It was only two years ago that the first confirmed interstellar object was identified passing through the solar system. The fact that the asteroid-like object ‘Oumuamua appeared to be very elongated – about 10 times as long as it was wide – made for some great science fiction speculation. In spite of the interesting theories that this might have been some sort of alien spaceship, there has never been any convincing evidence that it was anything more than just a big rock passing through the solar system.
This month our solar system has its second confirmed visiting object – comet 2I/Borisov. Named after its discoverer, it made its closest approach to the sun (somewhat outside the orbit of Mars) on Dec. 8. It will make its closest approach to the Earth on Dec. 28. Because it is only barely in what is considered the inner solar system, you would need at least a 10-inch-diameter telescope to see it.
These two objects are identified as originating outside the solar system because of their speed. They are just going too fast to be in orbit around the sun. Comets that start in the Oort cloud, thousands of times farther from the sun than the outer planets, have very nearly this speed when they approach the sun. They get moving that fast simply from falling toward the sun, accelerating all the way in. If their path doesn’t hit the sun, the sun’s gravity will whip it around and send it back in the same direction it came from. The part of the orbit we can see is shaped like a parabola.
However, the interstellar objects have speeds that are too high to be accounted for only by falling. The sun’s gravity will deflect their path, but they will leave the solar system in a very different direction than they entered.
The fact that it took so long to find the first interstellar asteroid or comet doesn’t mean these things haven’t been there all along. It just means they are hard to see and our technology has finally allowed us to start finding them. Equipment available to today’s amateur astronomers is far more sensitive than even the best professional equipment was only a few decades ago. Some astronomers have estimated there are always thousands of these visiting objects, but they have been too hard to detect. I expect that many more will be found in the next few years, and I hope they can help us learn more about the galactic environment surrounding our solar system.
This monthHello, Venus; goodbye, Jupiter. In December, Venus becomes much more prominent as the evening star. Seen in the western sky at sunset, it will be the brightest thing in the sky for the next few months. Meanwhile, Jupiter is moving behind the sun and will become visible again in the morning sky by the end of January.
Saturn tonight (Saturday) is about 4 degrees to the right and a bit below Venus. We will still be able to see Saturn right after dark for another month or so, but Venus will be getting farther away and higher in the sky each night.
The Geminid meteor shower is often one of the best of the year. This shower lasts from Dec. 7 to Dec. 17, with the peak on the evening of Dec. 13. Unfortunately, the moon was full on Dec. 12, so the still-almost-full moon will wash out many of the dimmer meteors. Even if the night is brightly lit from the moon on snow, some of the meteors in the Geminid shower should be bright enough to see as long as there are no clouds.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at firstname.lastname@example.org.