Through binoculars, a few nights ago comet NEOWISE looked like a blurry star that had been smeared upward by the artist before the paint was dry.
At 3:45 a.m., the sky was already brightening and almost twilight. Against the deep blue, the comet was a beautiful sight, even to eyes that were still half-asleep.
What is a comet, and why do astronomers find them so interesting?
When our Solar System formed some 4.5 billion years ago, there was a lot of construction material left over. This is sitting out at the edge of the Solar System as a collection of millions or billions of lumps of dust and ice, typically a few kilometres in diameter, except that few, if any of them, are actually spherical in shape.
Occasionally, a collision or near collision between two of these bodies can result in one of them going into a new orbit, taking it into the inner Solar System.
Comet NEOWISE is one of those deflected objects.
In the outer Solar System, far beyond Pluto, it is dark and very cold. Bodies lying out there are in a cosmic deep-freeze, which can preserve them as unchanged pieces of Solar System construction material for billions of years.
It is still extremely difficult to send spacecraft into the outer Solar System. This is one of the reasons we are so interested when a lump of construction material comes into our neighbourhood, which is exactly what is happening now.
The new orbit of one of these deflected objects may take it close to the Sun, maybe closer than Mercury or Venus, and then back out to where it came from, and then back into the inner Solar System again.
The object starts its inward trip as a dark lump of dirty ice, containing trapped organic chemicals and other things from the Solar System's birth cloud.
As it gets closer to the Sun it warms up. The more volatile chemicals start to evaporate and the ice begins to melt. In the vacuum of space the ice converts directly to water vapour. Blasts of vapour erupt through the surface, blowing dust into space.
Light pressure from the Sun and the solar wind blow the vapours and dust outward from the object. This is lit up by the Sun, and that dark, almost invisible lump of dirty ice becomes surrounded by a glowing envelope and trailing a glowing tail millions of kilometres long. It has become a comet, or "hairy star,” one of the most beautiful objects we see in the sky.
In the past, the search for comets has been mostly the province of amateur astronomers. Many of them have had their names attached to the comets, following the tradition of Halley's Comet, Comet Arend-Roland, Hyakutake and Hale-Bopp.
The advent of large-field, survey and search instruments over the last decade or so, deployed on the Earth and in space, means that today the amateur comet hunters face some very strong competition. Most new comets are found using these instruments.
This new comet was discovered on March 27, by the Near Earth Object Wide-Field Survey Explorer, NEOWISE. The instrument's primary purpose is to search for near-Earth asteroids, but it is also a good comet finder.
Comets are usually discovered when they are far from the Sun, before they have got into full production of jets of vapour and dust.
At that time it is hard to tell whether it will only release a little, in which case it will remain hard to see, or if it will produce huge amounts, possibly making a spectacle visible to our unaided eyes.
Comet NEOWISE was widely expected to be a fizzler. It turns out to have become quite a spectacle.
Search for the comet with binoculars between 11 p.m. and midnight by scanning the sky below and either side of the Big Dipper.
Otherwise look in the northeast between 3 and 4 a.m.
- Jupiter and Saturn rise around midnight and Mars in the early hours.
- That searchlight in the east before dawn is Venus. Scan the sky to the left of Venus to find the comet.
- The Moon reaches Last Quarter on the 27th.
https://news.google.com/__i/rss/rd/articles/CBMiS2h0dHBzOi8vd3d3LmNhc3RhbmV0Lm5ldC9uZXdzL1NreXdhdGNoaW5nLzMwNTQ3Ny9IYWlyeS1zdGFyLXBhc3NpbmctdGhyb3VnaNIBAA?oc=5
2020-07-17 13:00:00Z
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