Neptune’s New Little Moon, Hippocamp
A recent paper in Nature (“The seventh
inner moon of Neptune”, Nature 21 Feb 2019) has reported the discovery of a new
moon orbiting Neptune, which has been given the name Hippocamp. It is one of now six smallish moons (moon
radius = 17 km, orbit radius = 105,000 km) found within the orbit of Proteus, which
is a fairly large moon (moon radius = 204 km, orbit radius = 118,000 km). They are all dwarfed by the very large moon
Triton (moon radius=1350 km, orbit radius = 355,000 km), which orbits yet farther
out.
You can read more about Triton, which has
several interesting features (e.g. retrograde motion), in the blog linked
below.
The researchers used a number of
observations of Neptune, taken by the Hubble Space Telescope to go moon hunting. One problem was that Neptune’s moons images
get smeared during the long exposure times normally needed to bring them out,
due to their proper motion. The team
came up with some new algorithms that helped to “stack” images, even with the
smearing effect, based on calculating where a given object would be on one
image compared to another one taken at an earlier or later time, given various
orbital parameters. Stacking the images
basically means putting one on top of the other, in the computer, to increase
the signal to noise ratio. By doing so,
it is possible for objects to be seen that otherwise would be lost in the noise
of the image (e.g. hot pixels, dim background stars, etc.). The researchers go into considerable detail
regarding their methods, in the paper.
Little Hippocamp was missed by Voyage 2 in
its 1989 flyby of Neptune, because it was just too small to register in that
craft’s cameras very well. Recall that
the Voyager flyby of Neptune was done at a high velocity, so there wasn’t much
time for sightseeing, and aiming the cameras wasn’t easy. The researchers used data
that they discovered about Hippocamp to retro-predict where it would have been
during the flyby, to see if they could located the moon in those images. However they were all badly smeared and/or completely
missed this small moon. Thus, this new imaging
method had to be developed.
Hippocamp orbits very close to the much larger Proteus, as the schematic picture above shows, as does the still from the video, just above. In fact, it was probably even closer to Proteus in the past, as both moons are moving away from Neptune, due to tidal interactions, but the larger Proteus is migrating faster than little Hippocamp, due to its greater mass, and therefore greater tidal interaction with Neptune.
That leads to the hypothesis the
Hippocampus might actually be a fragment of Proteus, broken off during a
collision, perhaps with a comet from the far outer solar system that came into
the region of Neptune, and collided with Proteus. That moon has a large crater, the Pharos
Crater, that might be the evidence of that disruption. In its turn, Proteus may have been created
from detritus of the capture of Triton by Neptune.
It is possible that Hippocamp might have
been hit and broke apart a number of times during its history. That would account for some problematic
features of the hypothesis, which are outlined in the paper.
Note, that though Hippocamp is small, it
is still about 3 times larger than the asteroid that collided with the Earth 65
million years ago, likely leading to the extinction of the dinosaurs. So, if Earth collided with a Hippocamp
sized object, it would be no small thing.
Given how far away Neptune is, we may not
get a good picture of these moons for quite a while. However, the New Horizons mission does show
that such missions are possible, if the money and political will are there. So, maybe we will get better information at
that time.
By the way, Hippocamp was a sea-horse in Greco-Roman
mythology, which is appropriate for Neptune, the god of the sea. Also, the hippocampus is an important part of
the brain, and it looks something like a seahorse (the actual animal). It is thought to be involved with memory, special
orientation and perhaps impulse control.
And impulse control sounds like something a spaceship needs too, so it
all fits together.
Sources:
The seventh inner moon of Neptune”, Nature
21 Feb 2019
A new moon for Neptune”, Nature 21 Feb 2019
Wiki, Proteus (moon)
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Now that you have read some real science
(astronomy and astrophysics), you should read some science fiction. Either of the Kati of Terra series or the
Witch’s Stones series would be excellent choices. Alternatively, you could try the short story
“The Magnetic Anomaly”, which has lots of physics, and plenty about magnetic
fields, perhaps affecting the brain’s hippocampus. J
Kati of Terra
How
about trying Kati of Terra, the 3-novel story of a feisty young Earth woman,
making her way in that big, bad, beautiful universe out there.
The Witches’ Stones
Or, you might
prefer, the trilogy of the Witches’ Stones (they’re psychic aliens, not actual
witches), which follows the interactions of a future Earth confederation, an
opposing galactic power, and the Witches of Kordea. It features Sarah Mackenzie, another feisty
young Earth woman (they’re the most interesting type – the novelist who wrote
the books is pretty feisty, too).
The Magnetic Anomaly: A Science Fiction Story
“A geophysical crew went into the Canadian
north. There were some regrettable accidents among a few ex-military who had
become geophysical contractors after their service in the forces. A young man
and young woman went temporarily mad from the stress of seeing that. They
imagined things, terrible things. But both are known to have vivid
imaginations; we have childhood records to verify that. It was all very sad.
That’s the official story.”
