Prime viewing time for Lyrid meteors this week
The Lyrid meteors (radiant in Lyra — the Lyre or Harp) are visible annually between April 10-30, with the peak usually arriving during the pre-dawn hours on April 22 and 23.
One of the oldest known meteor showers, the Lyrids were observed by the ancient Chinese 2,500 years ago. The Lyrids are debris left by Comet Thatcher (C/1861 G1), a comet with a 415-year orbital period; last seen in 1861, Comet Thatcher will not be visible again until 2276.
Thatcher is what is known as a "long-period" comet because its orbital period is greater than 200 years. Comets whose orbital period is less than 20 years are referred to as "short-period" comets, while those between 20 and 200 years are known as "intermediate-period" comets.
WHAT ARE METOERS?
While most meteors originate from comets that have swung in around the sun, streaming rocky debris behind them before heading back to outer space, some meteor showers can originate from asteroid debris.
Meteor showers have three stages, depending on where the shower particles are relative to the Earth and its atmosphere. "Meteoroids" is the name given to the particles when they are traveling through space outside the Earth's atmosphere (think "o" in "oid" for "outside"); "Meteors" when the particulars pass through the Earth's atmosphere; and "meteorites" when the particles strike the Earth's surface (think "i" in "ite" for "inside the ground").
It is only when the meteors pass through the Earth's atmosphere and burn up that we see them as "shooting stars." The burning up is caused when the air ahead of the travelling meteor particle gets compressed at such a high rate that it forms a super-heated shockwave which, in turn, causes the dust or rock particle to glow, and, in most cases, disintegrate (what we see as the flash of the meteor). Some particles are large enough to survive the atmospheric entry and make it to the Earth's surface, where they may be found as meteorites. The larger the particle, the brighter the meteor, and the longer the trail that it leaves across the night sky.
Some meteors, usually very large ones, can fragment into numerous, smaller pieces, with each piece then leaving a flash and trail across the sky.
COMPOSITION OF METEORITES
Meteorites are classified according to their mineral composition. In appearance, stony meteorites (or "stones") look very much like any normal stone you see lying on the ground, except that they are usually covered with a fusion crust (the meteor's outer layers are melted by the heat generated by the meteor's fiery passage through the atmosphere). When stony meteorites are cut open and polished, they usually reveal tiny flecks of iron in the rock.
Although stony meteorites compose about 95 per cent of the meteorites that strike the Earth's surface annually, they are the most difficult type to find, as exposure to the elements over several years usually results in them becoming indistinguishable from ordinary rocks.
A second meteorite classification - stony-iron meteorites are composed of approximately equal amounts of iron and rock. This type of meteorite is extremely rare, comprising only about one per cent of all meteorites that survive to strike the Earth's surface.
The final class of meteorites are iron meteorites (or "irons") and comprise about four per cent of all meteors that fall to Earth. Composed of mostly iron with little or no stony material, these meteorites can also contain 10-20 per cent of the element nickel. Prior to the extraction of iron from ores around 2,000 BC, the only source of iron came from iron meteorites; as such, iron meteorites were valued more than gold and silver.
Meteors can display colours as they streak across the night sky, indicating the mineral and chemical composition of the meteor. As the meteor passes through the Earth's atmosphere, friction with the air molecules causes it to heat up. Depending on the mineral and chemical composition of the meteor, different wavelengths of light (colour) are emitted as those minerals and chemicals ignite and burn.
A green colour in the trail of a meteor indicates the presence of burning copper; the colour yellow, the presence of iron; while a reddish colour indicates elemental silicates. The presence of calcium produces a purplish or violet colour; magnesium, a green or teal colour; sodium, an orangish hue; and nitrogen and oxygen, the colour red.
Although it is usually very difficult to spot any of these colours with the naked eye (astrophotos can usually capture colours), with an exceptionally clear sky at a dark site away from lights, it is sometimes possible.
The next time you see a "shooting star" streak across your night sky, watch to see if you can observe any of these colours in the brief flash of the meteor; better yet, set up your camera and take a time-exposure shot of the night sky during a meteor shower. You just may be pleasantly surprised at what you capture in the photo.
HOW TO SPOT LYRA
On April 22 and 23, Lyra will be visible about halfway up the northeast sky shortly after midnight. Unfortunately, this time around, the waxing moon (77 per cent lit, and only five days from the full moon on April 27) will be up most of the night.
However, after it sets around 4:30 a.m. on April 22 (4:55 a.m. on April 23), you will have a brief period (about an hour) before the approaching dawn lightens the sky when you should, weather permitting, catch sight of some of the Lyrid fireballs streaking from the shower's radiant (directly overhead by then).
Just remember to dress warmly, let your eyes dark adjust, and, if possible, get away from city lights.
THIS WEEK'S SKY
Mercury and Venus remain too close to the sun to be seen this coming week.
Mars (magnitude +1.46, visible in Taurus - the Bull) is an early evening object, visible approximately 40 degrees above the western horizon around 9 p.m., before dropping towards the horizon and setting shortly after 1 a.m.
Saturn (magnitude +0.74, visible in Capricornus - the Sea Goat) rises around 3:45 a.m. in the pre-dawn, southeast sky, reaching an altitude of 16 degrees above the horizon before fading by about 5:40 a.m. with the approaching dawn.
Jupiter (magnitude -2.17, also visible in Capricornus) rises in the southeast around 4:25 a.m., reaching an altitude of 15 degrees above the horizon, before it, too, fades with the approaching dawn by about 6:30 a.m.
Watch for news of the maiden flight of NASA'S Ingenuity helicopter on Mars in the coming days.
Until next week, clear skies.
EVENTS
• April 20 — First quarter moon
• April 22-23 — Lyrid meteor shower peak, pre-dawn sky
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at glennkroberts@ gmail.com.