March 2025

Planets to look for

Mars and Venus continue to be visible in the north during March and make noticeable movement towards the west as the days go on. Mars is still easily spotted against Castor and Pollux of Gemini, and Jupiter holds its place underneath Aldebaran in Taurus. An excellent way to begin an evening of skywatching.

Image: Jupiter and Mars in the northern sky with prominent stars labelled. Credit: Stellarium 

Saturn, Venus and Mercury are all lost in the glare of the Sun this month. They are gone from the evening skies and reappear in the eastern sky just before sunrise in the late month, but you’re probably better just waiting until April for them to really be worth getting up early for. 

Constellation of the month

Lepus – the Hare 

Lepus is a middle sized constellation visible in the northwestern sky during March. It is related to the Orion group of constellationd and is interpreted as a hare, but doesn’t really have much deeper mythology associated with it. One day, Orion and his hunting dogs, Canis Major and Canis Minor, hunted a hare. 

The constellation is easy enough to spot unaided, located slightly above Orion in the sky, with the two brightest stars Alpha Leporis (Arneb) and Beta Leporis (Nihal), forming a notable line with several bright stars on either side. 

Image: Lepus in the sky relative to Orion and Sirius, with Arneb and Nihal labelled. Credit: Stellarium 

Alpha Leporis is a supergiant star about 14 times heavier and 30000 times brighter than the Sun. It is only expected to live for another million years or so before exploding in a supernova event. This star expemplifies the unintuitive result that heavier stars live shorter lives than lighter ones. The ~5 billion year old, less-massive Sun requires less fuel to maintain gravitational equilibrium compared to heavier stars. The more massive Alpha Leporis must burn through fuel much faster to maintain equilibrium, and hence shines for nowhere near as long as the Sun, despite its much larger mass. 

Beta Leporis is also heavier than the Sun, though its more modest 3.5 solar masses has given it a lifespan of about 240 million years so far. The star seems to be in the process of swelling to become a red giant. 

M79 – A celestial snow globe 

Messier 79 is a magnitude 8 globular cluster located in the constellation of Lepus. The cluster contains about 150000 stars packed in a region about 100 lightyears across. 

Image: Messier 79 in all its glory. Credit: NASA, ESA, STScI, F. Ferraro (Universita di Bologna) and S. Djorgovski (California Institute of Technology) 

There are about 150 globular clusters orbiting the Milky Way and most of them are located in the direction of the centre of the galaxy. Interestingly, M79 is located in almost the exact opposite direction, and is ultimately about 60 000 lightyears from the galactic core. 

This has led astronomers to debate whether M79 was in fact once a globular cluster belonging to the Canis Major Dwarf Galaxy (which we mentioned last month). This argument is compounded by the fact that the Canis Major Dwarf Galaxy’s very existence is still unclear. 

Will Earth get hit by an asteroid in 2032? 

Betteridge’s law of headlines states that “Any headline that ends in a question mark can be answered by the word no.” And that is likely the case here, but not yet entirely confirmed.  

In December 2024 astronomers discovered a new asteroid, now attractively named 2024 YR4, and upon calculating its future orbit, determined that there was about a 1.5% chance it would collide with Earth in December 2032.  

Image: Current predicted location of 2024 YR4 in December 2032. Credit: NASA Eyes on Asteroids 

Estimated to be between 40-90m across, an impact on Earth would cause major damage to a city sized area, or large tsunamis if impacting over ocean, so astronomers are working to obtain more data to precisely define the asteroid’s orbit as soon as possible. 

In 2013, an 18m asteroid exploded over Chelyabinsk in Russia and caused significant structural damage to the city, but no fatalities.  

Video: Dashcam footage of the Chelyabinsk meteor explosion.  

Getting a precise prediction of an asteroids’ location years in advance is quite difficult, requiring many observations over an extended period of time. Even though the asteroid was ‘discovered’ in Dec 2024, astronomers have since realised it had already been observed in previous historical sky surveys. Using this older data to more precisely refine the asteroid’s location, by Feb 18 astronomers had calculated that the chance of collision had gone up to 2.8%, giving 2024 YR4 the highest probability of impact of any asteroid we currently know about. 

Rather than alarm, most astronomers are in fact unconcerned. This is because of the unintuitive nature of impact probabilities as orbits are refined.

Image: Approximate size of asteroid 2024 YR4. Credit: Sinucep – Own work, CC0 

You see, without knowing the exact orbit of an object (essentially impossible) astronomers can instead only give regions of space where an asteroid might be located in the future. If that region includes Earth, then we can calculate the probability of impact. 

Image: The calculated possible locations of asteroid 2024 YR4, published Feb 6 2025. Credit: ESA 

As astronomers gather more data, they can more precisely calculate the orbit, so the region of space of where the asteroid might be located becomes smaller, signifying our better understanding. However, if that region still includes the Earth, then our planet now covers a larger fraction of that region, and so the probability of impact increases.  

Image: The calculated possible locations of asteroid 2024 YR4, published Feb 18 2025. Notice that the region is much smaller than on Feb 6, so Earth occupies a larger fraction of it. Credit: ESA 

This is an important distinction. The probability increases because the region of space where the asteroid might be located is smaller, not because we are definitely more certain that it will hit our planet. What usually happens, and what astronomers expect to happen in this case, is that the orbit will soon be so precisely calculated that the region of space where it might be located will shrink to be so small that it no longer overlaps with Earth, at which time the probability of impact drops abruptly to 0%. All we need is more observations.  

Unfortunately, the asteroid is currently heading away from Earth at a particularly difficult angle to study, so astronomers are hoping to use the James Webb Space Telescope to observe it in early March. Hopefully this will provide enough high-quality data that the orbit can be calculated to better precision. If not, we won’t get another good look at it until 2028. 

This does raise the question: is there anything we can do if the asteroid is definitely going to hit Earth? And the answer is a resounding Yes! (and doesn’t even need Bruce Willis). In September 2022, NASA’s DART mission deliberately slammed into the asteroid Dimorphous and significantly altered the orbit of this asteroid. 

Video: A summary of NASA’s DART mission. Credit: NASA 

At 177m across, Dimorphous is considerably larger than 2024 YR4, so in principle this kinetic impactor approach could slow the asteroid down enough for it to miss Earth. The theory has been demonstrated, whether we will need to put it into practice, time will tell. 

Image: The most desirable outcome for asteroid 2024 YR4. Credit: NASA Eyes on Asteroids