of a distant planetary system.
However, it is impossible to imagine sending a space probe to explore C/2019 Q4 Borisov. The interstellar object was discovered early compared to Oumuamua, but not early enough. A 2-ton spacecraft powered by a Falcon Heavy rocket could intercept it if it took off in July 2018, more than a year before the discovery of the interstellar object. All we can hope for is to try to catch up with it. Theoretically, a 3 kg CubeSat powered by an SLS should still be able to intercept the interstellar object, but we have to be reasonable. Telescope observations made from the ground or the orbit should already tell us a lot about this interstellar object.
If we want to intercept an interstellar object, we will probably have to wait for a new discovery to react quickly. The European Space Agency is working on a mission called Comet Interceptor, a set of three space probes that could be launched before even having a target. It would be located at the L2 Lagrange point of the Sun-Earth system. When a particularly interesting object will be discovered, for example a new interstellar object, the three space probes will immediately start going on an interception trajectory. The mission is originally intended to study a new comet but we can imagine that ESA might be tempted to target an interstellar object.
The asteroid 2015 BZ509 could come from another system
– News of May 22, 2018 –
Oumuamua is the first interstellar object detected in the solar system. Its shape and color challenged astrophysicists. It will however be difficult to learn more about this object as it moves quickly to a new destination. To one day have the chance to study an interstellar object, it will be necessary to prepare well in advance and to have a space probe fast enough to go to meet it. In addition, these objects will likely be detected late, and a space probe and its space rocket must be ready to leave for a chance to intercept it. But are all interstellar objects fast enough to escape the gravitational pull of our sun ? The 2015 BZ509 was discovered at the end of 2014 by the Pan-STARRS telescope, the same telescope that detected Oumuamua. Its orbit is astonishing : it is in a co-orbital configuration with Jupiter, but retrograde, that is to say that it orbits the sun in resonance with Jupiter, but in the opposite direction of the giant planet, and of almost all known objects in the solar system.
The causes of this retrograde orbit are difficult to identify. In a study that has just been published, a team from the Côte d’Azur observatory in France suggests that this orbit should be attributed to the interstellar origin of 2015 BZ509. Indeed, we model the creation of the solar system with a protoplanetary disk from which planets, moons and asteroids were created. But in the protoplanetary disc all the material would turn in the same direction. The objects created from this protoplanetary disk would thus retain this movement. When we discover an object turning in the opposite direction, it is because it has a particular story. We know about a hundred asteroids in retrograde orbits. This anomaly is most often explained by collisions or gravitational interactions with Jupiter, but the co-orbital configuration of 2015 BZ509 with Jupiter makes it unique.
Using computer simulations, the Côte d’Azur observatory team reproduced the orbital parameters of the asteroid going back in time. Their simulations show that even 4.5 billion years ago, while the solar system was in formation, 2015 BZ509 already had this retrograde orbit resonating with Jupiter. The possible explanations are therefore greatly reduced. The only way the asteroid could have been in this orbit is because it did not form in our solar system. It would have traveled to us. The explanation is plausible because our sun would have formed in the middle of a hundred similar stars. The proximity between the stars would have facilitated this kind of exchange of objects between the young stars. Today, the sun and its brothers are scattered in the galaxy. But 2015 BZ509 could be a testimony of that time. The big advantage with this asteroid is that it is not going anywhere. We can therefore take time to think of a way to confirm or invalidate the interstellar hypothesis. If its extra-solar origin is confirmed, 2015 BZ509 could become a very interesting target to explore. In the meantime, we may be able to identify other asteroids of this type.
New observations of Oumuamua provide information about its past
– News of February 20, 2018 –
In October 2017, the strange object Oumuamua was discovered. It was the first interstellar object observed. But it is not only the source of Oumuamua that intrigues. Its elongated shape and its dark red color make it a very unusual object compared to what we are used to see in our solar system. In a new study, an Irish university team studied the brightness of the object. The team of researchers was able to determine its axis of rotation or rather its axes of rotation. Indeed, unlike the asteroids that we know, Oumuamua’s rotation is very chaotic, which is probably a testimony to the very violent past of the object. This suggests that Oumuamua escaped from its original system following a collision. It will probably take billions of years for Oumuamua to return to a more traditional rotation. The study also shows that the surface of the object would be stained, which means that its composition would experience local variations, which is rather surprising for such a small object.
Oumuamua is moving away from us at high speed. It will become more and more difficult to continue the observations. It is now a question of preparing to observe the next interstellar object. This is a whole new field of study for astronomers. If the interstellar origin of Oumuamua is evident because of its great originality and high speed, other interstellar objects might be more difficult to spot. Some past observations of objects with a hyperbolic trajectory could thus be reinterpreted as interstellar objects. Likewise, a whole population of these objects might have been captured by the gravity of the sun or Jupiter. They would then behave like classic asteroids, so they would be very hard to distinguish from them. The PANSTARRS tool that had observed Oumuamua first has a good chance of discovering other objects. It is monitoring a large part of the sky continuously, which should allow it to identify new asteroids of the main belt, asteroids of gas giants, objects of the Kuiper belt and with some luck some new interstellar objects.
An asteroid of unusual shape passed near the Earth in October
– News of November 21, 2017 –
On October 19, an asteroid 400 meters in diameter was discovered as it passed only 30 million km from Earth. Its hyperbolic trajectory seems to indicate that it does not come from our solar system. This is the first time that an interstellar object has been discovered. It was called Oumuamua. It would have been ejected from the solar system where it was born. Very many asteroids would have the same destiny with each creation of a solar system. Oumuamua could therefore be the first in a long series. The identity of the solar system that ejected this asteroid is not yet sure. Some evoke Vega, a star located 25 light years from our solar system, or the stellar association Karina located between 163 and 277 light years from our sun. Oumuamua made a very long trip before visiting our solar system.
Oumuamua is a dark red object, and has a very elongated shape, a bit like a baguette. This suggests that the object is very dense, probably made up of rocks or metals. In any case, the discovery of this asteroid puts an end to several decades of waiting for astronomers. For a long time they have been supposed to exist but never have been observed yet. Now that the first detection has taken place, the methods should be refined in order to discover more, and perhaps even study them in detail.
The commissioning of the telescope LSST (Large Synoptic Survey Telescope) in 2022 should make it possible to multiply this kind of discoveries. It has been under construction in Chile since 2015. It is composed of three mirrors, including the largest convex mirror in the world. It should be able to observe large parts of the sky. Its 3.2 gigapixel digital sensor will collect a lot of data every night. Its extraordinary abilities will allow it to photograph very regularly all the observable sky from its position. This continuous monitoring of a large part of the sky should allow to discover other interstellar asteroids, and also the asteroids from the solar system.
We can imagine that one day a space probe could intercept an asteroid, but it seems quite difficult. It must indeed be able to detect it a long time in advance because they are very fast objects that only pass once. Current models estimate that such an object passes through the solar system about once a year, at most. We were lucky with Oumuamua because the asteroid passed relatively close to the Earth. To be able to make more observations, it is necessary to be patient and have good instruments.
Image by ESO / M. Kornmesser (http://www.eso.org/public/images/eso1737e/) [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
