The Chronicle Herald (Provincial)
How much do you know about our interstellar visitors?
Interstellar visitors often pay our solar system a visit.
No, I'm not talking about little green men from Mars or an alien race from another dimension popularized in sci-fi comics and movies, but rather those celestial objects primarily asteroids - that have entered our solar system from interstellar space, the vast, distant regions between the stars.
There are hundreds of thousands of asteroids orbiting the neighbourhood of our solar system: debris left over from when the planets formed some 4.5 billion years ago, from the protoplanetary disk of dust and gas surrounding our newly-formed sun.
The vast majority of these asteroids are concentrated in either the Main Asteroid Belt (between Mars and Jupiter), or the Kuiper Belt, a vast circumstellar disk of dust, gas, small planetesimals and asteroids out beyond the orbit of Neptune.
Asteroids are initially sorted into two distinct, though somewhat over-lapping, categories. The first, NEOS (Near Earth Objects), are asteroids whose orbit crosses that of Mars, or whose orbit lies within that of Mars. There are some 2,500+ NEOS currently catalogued and being monitored by NASA, the majority of which are under one kilometre wide. The second category, PHOS (Potentially Hazardous Objects) is reserved for those NEOS whose orbit has the potential to bring them within less than 0.05 AU (approximately 7.4 million kms or 19.5x lunar distance) of Earth.
The majority of the known PHOS are asteroids (currently 2,000+, 157 of which are more than one kilometre in size), although there are a few comets. If their projected orbit within the next 100 years has them possibly impacting Earth, they become of utmost concern to astronomers and global governments due to the potential, if very large, to cause cataclysmic damage on a massive scale, possibly to a civilization-ending extent. These get listed on NASA'S Sentry Risk Table for special monitoring; to date, there are under 50 PHOS on the list.
However, some of these celestial travellers visiting our solar system are merely tourists, just passing through our part of the galaxy bound for a celestial vacation elsewhere.
In 2017, the first interstellar object, Oumuamua (Hawaiian for "a messenger from afar arriving first") passed through our solar system on its journey through space. Studies of the asteroid, as it swept past Earth, suggest it was a shard of a planet that fragmented due to the tidal forces of its distant host star.
Comet Q4 Borisov was the next interstellar tourist to visit our solar system in 2019. Analysis of the chemical composition of the comet's coma/ halo, as it hurled inward to dive around our sun last December, indicate it likely originated around a distant red dwarf star somewhere in our galaxy before being cast outward towards our solar system.
Our most recent visitor was 1998 OR2, a two to threekilometre-wide asteroid, which zipped past our planet Wednesday, April 29. Astronomers had been following this asteroid for over 20 years and knew its trajectory would safely carry it some 6.3 million kms (about 16x lunar distances) beyond our planet. Hopes for a naked-eye comet were dashed in March when the comet fragmented into numerous pieces and faded in magnitude. What remains of the asteroid is now continuing its celestial voyage to another port of call.
Not all interstellar objects simply pay a quick visit before moving on. In 2014, BZ509, a three-kilometer-wide asteroid, the first object known to have an extra-solar (outside our solar system) origin, became the first such object to become a permanent member of our solar system. This asteroid currently orbits our sun within the orbit of Jupiter (although with a retrograde and highly eccentric orbit relative to Jupiter's). Astronomers recently discovered that, in the distant past - around the time that our solar system was taking shape - 19 small interstellar asteroids liked the climate and scenery of our solar system so much that they decided to stay, taking up residency in the suburbs between Jupiter and Neptune. These asteroids belong to a group of objects called Centaurs (named for the wild, untamed half-man, half-horse creatures of Greek mythology), small rocky bodies orbiting between the Trojan asteroids (each named for a figure in the famous story of the Trojan War) that share Jupiter's orbit around the sun and the distant Kuiper Belt. They are thought to have been asteroids from a distant planetary system captured by Jupiter's gravity.
WHAT TO WATCH FOR
Mercury, still very close to the sun, cannot be seen at present. On May 4, it reached superior conjunction with the sun, and on May 10, it's at perihelion.
Jupiter (magnitude -2.2) rises in the southeastern sky shortly before 2 a.m., reaching about 22 degrees above the southern horizon, before fading from view around 5:30 a.m.
Saturn (magnitude +0.8) is next up in the southeastern pre-dawn sky shortly after 2 a.m., fading from view as dawn brightens the eastern sky around 5 a.m.
Mars (magnitude +0.4) makes an appearance in the southeastern sky shortly after 3 a.m., before fading from view shortly after 5.a.m.
Venus, still our "evening star," is visible high in the southwestern sky