How Many “Official” Moons Does Jupiter Have?

Question: I am writing a piece for middle school students about the planets of our solar system and am trying to find an up-to-date figure for how many moons are officially recognized around Jupiter. I’ve seen figures from 23 to 49 and more pending.  — Fiona

Answer: There are two good references to the number of “official” moons orbiting Jupiter.  The first is a listing from NASA’s Jupiter Moons page, which lists 50 named moons plus an additional 17 “provisional” moon.  I believe that provisional moons have not yet been assigned permanent names.  The second source of this listing is on Scott Sheppard’s Jupiter Satellite and Moon page, which lists 67 as of March 2015.  This second reference appears to list the sum of the 50 named moons plus the 17 provisional moons.  For your summary I would use the total of named and provisional moons, or 67.

Jeff Mangum

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Why is the New Horizons Flyby of 2014 MU69 So Much Closer than its Pluto Flyby Distance?

Question: There is talk of New Horizons travelling to another object in the Kuiper Belt. The fly by will be at about 1900 miles, much closer than the fly by of Pluto. How are the distances of observation of these type of objects arrived at and why would there be a difference between the two?  — Robert

Answer: The closest approach of New Horizons to Pluto was about 7800 miles.  Spacecraft trajectories and flyby distances are determined by a number of factors, including the measurement system used for determining the position and speed of a probe, the location from which these measurements are obtained, an accurate model of the solar system, and the availability of accurate models of the motion of a probe.  There is a nice Scientific American article on this subject with more details.  As for why the flyby of this new object, which is I believe a Kuiper Belt Object (KBO) called 2014 MU69, it is difficult to say.  My guess is that the Pluto/Charon flyby imposed restrictions on how close New Horizons could get to Pluto based on the fact that it also had to get close to Charon, while the flyby of 2014 MU69 is a flyby of just one object, so they can get as close as they can.

Jeff Mangum

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What Happens to the Other Planets in the Solar System if Jupiter is Removed?

Question: What would happen to the other planets in the solar system if we removed Jupiter?  — Willie

Answer: Instantaneously removing Jupiter from the solar system would have little effect on Earth and the other planets.  This is due to the fact that Jupiter is about 1/1000th the mass of the Sun, and it is about 5 times further away from us than the Sun.  Since the gravitational force on a planet of mass M due to an object of mass m and distance R is given by F=GMm/(R^2) (this is Newton’s Law), the effect of Jupiter on the Earth is 1/25000 that of the effect of the Sun on Earth.  This effect is negligibly small.  Now, Jupiter’s removal would have a big effect on its moons and the asteroids which inhabit the asteroid belt since Jupiter’s mass has a big effect on their orbits.

Jeff Mangum

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Will a Comet Skim the Earth’s Atmosphere in 2016?

Question: I’ve been dealing with a false prophet who says that a comet is coming and is going to skim the earth, as if to skip off of it, like a stone skipping on water.  Is this even possible?  She says it will skip off of the earth and keep going into space.  Please let me know if this is even possible?  Thanks so much.  — Andrew

Answer: The scenario you describe is physically possible, but the actual event described by this “false prophet” is misrepresented.  A pair of comets, known as 252P/LINEAR and P/2016 BA14, passed close to Earth on March 21 and 22, 2016.  The closest that they came was within 2.2 million miles of the Earth, or about 9.6 times the distance between the Earth and the Moon.  Comets have on a couple of occasions passed closer to Earth, and as was the case with the most recent comet passage we survived.

Jeff Mangum

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Double Shadow Transits of the Galilean Satellites of Jupiter

Question: Why is there a double shadow transit (i.e. of the Galilean moons) season for Jupiter and why does it occur during the opposition of the Jupiter relative to Earth?  Do DST occur all the time, but during opposition, it is more apparent to us on Earth?  — Grace

Answer: Double Shadow Transits (DST), or the shadow of two Galilean satellites on the cloud tops of Jupiter, occur during Jupiter opposition due to the relative alignment of the Sun, Earth, and Jupiter during opposition.  Jupiter opposition makes these events visible to us on Earth, but in fact Galilean shadow events can occur any time that a Galilean satellite passes between Jupiter and the Sun as viewed along a line connecting the Sun and Jupiter.

Jeff Mangum

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Which Planet Orbits our Sun the Fastest?

Question: Which planet in our solar system is orbiting the sun at the fastest speed?  — Mike

Answer: Mercury is the winner at an orbital speed of about 47.87 km/s (107,082 miles per hour), which is a period of about 87.97 Earth days.  Just for your information, here is a list of the orbital speeds (and periods) for all 8 (plus Pluto) planets:

  • Mercury: 47.87 km/s (107,082 miles per hour), or a period of about 87.97 days
  • Venus: 35.02 km/s (78,337 miles per hour), or a period of about 224.7 days
  • Earth: 29.78 km/s (66,615 miles per hour), or a period of about 365.256365 days
  • Mars: 24.077 km/s (53,853 miles per hour), or a period of about 686.93 days
  • Jupiter: 13.07 km/s (29,236 miles per hour), or a period of about 11.86 years
  • Saturn: 9.69 km/s (21,675 miles per hour), or a period of about 29.42 years
  • Uranus: 6.81 km/s (15,233 miles per hour), or a period of about 83.75 years
  • Neptune: 5.43 km/s (12,146 miles per hour), or a period of about 163.72 years
  • Pluto: 4.74 km/s (10,603 miles per hour), or a period of about 247.92 years

Jeff Mangum

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Does the Coriolis Effect From the Milky Way Rotation Influence the Orbits of Planets in our Solar System?

Question: I read that our solar system orbits the center of the Milky Way Galaxy and am wondering if orbital direction of the planets was initially influenced by the Coriolis effect.  — Jack

Answer: Two comments to your question.  First, the net orbital angular momentum axis of the Sun or the Solar System turns out not to be aligned with the “spin axis” of our Milky Way galaxy.  The plane of the disk of our Solar System is inclined by an angle of about 63 degrees relative to the plane of the Milky Way within which our Solar System resides.  Second, the Coriolis effect, sometimes referred to as a “fictitious force”, refers to motion which is really in a straight line, but which appears to be curved when viewed from a reference frame that is accelerating (i.e. rotating).  So, if you are in a rotating disk, like our Solar System, you experience a Coriolis force, but if you step away from our Solar System and look at it from a point that is not accelerating, you would see planets in motion but with no Coriolis effect.

Jeff Mangum

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Building a Pyramid on Io for Maximum Durability

Question: If I were to build a 146 meter tall pyramid on the surface of Io, what material(s) would you recommend I use for maximum durability with respect to the satellite’s terrain and climate?  — Evan

Answer: I think that steel would be a good material to use to build a pyramid on Io.  It has good strength-to-weight, and it would likely be resistant to the sulfur-heavy conditions on Io.

Jeff Mangum

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Did Our Solar System Originally Include a Fifth Gas Giant?

Question: Read an article that researchers at the University of Toronto believe that Jupiter in the solar system early days kicked a planet out of the solar system . Do you believe this could be the one? Many thanks  — Bill

Answer: This scenario, whereby the early solar system was comprised of more than the four gas giants we have today, has been hypothesized since 2011.  The early solar system was an evolving place, with lots of gas and dust forming young planets which ultimately have to compete, in a gravitational sense, for a place in the solar system.  It is entirely plausible that a fifth gas giant existed early in the evolution of the solar system, but was ejected by the heavyweights in this environment, Jupiter and Saturn.  In fact, the prime suspect from the University of Toronto study appears to be Jupiter.

Jeff Mangum

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How Many Earths Can Fit in the Volume of the Milky Way?

Question: How many earths can fit in one galaxy and if the earth were about the size of galaxy how many planets, moons, etc. would be able to fit on earth.  — Xondra

Answer: To answer this question you need to know the volume of the Earth and a galaxy.  Let’s take the Milky Way as our galaxy example.  The Earth’s volume is a little over 10^(12) km^3.  To calculate the volume of the Milky Way, we assume that it can be approximated by a disk with a thickness of 1000 light years and a radius of 50,000 light years.  A light year is about 9.5 x 10^(12) km, while the volume of a disk is pi*(thickness)*(radius)^2.  Plugging in our thickness and radius we get about 6.7 X 10^(51) km^3.  Dividing the volume of the Milky Way by the volume of the Earth, you get (6.7 X 10^(51))/(10^12) =~ 6.7 X 10^(39) Earths that can fit in the volume of the Milky Way galaxy.

Jeff Mangum

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