Hypothetical moon of Mercury
A suspected moon of Mercury was actually a star.
The hypothetical moon of Mercury refers to a briefly suspected natural satellite of the innermost planet, hypothesized in the early 1970s based on ultraviolet radiation detected by instruments before the Mariner 10 flyby. The claim was later resolved as a misinterpretation of data from the background star 31 Crateris, but the episode contributed to revised understanding of ultraviolet radiation's behavior in the interstellar medium.
- field
- Planetary astronomy
- known_for
- Briefly hypothesized moon of Mercury, later identified as star 31 Crateris
- discovery_year
- 1974
- misidentification
- 31 Crateris
- mission_involved
- Mariner 10, MESSENGER
Lore & Background
On March 27, 1974, two days before Mariner 10 made its flyby of Mercury, instruments began registering large amounts of ultraviolet radiation in the vicinity of Mercury that, according to one astronomer, 'had no right to be there.' By the next day, the radiation had disappeared; it reappeared three days later, appearing to originate from an object seemingly detached from Mercury. Some astronomers speculated they had detected a star, but others argued the object must be a moon, citing the two different directions from which the radiation had emanated and the belief that such high-energy radiation could not penetrate very far through the interstellar medium. Arguments in favor of a moon were strengthened when the object's speed was calculated to be 4 kilometers per second, which matched the expected speed of a moon.
Reader's Guide
The hypothetical moon of Mercury, though ultimately a misidentification, holds significance for two reasons. First, it illustrates the challenges of observing objects near the Sun: Mercury was not observed in the infrared spectrum until 1995, and its small Hill sphere limits the potential for natural satellites. Second, the 1974 episode sparked an important discovery in astronomy: ultraviolet radiation is not as completely absorbed by the interstellar medium as was formerly thought. The star 31 Crateris, a spectroscopic binary with a period of 2.9 days, was identified as the source of the ultraviolet radiation. Later, NASA's MESSENGER spacecraft, which orbited Mercury from 2011 to 2015, did not detect any moon, and a 2012 April Fools' Day joke by NASA claimed a moon named Caduceus. Searches in 2011 and 2013 confirmed Mercury has no moons.
Did You Know?
- The hypothesized moon was based on ultraviolet radiation detected two days before Mariner 10's flyby of Mercury in 1974.
- The object's speed was calculated at 4 kilometers per second, matching the expected speed of a moon.
- The misidentification led to the discovery that ultraviolet radiation is not as completely absorbed by the interstellar medium as formerly thought.
- NASA's MESSENGER spacecraft searched for moons in 2011 and 2013 and confirmed Mercury has none.
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