What if The Moon Had a Moon?

 

moons are some of the most intriguing  objects in space  as they orbit planets larger than them  by several times  yet one question has always intrigued  scientists about the nature of moons  can moons have a moon of their own today  we'll be finding out  most planets in our solar system and  even exoplanets in other star systems  have their own natural satellites what  we know as moons  as defined by the nasa and other expert  astronomers  a moon is a celestial body that makes an  orbit around a planet  moon materials range from rocky material  to icy bodies and mixtures of both  earth's moon is made of rock while mars  moons are the same material as  rocky asteroids saturn's moons are  mostly ice with rocky coarse  its largest moon titan is predominantly  rocky with an icy surface  the moons of uranus and neptune are  largely icy  pluto's binary companion cheron is  mostly rocky with an icy covering  the exact makeup of its smaller moons  which were likely captured after a  collision  is still being worked out by scientists  however can any of these satellites have  a moon of their own  short answer yes however that doesn't  necessarily mean that a sub moon or  moon moon can easily survive in any kind  of environment  without any sort of help in most cases  there's not enough space for a sub  sub-moon to orbit another moon  that's sort of the reason why there  aren't many sub-moons out there  the region of space around a satellite  where a sub-satellite can exist  is called the hill sphere outside the  hill sphere  a sub-satellite would be lost from its  orbit about the satellite  for example let's analyze the already  existing  sun-earth moon system earth is a  satellite of the sun  and the moon is a sub-satellite orbiting  earth  the moon orbits the earth because the  moon is about 380  000 kilometers from earth well within  earth's hill sphere  which has a radius of 1.5 million  kilometers  the hill sphere is basically the space  where the earth's gravity is more  important than the sun's gravity on  another object  if the moon ended up outside earth's  hill sphere  then the moon would orbit the sun  instead of the earth  similar to other planets asteroids and  comets  which is sort of fun when you think  about it jupiter's hill sphere has a  radius of 0.36  astronomical unit that's of course much  bigger than the earth's hill sphere  and that's because jupiter is several  times more massive than the earth  and also has a stronger gravitational  pull on top of that it's further from  the sun than earth  so the sun's gravity is weaker at  jupiter than at earth  tidal forces would pull the little moon  towards the host planet  and rip the mini moon to pieces  for it to survive it would have to be a  small mini moon  roughly six miles in diameter or less it  would also need to orbit a large moon  with enough gravity to hold it in place  most moons in our solar system would be  unable to do that  regarding the naming trend of sub-moons  scientists use the term  moon moon for referring to a sub-moon  others prefer sub-moon or metamoon as  the prefix of choice for the moon's moon  can our moon have a moon well yes  the moon could have a sub-satellite with  no problem  if we look at a system of the earth moon  and a sub-satellite  the same idea as above applies the moon  has its own hill sphere  with a radius of sixty thousand  kilometers which is roughly one-sixth of  the distance between the earth and the  moon  that's where the sub-satellite could  exist should an object  lie outside the moon's hill sphere it  would orbit the earth instead of the  moon  however a sub-satellite cannot  indefinitely stay in orbit  around the moon due to the tidal effect  the moon  like almost all other moons in the solar  system is in synchronous rotation around  the earth  meaning it shows the same face to earth  at all times  a side effect of the tidal forces  between the earth and moon  these are the same tidal forces that  cause the high and low tides on earth  and using this configuration any kind of  object within the hill sphere of the  moon  will have a decaying orbit due to tides  the orbit of any sub-satellite of the  moon will shrink over time  and the distance between this  sub-satellite and the moon  will get smaller and smaller until it  crashes into the moon  or the lunar tide rips it apart  on another note can a satellite like an  artificial one  be considered a submoon or eventually  become one  well lunar orbiting satellites only  orbit it for a few years  which is a very short time in  astronomical standards where planetary  or galactic movements can easily span  millions of years  man-made satellites can stay in orbit  around the moon or  any moon for the duration of a mission  because tidal effects require thousands  or millions  or more years depending on the system to  cause the loss of a sub-satellite  because of this we can leave a man-made  satellite in stable orbit around the  moon for a few years  using the spacecraft's rocket thrusters  to correct for any changes in its orbit  even though earth's moon doesn't have a  sub-moon now it may in the future  according to the researchers an  artificial one  perhaps nasa's planned at lunar gateway  the lunar gateway would help to  establish humanity's presence  in deep space as outlined by william  gerstenmaier  associate administrator of human  exploration and  operations mission directorate at nasa  headquarters  the possibility of moons having their  own moons is a fascinating one  even though we haven't found any  examples yet the majority of detected  exoplanets are giant planets  at least one kepler-1625b  may have a very large exomoon  nonetheless  no notable moon of a moon or  sub-satellite is known in the solar  system or beyond  there are of course human-launched  satellites in temporary lunar orbit  the planets with the highest potential  to host long-lived submoons around their  current moons  are jupiter saturn and earth based on  its orbital separation  and inferred mass and size the new  exomoon candidate  kepler-1625b dash i  appears capable of hosting a large  sub-moon however  it is worth noting that kepler-1625b-i  has a significant orbital inclination  which may affect the stability of  sub-moons  there have been some possible cases of  sub-moons  in natural cases the first notable one  is in rhea  which has a possible ring system and has  led to calculations that indicated that  satellites orbiting raya would have  stable orbits  furthermore the suspected rings are  thought to be narrow which is something  caused by shepherd moons  a shepherd moon is a small natural  satellite that basically creates a gap  in planetary ring material  or keeps particles within a ring  contained we  find that 10 kilometer scale submoons  can only survive around large moons on  wide separation orbits  on top of that tidal dissipation  destabilizes the orbits of submoons  around moons  that are small or too close to their  host planet  this is the case for most of the solar  system's moons  next up we have iapetus it's been  proposed that saturn satellite possessed  a  sub-satellite in the past this is one of  several hypotheses that have been put  forward  to account for its unusual equatorial  ridge  it may be that in many or even most  cases  there are multiple factors that make the  orbits of sub-moons  inherently unstable knowing whether that  is true or not  may have to wait for discoveries of  moons orbiting distant exoplanets  so that's why a moon can have a moon yet  we don't see many moons with sub-moons  in their orbit  don't forget to subscribe and to turn  notifications on  for more articles like this one