A comet from another Sun
Kamal Lodaya
On 1 July, 2025, a new comet was discovered by the Atlas telescope in Rio Hurtado, Chile, South America. (This has been set up to catch asteroids approaching Earth). The comet was approaching from the direction of the Milky Way, in the constellation of Sagittarius (Dhanu).
It had been accidentally observed earlier during May and June. From this data we know two amazing things. First, that it is the fastest moving object we have seen in our solar system, travelling at over 2 lakh km per hour. This is faster than the speed of our fastest planet Mercury going around the Sun. Second, this means that it is coming from outside the solar system. Sometimes the Sun sends comets outside the solar system when they venture too close. This comet has possibly been sent off by the star which it is coming from. The star cannot be identified since there are too many stars in the direction of the Milky Way.
The comet has the name "3I/Atlas". It is not coming close to Earth as it shoots through our solar system (closest on 29 October). It passes closer to Venus, Mars and Jupiter, which are on the other side of the Sun then. Already Nasa's Hubble telescope has seen the usual fuzzy head (where ices are vaporizing) and a faint tail of dust. These ices are from the planetary system where the comet came from. They are now vaporizing because the now-nearest star, our Sun, is heating them.
The Webb telescope shows an unusually large amount of carbon dioxide vapour, 8 times that of water. Along with carbon dioxide escaping the comet, grains of dust and water-ice grains are thrown into space. The dust grains form a tail. The Swift telescope suggests that hundreds of kilometres out in space in the comet's fuzzy head, the water in the ice grains vaporizes. This vaporization outside the comet's nucleus has not been seen in most comets.
Cyanide and nickel, unusually without iron, are seen in the ultraviolet spectrum from the Very Large Telescope in Antofagasta, Chile. US graduate student Rohan Rahatgaonkar and colleagues speculate weathering of nickel tetracarbonyl, an organic compound not seen in comets earlier, to account for the nickel detection. Hydrogen cyanide is seen by the Maxwell radio telescope in Mauna Kea, Hawaii. In the observed spectrum from this comet, traces of metallic elements (including nickel) are lower than in the Sun, so the comet could be from a star older than the Sun. It would be wonderful (but unlikely) if from a detailed spectrum we can identify the originating star.
The comet's nucleus is not yet visible. It is estimated between 0.3 and 5 km in diameter, most likely below 1 km. The tail will grow but the comet will not be visible to the naked eye.
The high speed means that it would take too much fuel to launch a space probe which can catch up with the comet and photograph it. There are proposals to redirect probes already in space, such as Nasa's Juno spacecraft which has been orbiting Jupiter for a long time. Again Juno does not have enough fuel to power its way to the comet. Other probes like Psyche (on its way to asteroid Psyche) and Juice (going to the moons of Jupiter), or those orbiting Mars, can try to take pictures of this comet. Psyche was 4 crore km away on 4 September. NASA has not released any pictures. Mars Reconnaissance Orbiter, Mars Express and ExoMars attempted observation on 2 and 3 October. Juice is closest on 4 November.
If you can think of a cleverer idea to snap this racing object, please write to us and we will send your idea across to our friendly neighbourhood astronomer. Wouldn't we all like to see a close up of a visitor from another Sun?
Astronomer Rosemary Dorsey of the University of Helsinki has a different thought. She thinks interstellar objects (which come from outside our Solar System) like 3I/Atlas are quite common in the Milky Way. We found one in 2017 and then a comet in 2019. (That's why this comet is labelled "3I", the third visitor from interstellar space!) Possibly there are such objects at any time in our solar system. Because they are dark we find it difficult to see them. She estimates that during the next 10 years the new Vera Rubin telescope located on the mountain tops east of La Serena, Chile, will find as many as 20 such objects.