A vapour trail streaks behind the Chelyabinsk meteor, seen about a minute after the asteroid entered Earth’s atmosphere. (Alex Alishevskikh)

Sky News: Exploding asteroids and a chance to spot Pluto

By Samantha Bell

Happy Asteroid Day! The third annual Asteroid Day is June 30, the 110th anniversary of the Tunguska impact in Eastern Russia. This asteroid/meteor is thought to have been about 40 metres in diameter and exploded in the air with the force of about 15 megatons of TNT, levelling about 2,000 km2 of forest. This airburst remains the largest impact in modern human history.

Another asteroid impact in Russia, the Chelyabinsk impact of Feb. 15, 2013, was by an object of about half the size, and it released energy equivalent to about 1 megaton of TNT. An airburst over Southeast Michigan on Jan. 16, 2018 air was caused by an object that NASA estimates to be about two meters in diameter, and released energy equal to about 100 tons of TNT. The sonic boom was powerful enough to register as a magnitude 2.0 earthquake on US Geological Survey equipment.

Of course, we can go to extremes and recall the Chicxulub impact on the Yucatan Peninsula 66 million years ago, made by an object 10 km across that released an estimated 100 million megatons of energy. Actually, that last one was good for humans as I don’t think that we would be here if the dinosaurs were still around, but I digress.

You can find out more at the Asteroid Day website, which recently added new material, including a 360-degree VR experience of the Tunguska impact.

Speaking of falling objects, we have the Southern Delta Aquariid meteor shower from July 12 to August 23. The shower peaks July 30 with a maximum rate of 25 meteors per hour. Unfortunately, this is only three days after the full moon, so moonlight will block out many of the fainter meteors. To see them, look south-southeast towards Aquarius in the early morning hours. The further south you are the better, as that will bring the radiant higher in the sky (the radiant is the area from which the meteors appear to come from).

I mention the Perseids meteor shower only because it starts July 17 and runs until August 24. It peaks Aug. 12 with between 50 to 80 meteors per hour. These meteors radiate from the constellation of Perseus, so don’t mix them up with the Aquariid meteors. I will talk more about this shower in my next article, but keep an eye out for the occasional Perseid meteor.

Mercury can be found very low in the east during the evening twilight. It will pass very close to the Beehive star cluster on July 3 and 4. The speedy planet will move from the constellation of Cancer the Crab to Leo the Lion on July 14, which will also be when a thin crescent moon passes just above it. This should make it slightly easier to spot. The innermost planet will disappear into the evening twilight about a week later.

Venus appears in the west as evening begins and reigns as the “Evening Star” all summer. This planet will spend July in Leo and will stand just to the upper left of Mercury in the second half of the month, making it a handy aid to spotting the smaller planet. On July 15, a crescent moon will be just to the right of Venus, providing a nice photo opportunity. When Venus and the moon slip towards the horizon look to their left and you will see Jupiter, Saturn, then Mars.

Jupiter rises in the South in the early evening and resides in the Western part of Libra in July. Jupiter is one of the brighter objects in the sky, but will be outshone by both Venus and Mars (the brightest and second brightest “stars” respectively) this month. The best time to catch this gas giant will be early in the evening while it is still high in the sky. It will shrink in size slightly during the month as it moves further away from Earth, but will still be a good telescope or binocular object.

Look slightly east of Jupiter and you will find Saturn residing in Sagittarius. The ringed world will be visible all night, low in the south, and will drift slowly westward in July. The planet rests in a section of sky filled with interesting objects. In early July, Saturn rests just east of the Trifid Nebula (M20) with the brighter Lagoon Nebula (M8) just south of that. The open star cluster M25 lies to the northeast of Saturn and the globular cluster M22 resides just to the southeast. Even with this backdrop, the view of Saturn through a telescope can’t be beaten.

Mars rises in the southeast in the early evening and remains in the sky all night, its red glow outshining Jupiter. Mars reaches opposition (the point where it is opposite the sun from Earth’s point of view) on July 26 around 10 p.m., but it reaches its closest point to Earth on July 31 when it is only 57.6 million km away. This is the best month to view the red planet and it won’t be this good again until 2035.

Mars will spend July in Capricornus, but shines 100 times brighter than any of the sea goat’s stars. It will rise slightly earlier each night until it is rising around sunset at the end of July. A day on Mars lasts 37 minutes longer than on Earth, so if you are looking at the red planet through a telescope you will see the markings appear to shift westward slightly. You should be able to view the entire surface of the planet during the course of the month (provided the weather co-operates). This month’s SkyNews magazine has a great reference map of the Martian surface in the July/August issue. Mars will move slightly westward during the month, a motion called retrograde by astronomers and astrologers (but don’t get the two groups mixed up, it’s messy and painful).

Neptune rises around midnight in the east at the beginning of July and around 10 p.m. by the end of the month. It lurks in the eastern half of Aquarius for all of July. It is fairly dim, so you will need binoculars or a telescope to spot it. For the best viewing, wait until the planet is high in the southern sky, just before dawn. This will minimize the atmospheric distortion as you hunt for the blue-gray disk.

Uranus rises in the southeast during the early morning hours and lingers in southwestern Aries. The best time to spot it will be just before dawn when it is high in the eastern sky. You should be able to spot it with binoculars, but will need a telescope to confirm your sighting of the blue-green disk.

If you have an 8-inch or larger telescope you have a chance of spotting everyone’s favourite dwarf planet: Pluto. This tiny world is very dim and difficult to spot. It will start the month 15 degrees east of Saturn, about the distance between the tip of your index and pinky fingers when fully extended and held at arm’s length. Pluto will travel southwest through Sagittarius a about a 10-degree angle (as measured by my high school math instruments, so take it with a grain of salt). Happy hunting!

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