18 Melpomene – An Asteroid in Motion
Key Scientific Data
Object: (18) Melpomene
Object type: Main-belt asteroid
Location: Main Asteroid Belt, between Mars and Jupiter
Discovery: 24 June 1852
Discoverer: John Russell Hind
Composition: S-type, predominantly rocky
Diameter: approximately 140–150 km
Semi-major axis: approximately 2.30 AU
Orbital period: approximately 3.44 years
Name origin: Melpomene, the Muse of tragedy in Greek mythology
Description
This animation captures the movement of 18 Melpomene, a rocky asteroid orbiting the Sun within the main asteroid belt between Mars and Jupiter.
Unlike distant stars and galaxies, which appear stationary during a short observing session, Melpomene is sufficiently close to Earth for its movement against the stellar background to become detectable within minutes.
Across eight consecutive exposures, the asteroid visibly changes position while the surrounding stars remain aligned, providing a direct demonstration of orbital motion within our Solar System.
Eight Images – One Moving World
The animation consists of 8 luminance exposures of 180 seconds each, separated by approximately 30-second intervals.
The total integration time is 24 minutes, while the complete sequence spans approximately 27 minutes and 30 seconds.
During this short period, Melpomene moves sufficiently against the distant stellar background to reveal its changing position from frame to frame.
Each exposure captures the asteroid at a different point along its apparent trajectory, transforming a sequence of still images into a visual record of motion.
A Rocky World Between Mars and Jupiter
Discovered by John Russell Hind in 1852, Melpomene became the eighteenth numbered asteroid.
With an estimated diameter of 140–150 kilometres, it is a substantial rocky body, although its enormous distance makes it appear as an unresolved point of light even through a 250 mm telescope.
Its orbit has a semi-major axis of approximately 2.30 astronomical units, equivalent to about 344 million kilometres, and it completes one revolution around the Sun in approximately 3.44 years.
Melpomene belongs to the S-type asteroid population, characterised by predominantly silicate-rich, rocky material.
Its name comes from Melpomene, the Greek Muse of tragedy, traditionally depicted holding a theatrical mask.
Why Does the Asteroid Move?
The apparent motion of Melpomene results from its orbital movement around the Sun, combined with Earth's own orbital motion.
While the distant stars provide an effectively fixed reference over the duration of the animation, the asteroid changes its position sufficiently to produce a measurable displacement.
Melpomene does not generate visible light of its own. The bright point recorded in each frame is sunlight reflected from its rocky surface.
Although its physical disc cannot be resolved in this observation, its motion clearly distinguishes it from the surrounding stars.
From Animation to Astrometry
Sequences such as this are useful not only for visualising asteroid motion but also for making scientific measurements.
By combining accurate acquisition timestamps with plate solving, the asteroid's position can be determined in right ascension and declination for each exposure.
These measurements allow astronomers to calculate its apparent angular velocity and direction of movement.
Repeated astrometric observations can also contribute to refining asteroid orbital trajectories.
This is the same fundamental principle used by astronomers to discover and track minor planets: identifying objects that move against the distant stellar background.
Curiosities
Melpomene was discovered on 24 June 1852, during the early decades of asteroid exploration.
Its designation, (18), indicates that it was the eighteenth minor planet to receive a permanent number.
Despite its considerable size, Melpomene appears indistinguishable from a star in an individual exposure. Its true nature becomes apparent only when successive images reveal its movement.
The animation also illustrates an important difference between deep-sky astrophotography and Solar System imaging: while long integrations are generally used to reveal faint, apparently stationary objects, moving asteroids can be studied through their changing positions over comparatively short periods.
Looking Back in Time
Unlike distant galaxies, whose light may have travelled for millions of years, Melpomene is located within our own Solar System.
The sunlight visible in this animation first travelled from the Sun to the asteroid, reflected from its surface, and then continued towards Earth.
The exact light-travel time depends on Melpomene's distance from both the Sun and Earth at the moment of observation.
Although the light is only minutes old, the asteroid itself is a remnant of the material from which the Solar System formed approximately 4.6 billion years ago.
This animation therefore captures the present-day motion of a small world whose origins date back to the formation of our Solar System.
Image Data
Sequence: 8 frames
Exposure per frame: 180 s
Interval between exposures: approximately 30 s
Total integration time: 24 minutes
Approximate duration of the complete sequence: 27m 30s
Filter: Luminance
Equipment:
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Telescope: Artesky ARTEC 250 Pro
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Camera: Player One Poseidon-M Pro
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Mount: 10Micron GM2000 HPS II
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Filter: Antlia V-Pro Luminance 2"
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Accessories: Pegasus Astro Prodigy Microfocuser, Player One Phoenix Wheel 7×2", WandererAstro WandererCover V4-EC
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Software: Adobe Photoshop, Pleiades Astrophoto PixInsight, Stefan Berg Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA)
For further details and the animation, visit AstroBin: