Sky Diary: Observe the Moon Night coincides with launch of new Jupiter mission

An ambitious Nasa mission launched this weekend is set to reveal the secrets of a mysterious region in the vicinity of Jupiter.

Lucy – named for the hominoid – will embark on a tour of the two camps populated by Jupiter’s Trojan asteroids. These small bodies swarm around dynamically stable zones ahead of and behind Jupiter in its orbit around the sun.

The Atlas V rocket responsible for propelling the probe took off at 5.34am local time on Saturday (10.34am Irish time) from Cape Canaveral. It is a 12-year mission and is the furthest from the sun to be propelled by solar power.

The regions are at two of Jupiter’s five Lagrangian points, and are where the tug of the gas giant and the sun balance each other gravitationally.

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Earth has five similar zones, such as the 1.2 million km distant L2 point, where the soon-to-be-launched James Webb space telescope will be placed.

What we know to date about the Trojans has come solely from ground-based observations, but Lucy should change that.

The first Jupiter Trojan was found in 1906 and nearly 10,000 are now known, with the largest being the 225km wide Hektor.

The major Trojans are named for figures in the war, with those orbiting ahead of Jupiter belonging to the Greek army, and those following the planet falling into the Trojan group. The naming convention was only decided though after a couple of individuals infiltrated rival camps.

Astronomers estimate there are a million Trojans greater than 1km in diameter clustered at Jupiter’s L4 and L5 points – the same number of similarly sized objects are believed to reside in the main asteroid belt between Mars and Jupiter.

Lucy will use a series of complex manoeuvres to visit one main belt asteroid and six Trojans during the 12-year mission. Flybys of Earth will occur in 2022 and 2024 to slingshot the spacecraft towards the asteroid belt, where it will make a flyby of the object Donaldjohanson in April 2025.

The robotic explorer will continue to its first encounter with a Trojan at Jupiter’s L4 point in August 2027 and will visit three more over the next two years. Lucy will then depart the Greek faction to swing by Earth in 2030 for another gravitational assist, this time towards the Trojan camp, which it will reach in March 2033.

Lucy’s mission could revolutionise our theories about the formation of the solar system.

One model to explain its current arrangement suggests an outward migration of the giant planets from where they originally formed. The Trojans may have been shepherded into their present positions during this period or accumulated during the initial formation of the planets.

What’s certain is the Trojans are primordial relics and astronomers hope Lucy will help unlock the solar system’s early history, such as how its 3.2-million-year-old ape-like namesake helped paleoanthropologists rewrite some chapters of pre-human story.

The Planets

Mercury may be seen in the morning sky from the third week of October but your best change of spotting the fleet-footed world is towards the end of the month. By then it will have gained in height and rapidly brightened .

The innermost planet rises at 6.30am on October 25th, or about two hours before the sun, and is roughly a fist-width at arms-length above the eastern horizon at the beginning of civil twilight.

Venus is an evening sky object but is still rather low in the southwest. It sets at about 6.45pm mid-month, or just more than an hour after the sun.

Mars rises 50 minutes before the Sun at the end of October but is too deep in the dawn glow to be seen.

Jupiter sets around midnight at month’s close but look for it near the moon around this time. The solar system’s behemoth remains quite prominent in the southeast after dark among the dim stars of Capricornus.

Saturn, also in the sea goat, slips from view even earlier than Jupiter and dips below the horizon by late-evening on Halloween.

The annuals “Stargazing 2022” (published by Philips) and “Night Sky 2022” (from Collins) are both available. They retail for about €10 and give full details, along with charts, of what is happening in the sky for the year ahead.

The Moon

International Observe the Moon Night is a global event that encourages people to get acquainted with our companion world in space. This year it takes place on October 16th when the moon's phase is midway between first quarter and full.

The first rays of the lunar sunrise that day just brush the semi-circular Jura mountain range to the upper left of the moon's disk. The sight is sometimes called "the golden handle" and is a dramatic clair-obscure effect that can even be captured on a pocket digital camera, when you zoom in.

The full moon on October 20th is popularly known as ‘the hunter’s moon’. It floats amongst the stars of Pisces that night, in a region of sky known as the celestial sea for its predominance of watery star groups. Perhaps the hunter has gone fishing this time?

The last quarter moon on October 28th is just shy of the Celtic festival of Samhain on October 31st.

Halloween is now also circled in the cosmologist's diary as Dark Matter Day, which promotes the search for answers to some of the fundamental questions about our universe.

The Orionids

Light from the full moon during the early hours of October 21st washes out all but the brighter members of the Orionid meteor shower, which peaks then.

Orionids are quite swift, and their rates seem to spike every 12 years due to perturbations by Jupiter. The shower is associated with the dust trails laid down by comet Halley as it laps the sun.

A handful of Orionids might be bagged if you commit to a watch after the witching hour, as its host constellation only rises then.

[ Irish Astronomical Society ]