The Sky Tonight

Planets to look for

Venus and Saturn are in the west all month after sunset, getting lower to the horizon as the month progresses. They have a close approach on Jan 18, separated by just over 2 degrees, and are within a handspan of each other for a week either side of this date. They are both moving closer to the line of sight to the Sun, meaning January is really your best chance to get a last good look at them, especially Saturn.

Image: Venus and Saturn in the western sky on Jan 18. Credit: Stellarium

Mercury is visible above the eastern horizon in the hour or so before sunrise for the first half of the month before once again being lost in the glare of the rising Sun.

Mars is at opposition this month and makes for excellent viewing all night, while Jupiter continues to slowly move through Taurus in the north, contrasting nicely with nearby Aldebaran.

Image: Mars and Jupiter in the northern sky with Aldebaran for reference. Credit: Stellarium

Constellation of the month

Gemini – The Twins

Gemini is a medium sized constellation visible low in the northern skies during the early months of the year. It is most easliy spotted by looking for its two brightest stars – the red giant Pollux located 34 light years away, the closest red giant to the Sun in fact, which may have a planet orbiting it – and the 6-star system Castor, consisting of 3 pairs of stars orbiting each other in a complicated arrangement.

Image: Gemini in the northeast during January, with Mars nearby as well. Credit: Stellarium

The constellation is often associated with the twins of Greek mythology, the namesakes of these two brightest stars who appear in many stories including The Iliad – as soldiers beseiging legendary city of Troy – and in the story of Jason and the Argonauts on the quest for the golden fleece.

According to some myths, Castor was born mortal, son of Tyndareus, but Pollux was the son of Zeus and therefore immortal. When Castor was slain in battle, Pollux was inconsolable and asked to renounce his immortality. Zeus agreed, and now the two brothers are united in the night sky as the two brightest stars in the Gemini constellation.

Gemini is home to the Eskimo Nebula, a planetary nebula that apparently visually resembles the head of a person wearing hooded clothing.

Image: Eskimo Nebula. Credit: Credit: NASA, ESA, Andrew Fruchter (STScI), and the ERO team (STScI + ST-ECF)

Despite their name, planetary nebulae have nothing to do with planets. Instead, they are the last gasps of dying stars. As Sun-like stars run out of fuel in their core, they expel the outer layers of their swollen atmospheres into space – a bubble of this ejected material forms the ‘face’ of the Eskimo Nebula in this case, with the remaining dying star visible in the very centre. The ejected material exposes the white-hot core of the star, so hot its intense radiation ionises the ejected material causing it to glow as a nebula. Since stars are round and tend to eject matter in all directions, planetary nebulae are usually roundish in shape, leading early astronomers to confuse them as ‘fuzzy planets’, and hence where the misleading name comes from.

Mars

This is the best time to see the red planet for the next couple of years. At opposition not only are Earth and Mars as close together as they will be, but we are also seeing Mars face on, so its fully illuminated side is presented in our direction. Although opposition occurs on Jan 16, you may find it easier to wait a couple more days before observing while the pesky Waning Moon moves a bit further to the east away from Mars. Mars will continue to be visible in the evening sky for about the next 7 months, but it will only get less impressive after this month so make sure you seize the opportunity.

Image: Simulated image of Mars on Jan 18. Note the planet is face on and fully illuminated. Credit: Stellarium

Messier 35 and NCG 2158

On the far western edge of Gemini you can see two open clusters in the same line of sight. Messier 35 is a loosely bound open cluster containing more than 500 bright stars covering a region about the size of the Full Moon in the sky. In the same field of view is the older and more distant NGC 2158. Being older, all this cluster’s blue stars have expired, leaving the lighter and longer lived yellowish stars behind and giving the cluster a noticeably yellower colour than M35. Despite their line of sight, the two clusters are not related.

Image: Messier 35, mostly concentrated top left, and the yellower NGC 2158 bottom right. Credit: 2MASS

Bepi Colombo Waves at Mercury

The BepiColombo spacecraft, a joint European Space Agency (ESA) and Japanese Aerospace Exploration Agency (JAXA) will be completing its 6^th flyby of Mercury on Jan 8. The mission, launched in 2018, is actually two separate spacecraft tied together – the Mercury Planetary Orbiter (MPO) built by ESA and the Mercury Magnetospheric Orbiter (MMO) built by JAXA.

Image: Artist impression of BepiColombo. Credit: ESA

Mercury is a surprisingly under-studied planet because it is actually quite difficult to make detailed observations from Earth. It is only ever visible near the horizon at sunrise or sunset, and its proximity to the Sun means if you point a telescope at it, you’re also pointing close to the Sun, confounding observations.

Sending a spacecraft to Mercury is not any easier. Since going to Mercury means getting closer to the Sun, the journey is basically “downhill” the whole way and the Sun’s strong gravity imparts energy to an approaching vessel.  Spacecraft pick up an enormous amount of speed as they approach Mercury, and you have to find a way to lose that excess velocity. Think of it like trying to safely jump off the roof of a house – you need to lose the speed before you hit the ground or bad things will happen. Some quick maths shows that if a spacecraft travelling to Mercury was to try to lose this speed simply by burning chemical fuel in its engines, it would take more fuel than it takes to travel to Pluto. The dwarf planet Pluto is about 40 times further away from Earth than Mercury, but it takes less fuel to get there. Orbital mechanics is weird like that (you should really click that link)

For this reason Mariner 10 – the first of only two spacecraft to ever visit Mercury previously – was only intended to do a single flyby of the planet in 1974. That was until Italian scientist Guiseppe ‘Bepi’ Colombo cleverly calculated how the spacecraft could instead fly close to Venus and use the planet’s gravity to ‘slingshot’ toward Mercury, losing some of its excess speed and saving a lot of fuel, ultimately allowing three flybys of Mercury instead of one. This technique, called a gravity assist manoeuvre, is commonplace these days but was the first of its kind in the 1970s. It was by using multiple gravity assist manoeuvres that NASA’s MESSENGER spacecraft became Mercury’s second visitor in 2011, ultimately orbiting the planet until 2015.

An enormously complicated series of gravity assist manoeuvres is allowing BepiColombo to approach Mercury today.

Video: The path of BepiColombo toward Mercury, making use multiple gravity assists from various planets. Credit: ESA

Ultimately, BepiColombo will have made use of 9 gravity assists: 1 Earth, 2 Venus and 6 Mercury, and on Jan 8, 2025, it will complete its 6^th and final Mercury flyby. This last flyby of Mercury will rob the spacecraft of enough speed that its miniscule ion engines (providing a whopping 30 grams of thrust) will be able to put the spacecraft into orbit around Mercury by December 2025. After this seven-year journey to lose enough speed to get into orbit, the BepiColombo science mission begins.

At this stage the MPO and the MMO will separate and use their own propulsion systems to go into slightly separate orbits. The MPO will go in a close orbit to get good views of the surface of the planet, while the MMO will stay in a higher orbit to better sample different parts of Mercury’s magnetic environment.

Image: The orbits of the Mercury Planetary Orbiter and the Mercury Magnetospheric Orbiter

The Mercury Planetary Orbiter carries a wide range of instruments to study the planet. Several cameras and a LiDAR system will give detailed information of the surface, while 4 different spectrometers will analyse the chemical composition at the same time, and all these things combined will give a comprehensive 3D map of the surface of Mercury, complete with mineral distribution and chemical composition. Meanwhile, particle detectors will sample Mercury’s miniscule atmosphere to see how it behaves, and as this is happening, accelerometers on board the MPO will also detect tiny changes in the spacecraft’s motion as the planet’s gravity tugs it to and fro, allowing scientists to work backwards and map the interior distribution of mass inside the planet.

While the MPO is busy studying the surface, the Mercury Magnetospheric Orbiter will be studying charged particles, plasma waves, dust and the general magnetic environment surrounding the planet, to determine the origin and behaviour of Mercury’s magnetic field. During BepiColombo’s 3^rd flyby in June 2023, instruments on the MPO collected enough data in 30 minutes to provide fascinating early models of Mercury’s magnetic environment.

Image: Mercury’s magnetic environment, as mapped from the June 2023 flyby. Credit: ESA

Ultimately the goal of the mission is to learn a lot more about the planet in general. Does Mercury have a solid or liquid core, or both? Where are the predicted large amounts of iron? What is the origin and behaviour of its magnetic field? Is the planet geologically active today? Do those permanently shadowed craters at the north pole of Mercury actually contain water ice? Yes, seriously, the closest planet to the Sun may have polar ice ‘caps’.

Image: MESSENGER data shows some craters at the poles of Mercury are in permanent shadow and cold enough for ice to be stable. Credit: NASA

Time will tell what BepiColombo will discover. For now, it will wave at Mercury as it completes its 6^th and final flyby on Jan 8. We’ll catch up with it later next year when it goes into orbit.

Video: Timelapse of BepiColombo’s fourth flyby of Mercury. Credit: ESA