18 Melpomene – An Asteroid in Motion
Certo. Qui imposterei la pagina in modo diverso dalle immagini deep-sky: il protagonista è il movimento. La GIF mostra 18 Melpomene che cambia chiaramente posizione rispetto alle stelle, con 8 esposizioni da 180 s intervallate di 30 s.
18 Melpomene – An Asteroid in Motion
Key Scientific Data
Object: 18 Melpomene
Object type: Main-belt asteroid
Designation: (18) Melpomene
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
Average distance from the Sun: approximately 2.3 AU
Orbital period: approximately 3.5 years
Name origin: Melpomene, the Muse of tragedy in Greek mythology
Description
This animation captures the motion of 18 Melpomene, one of the larger asteroids of the main asteroid belt.
Unlike galaxies, nebulae and stars, which appear essentially stationary during a single observing session, an asteroid is close enough to Earth for its movement against the distant stellar background to become detectable over a surprisingly short period of time.
In the animation, Melpomene can be seen shifting from frame to frame while the surrounding stars remain fixed.
The effect provides a direct visual demonstration that the apparently static night sky contains objects travelling continuously through the Solar System.
Eight Images – One Moving World
The sequence consists of 8 luminance exposures of 180 seconds each, with approximately 30 seconds between successive exposures.
The total exposure time is therefore:
8 × 180 s = 1,440 seconds = 24 minutes
Including the seven 30-second intervals between frames, the sequence spans approximately 27 minutes 30 secondsfrom the beginning of the first exposure to the end of the last.
During that short interval, Melpomene's displacement against the background stars is already clearly visible.
Each frame therefore records the asteroid at a slightly different point along its orbit.
Why Does the Asteroid Move While the Stars Do Not?
Melpomene is vastly closer to us than the stars visible in the same field.
It orbits within the main asteroid belt between Mars and Jupiter, whereas even the nearest stars lie many trillions of kilometres beyond the Solar System. Main-belt objects typically occupy the region roughly 2.2–3.3 AU from the Sun.
Over half an hour, the apparent movement of the distant stars is effectively negligible at this image scale. Melpomene, however, changes its apparent position enough to be measured.
The animation therefore uses the distant stellar background as a nearly fixed reference against which the asteroid's motion becomes obvious.
A Small World Between Mars and Jupiter
Melpomene was discovered in 1852 and became the eighteenth numbered minor planet.
Its orbit has a semi-major axis of approximately 2.30 astronomical units, giving it an orbital period of about 3.44 years.
One astronomical unit is the average Earth–Sun distance, about 150 million kilometres. Melpomene therefore spends its orbit hundreds of millions of kilometres from the Sun.
Despite appearing as nothing more than a point of light in this image, it is not a star. It is a substantial rocky body. Published diameter estimates have varied; historical NASA compilations give values around 150 km, illustrating the difficulty of determining asteroid sizes precisely from remote observations.
Why Does It Still Look Like a Star?
Even with a 250 mm telescope, Melpomene is far too small and distant for its surface to be resolved in this observation.
The asteroid therefore appears as a point source, almost indistinguishable from an ordinary star in any individual frame.
Only when the images are compared over time does its identity become obvious.
This is essentially the same principle historically used to discover asteroids: astronomers compared the positions of point-like objects on successive observations and searched for the one that moved against the fixed stars.
What Are We Actually Seeing?
Melpomene does not produce visible light of its own.
The small bright point in the animation is sunlight reflected from the asteroid's surface, travelling from the Sun to Melpomene and then towards Earth.
This makes the observation fundamentally different from imaging a star, which generates its own light through nuclear fusion.
The animation is therefore tracking sunlight reflected by a rocky world only a fraction of the size of the Moon as it travels around the Sun.
From Still Image to Measurement
A sequence like this can provide more than a visual animation.
If the images are accurately plate-solved and their acquisition times are known, the asteroid's position can be measured in right ascension and declination in each frame. From these measurements, its apparent angular velocity and direction of motion across the sky can be calculated.
Repeated astrometric measurements of minor planets are precisely what allow astronomers to refine their orbital trajectories.
In this case, the 10Micron mount keeps the stellar field aligned while Melpomene reveals its own independent movement through it.
Melpomene – The Muse of Tragedy
The asteroid was named after Melpomene, one of the nine Muses of Greek mythology.
Melpomene became associated with tragedy and is traditionally represented holding the tragic theatrical mask.
The name follows a strong tradition in early asteroid astronomy: many of the first minor planets discovered in the nineteenth century were named after female figures from Greek and Roman mythology.
A Different Kind of Astrophotography
Most deep-sky photographs attempt to accumulate hours of exposure while keeping the celestial target perfectly stationary.
Here, time itself becomes part of the image.
Each individual exposure freezes Melpomene at one location. When the eight frames are displayed sequentially, those separate positions reveal a trajectory.
The animation transforms a point of light into a moving world.
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: