Telescopes

Telescopes come in two basic types: refractors and reflectors. Refractors, or refracting telescopes, use lenses to focus electromagnetic radiation to a focal point to be detected by some sort of detector: eye, CCD computer, camera. Reflectors use mirrors to reflect light to a focal point to be captured by the detector.

Refracting telescopes are basically used to observe one particular type of electromagnetic radiation, visible light. The telescope has two basic pieces; the aperature or objective lens, and the eyepiece or detector. The eyepiece was used by up until the invention of photgraphic astronomy, and now can be replaced by a camera or a computer to take images. Lenses have a property called focal length which is the distance parallel light coming into a lens must travel after passing through the lens to come to a point. The focal length depends on the curvature of the lens.

Knowing the focal length, we can define a couple of terms.

• telescopic magnification power is the ratio between the focal lenght of the objective lens and the focal length of the detector. The larger this ratio, the stronger the telescopic power
• light-gathering area is only dependent on the area of the objective lens. With more area, more light can be gathered and therefore a distant object will be brighter than with a smaller lens. Compare images of Jupiter with a telescope to just your eye. The aperature of a telescope is much bigger than that of the human eye.
• Resolution is how small a detail a telescope can make out. Depends on both the diameter of the objective and the wavelength of light. The longer the wavelength, the bigger the diameter of the telescope's objective to have good resolution. This is why visible light telescopes have smaller diameters than radio telescopes.

Refracting telescopes can only get so big before the lenses cannot support their own weight and will sag in the center. To resolve this problem, reflecting telescopes were invented. There are typically two types: a Newtonian reflector and a Cassegrain reflector. Newtonian reflecting reflect light to a secondary mirror which then reflects the light off to the side of the telescope. Cassegrains reflect light to a secondary mirror which then reflects the light back down through a hole in the primary mirror (or objective mirror).

The Three Types of Telescopes mention in This Blog

Image Credit:

Me

  

Arecibo Observatory in Puerto Rico. This is a radio telescope which is basically a giant reflecting telescope. The detector is at the top, at the focal point of the mirror.

Image Credit

NAIC - Arecibo Observatory, a facility of the NSF

The Cassini Division

Image Credit: NASA

The Cassini Division is a gap in the ring system of Saturn between the B Ring and the A Ring. It was discovered in 1675 by Giovanni Cassini, hence its name. From Earth, it appears dark and therefore, it was believed that this part of the ring system did not contain any particles. However, when Voyager passed by Saturn, it discovered that there were particles in the division, similar to those in the C Ring. However, they are much more dispersed than in the C Ring, so the division looks empty from Earth.

There are ringlets in the Cassini Division which is caused by resonance with two moons: Mimas which shares a 2:1 resonance with the division (for every one orbit of Mimas, the ringlet makes two) and Enceladus which shares a 3:1 resonance. Mimas and Enceladus tug on the particles in the Cassinin Division, keeping it relatively clear of any particles.

Also, there is a gap in the division called the Huygens Gap. The Huygens Gap contains the ringlet created by the resonance with Mimas.

The B Ring of Saturn

Saturn's rings dark side mosaic

Image Credit:

NASA/JPL/Space Science Institute

Saturn's Ring Plane

Image Credit:

NASA/JPL/Space Science Institute

 

The B Ring is the second ring of Saturn discovered, and the third ring from Saturn. It is composed mostly of golf ball-sized and smaller ice particles, making it very reflective and bright, compared to all the other rings. It is much brighter than the C Ring and the A Ring, and is by far the widest of all the main rings. Because of its width and its depth of 5 to 15 km, it is also the heaviest ring.

 

Unlike the C Ring, which is very transparent, the B Ring blocks 91% of all light incident on it, reflecting most of it, allowing us to see it very easily.

 

Also unlike the C Ring, which has small scale structures inside the ring itself, the B Ring does not contain gaps, but only small ringlets within its structure. The unique feature of the B Ring, however, is the radial lines evident in the rings. These spokes, as they are referred to, are not from gravity, but from Saturn's magnetic field. If the spokes were due to gravity, they would remain in place, even as the planet rotated. But instead, they have a period the same as the magnetic field of Saturn, so we know that they are created by magnetism.

Dark Spokes in the B Ring

Image Credit:

NASA/JPL/Space Science Institute

Rotation of the Spokes

Image Credit:

NASA/JPL/Space Science Institute

 

 

The C Ring of Saturn

Saturn's rings dark side mosaic

Image Credit:

NASA/JPL/Space Science Institute

Saturn's Ring Plane

Image Credit:

NASA/JPL/Space Science Institute

 

The C Ring was the third ring of Saturn's ring system to be discovered and as shown in the above photos, is the closest of the three main rings. It is fainter than both the B Ring and the A Ring, and is between the B ring (25,500 km) and the A ring (14,600 km) in width at 17,500 km. It was first discovered by William and George Bond in 1850, though William R. Dawes and Johann Galle also independently saw it. William Lassell nicknamed the ring, "Crepe Ring" as it is darker than the A Ring and the B Ring, and to him, resembled the black cloth associated with funerals.

 

It is only five meters thick from top to bottom, and even though it is dark, it is relatively transparent. Between 5% to 12% of light incident on it well be blocked, so it is very easy to see though. It is composed of boulder-sized ice chunks, while the A Ring and B Ring are golf ball sized and smaller. Even though it is made up of ice, it is still darker, meaning that the ice may be covered with a crust of dust, prevently light from being reflected efficiently.

 

The C Ring has smaller parts to it, including the Colombo Gap and Titan Ringlet, the Maxwell Gap and Maxwell Ringlet, the Bond Gap, 1.470 Rs Ringlet, 1.495 Rs Ringlet, and the Dawes Gap. The Titan Ringlet is unique in that it shares a resonance with Titan, so that Titan somewhat controls the rotation of the Ringlet. The Maxwell Gap and Ringlet are named after James Clerk Maxwell, who had mathematically calculated that the rings could not be solid disks, the Bond Gap named after William and George Bond, and the Dawes Gap is named after William R. Dawes.

The Rings of Saturn

PIA17172 Saturn And Its Rings with Earth, Mars, and Venus

Image Credit:

NASA / JPL-Caltech / Space Science Institute

 

The first image that pops in people's heads when they think of Saturn is probably a planet with rings. And they wouldn't be wrong. Saturn's most famous feature are its impressive rings and in my opinion, the most striking feature of any planet in our Solar System. So what exactly are the rings and where did they come from?

The rings were first discovered by Galileo in 1609 when he saw these objects on the sides of Saturn that he called "ears". His telescope was not good enough to resolve the ears into a disk and to see the rings clearly. It wasn't until 1659 when Christian Huygens was able to resolve the rings into a disk and see that the rings were not attached to Saturn physically.

 For the next few centuries, it was believed that the rings were a solid torus around Saturn. No evidence was observed to make anyone think differently. In 1859, James Clerk Maxwell, famous for his four equations of electromagnetism (which you can learn about here), proved mathematically that a solid ring would be unstable and not be able to orbit around Saturn. It wasn't until 1895 that two astronomers, James Keeler at Allegheny Observatory outside of Pittsburgh, PA (where I used to work) and Aristarkh Belopolsky of Pulkovo Observatory near Saint Petersburg, Russia, independently spectroscopically determined that the rings were not solid, but made up of many particles. Using the Doppler effect, which is that effect that causes waves to change wavelengths based on the speed of the observer, the source of the wave, or both, they both were able to show that the outer rings travel slower than the inner rings. This would not be possible if the rings were solid. If the rings were solid, the angular velocity of the inner parts of the rings and the outer parts of the ring would have to be the same, and they showed that this was not true. Maxwell's mathematical prediction was true.

The rings themselves are made up of both rocky dust and ice particles, depending on where in the ring structure the particles are located. The rings orbit (in general) above Saturn's equator. Much like the rings of Jupiter, the particles must be continually replenished by micrometeorite collisions with the moons of Saturn, adding particles to the rings while parts of the rings are dissipated by Saturn's gravity or the gravity of the nearby moons. The rings are believed to have been first formed when Saturn was formed when small planetessimals were within the Roche limit of Saturn and could not consolidate into moons.

The rings were originally named in the order in which they were discovered, starting with A. But as more were found, the newer ones were given proper names. Starting with the innermost ring, the ring system is broken down in this manner:

• The D Ring: the fourth ring discovered in 1980 by Voyager 1 is a very faint ring system. Its distance from Saturn ranges from 66,900 km to 74,510 km
• The C Ring: the third ring discovered in 1850 by George and William Bond. Its distance ranges from 74,658 km to 92,000 km and will be discussed in more detail in its own post.
• The B Ring: the second ring discovered and the most massive of the rings. Its distance ranges from 92,000 km to 117,580 km and will be discussed in more detail in its own post
• The Cassini Division: a space between the B ring and the A ring discovered by Giovanni Cassini in 1675. Its range is from 117,580 km to 122,170 km and again, will be discussed further in its own post.
• The A Ring: the first ring to be discovered when Huygens first detected the rings as rings. It ranges from 122,170 km to 136,775 km and will be its own post
• The Roche Division: the gap between the A Ring and the fainter F Ring. There is material in this division, but is so thinly populated that we do not see it very well. The moon Atlas orbits in this division. Its range is 136,775 km to 139,380 km.
• The F Ring: thin ring orbiting outside the Roche Division. It has a small range or 30 to 500 km but orbits around 140,180 km from Saturn. It is kept in place by two small moons, Pandora and Prometheus and will be discussed in detail when talking about those two moons.
• The Janus/Ephimetheus Ring: a ring that is maintained by the moons Janus and Ephimetheus. It was discovered by the Cassini spacecraft in 2006. It ranges from 149,000 km to 154,000 km.
• The G Ring: faint ring with a bright inner edge. Halfway between the F Ring and the E Ring, it has the moonlet Aegaeon orbiting nearby. It ranges from 166,000 km to 175,000 km.
• The Methone Ring Arc: not a full ring, but a 10° arc orbiting around Saturn. It shares an orbit with Methone and was detected for the first time in September of 2006. It orbits abour 194,230 km from Saturn
• The Anthe Ring Arc: not a full ring, but a 20° arc orbiting around Saturn, much like the Methone Ring Arc. It shares an orbit with Anthe and was detected for the first time in June of 2007. It orbits abour 197,665 km from Saturn
• The Pallene Ring: shares an orbit with the moon Pallene at around 211,000 km to 213,500 km. It was discovered by Cassini in 2006.
• The E Ring: the last of the lettered rings, though the fifth discovered. The second outermost ring, but the outermost orbiting equatorially with Saturn. It is very wide and is between the orbits of Mimas and Titan. There are moons that orbit within the ring and they are tinted by particles from the ring. It orbits between 180,000 km to 480,000 km, by far the widest of the rings.
• The Phoebe Ring: the outermost ring orbiting just to the interior of the moon Phoebe. It was discovered in October of 2009 by NASA's infra-red Spitzer Space Telescope and orbits at an angle of 175° to the equator of Saturn, so it also orbits retrograde. It orbits between 4 million and 13 million km from Saturn and will be discussed more in its own post.

Saturn

Image Credit:

NASA/Voyager 2

 

Saturn is the second largest planet in the Solar System, about 95 times the mass of the Earth. However, compared to Jupiter, Saturn is tiny. It is only 30% the mass of Jupiter. Its radius at the equator is 9.44 times that of Earth and its polar radius is only 8.5 times Earth's. Despite this, if you could stand on Saturn, you would feel the same gravity as you do on Earth. Its day is just a little longer than Jupiter's at 10.57 Earth hours. Its average distance from the Sun is 9.5 AU which gives it an orbital period of 29.46 Earth years. It has an inclination of 26.5° with respect to its orbit and its orbit is only tilted at 2.5° to the ecliptic (the orbit of the Earth). With respect to the Sun's equator, it is tilted at 5.51°.

 

The thing Saturn is most known for is seen in the above image from Voyager 2. Its ring system is the most extensive of all the Jovian planets and Saturn has been known to have rings since the 1600s. The rings are a fascinating aspect of the most beautiful planet (in my opinion) in our Solar System, so that I can't talk about all rings in only one post. Stay tuned to learn a lot about Saturn's rings.

 

Saturn also has many moons, almost three times as many as Jupiter with 150 known, though only 51 have formal names. The largest satellite in the Solar System, Titan, belongs to Saturn and could be considered a mini-world in its own right. It also has moons that keep some of Saturn's rings in line, called shepherd satellites. It has a moon that doesn't look like a moon and moons that share an orbit.

 

Saturn's composition is similar to Jupiter, containing the same gases, but in different concentrations. This difference in concentrations, the thickness of its atmosphere, and the size of its heavy element core give rise to a strange phenomenon when looking at Saturn's density.

Celestial Sphere

The celestial sphere is the imaginary sphere covering the sky surrounding the Earth with the Earth at the center of the sphere. It is a way to map the location of stars, galaxies, and extra-solar objects in the sky, as well as track the movements of the Sun, Moon, and planets among the background stars.

There are a few important locations on the sphere that should be defined to help get a sense of what to look for in the sky.

Zenith: the point on the celestial sphere directly above your location on Earth. This point is dependent on where you are on Earth. To someone in Pittsburgh, PA, USA, their zenith is not that same as to someone in Sydney, New South Wales, Australia.

Nadir: the point on the celestrial sphere directly below your location on Earth. Like the zenith, this is location dependent.

The North (South) Celestial Pole: the point on the celestial sphere directly above the North (South) Pole on Earth. The North (South) Celestial Pole is the zenith for the North (South) Pole and the The South (North) Celestial Pole is the nadir for the South (North) Pole.
 
The Celestial Equator: similar to the equator on Earth. In fact, it is the imaginary line on the celestial sphere is directly above the Earth's equator. It is used to help explain the equinoxes as well as the solstices.

Celestial Prime Meridian: the line on the celestial sphere equally 0h Right Ascension. It is the line directly corresponding to 0° longitude on Earth, i.e. the prime meridian running through Greenwich, England.

Right Ascension: equivalent to longitude on Earth, this is an angle measured in hours, minutes, and seconds from 0h to 24h. The angle increases from east to west, from the Celestial Prime Meridian

Declination: equivalent to latitude on Earth, also measured in degrees, arcminutes, and arcseconds from 0° to 90° from the Celestial Equator to the Celestial Poles. Travelling north from the Celestial Equator to the North Celestial Pole, the angles are positive, and travelling south from the Celestial Equator to the South Celestial Pole, the angles are negative.

Ecliptic: this is defined in another page, located here. It is the imaginary path the Sun traces over a course of the year on the Celestial Sphere. It is tilted at 23.5° with respect to the Celestial Equator and when it crosses the Celestial Equator, the equinoxes occur. At the Sun's northernmost point on the ecliptic, we have the summer solstice in the Earth's northern hemisphere and the winter solstice in the southern hemiphere. At the Sun's southermost point, the solstices are switched.

Composition of Jupiter

Jupiter's density is about 1.3 times that of water. So even though we know that Jupiter has a heavy element core, we know that the composition of Jupiter is mostly hydrogen and helium like the Sun. The outer layers of Jupiter are the lighter elements, like hydrogen and helium.

Jupiter itself is about 78% molecular hydrogen (H₂) and 19% helium with trace amounts of methane (CH₄), ammonia (NH₃) and water (H₂O). The pressure on Jupiter is so great that the gases which are gas at normal pressure and temperatures on Earth are liquid on Jupiter. Really deep in the interior, something even stranger happens to hydrogren. Not only is it liquid, but it becomes something called liquid metallic hydrogen. Metallic hydrogen just means that the molecules are organized in such a way that the atoms share electrons easily, allowing the liquid hydrogen to be electrically conductive. Try that with normal liquid hydrogen (though there really isn't anything normal about liquid hydrogen because it needs to be at 14 Kelvin (-253°C or -423°F) to be liquid. Obviously, liquid metallic hydrogen does not occur naturally on Earth, but can be created in labs under the correct pressure and temperature conditions.

The outer layers of Jupiter are its atmosphere which is "only" 1000 km thick, but that is only 1% of Jupiter's radius of 72,000 km. Compare that to Earth which has an atmosphere that is 2.5% of the Earth's radius. The atmosphere is composed of hydrogen gas with different cloud layers. The topmost cloud layer is ammonia crystals at a temperature of 150K, followed by ammonia hydrosulfide at 200 K, with the lowest cloud layer being water at 280K. The Galileo probe, whose images have been included in some of these posts, was also used to explore the atmosphere of Jupiter. After entering the atmosphere, the probe reached darkness about 80km down and was completely destroyed by the high pressure and heat at only 130 km down.

The Autumnal Equinox

September 23, 2014 at 02:29 UTC (10:29 EDT) marks the official beginning of autumn (autumnal equinox) in the northern hemisphere and spring (spring equinox) in the southern hemisphere.

If you were to imagine the sky above the earth to be a giant sphere with the Earth at the center, the path the Sun travels on this sphere is the ecliptic. The sphere itself is the celestial sphere, and is like a map of the nighttime sky. The line direction above the equator is the celestial equator. When the ecliptic and the celestial equator intersect, we have the equinoxes. If the Sun is travelling north on the celestial sphere when it crosses the equator, we have the northern hemisphere's vernal, or spring, equinox. When the Sun is travelling south on the ecliptic when it crosses the celestial equator, the autumnal equinox occurs, like it is doing today. If the Earth were not tilted with respect to the ecliptic, the celestial equator and the ecliptic would coincide and we would not have the seasons we have. But because it is tilted at 23.5° with respect to the ecliptic, we have the four seasons we know and love (unless you are like me and hate winter).

This is the time of year for us in the northern hemisphere start cursing the weather getting colder, especially those of us in the northern latitudes, and those in the southern hemisphere are beginning to get warmer weather.

Comet Shoemaker-Levy 9

Hubble Telescope Image of the Comet Shoemaker-Levy 9 before collision with Jupiter

Image Credit:

H.A. Weaver, T. E. Smith (Space Telescope Science Institute), and NASA

 

In 1993, Comet Shoemaker-Levy 9 was discovered by Eugene and Caroline Shoemaker and David Levy in photographs taken of Jupiter. Sometime before its discovery, it passed within the Roche limit of Jupiter and ripped apart by tidal forces from Jupiter's gravity and broke up into 23 individual pieces.

 

Because of its discovery and by following the trajectories of the pieces, the orbits of the pieces could be calculated accurately. From observations, it was determined that the comet would impact Jupiter in July of 1994. This was the first impact that could be accurately predicted and be observed safely as it occured on another planet. The impact of Comet Shoemaker-Levy 9 was going to televised worldwide as this was before the invention of the internet.

 

Over the course of six days, the 23 different pieces of the comet fell on Jupiter. Unfortunately, the impact sites were on the side of Jupiter away from Earth, but luckily on the side that was rotating towards Earth. We might not have seen the impacts directly, but we could see the aftermath on the surface of Jupiter as well as plumes that could be seen over the horizon, if the pieces were large enough.

 

Galileo images of the impact sites

Dept of Physics and Astronomy, University of Tennessee at Knoxville

 

Image of Jupiter with 21 of the fragments

Image Credit:

NASA

MIRAC2 103 micron (one-millionth of a meter) Image of one of the impact

Image Credit:

MIRAC2/NASA/Harvard

 

Watching a comet fall onto Jupiter told us a lot about impacts in the Earth's distant past. It also helped us learn more about Jupiter's atmosphere, which will be addressed in the next post. Early in our history, Earth was continuously bombarded by asteroids and comets, which helped build our planet. In fact, it is believed that the majority of the water on our surface came from the impact of comets with Earth. However, we are not prone to impacts today because there are less asteroids and comets and because Jupiter captures many with its immense gravity. If there was no Jupiter and no large planets, we would be more likely to be impacted by comets and asteroids. If Comet Shoemaker-Levy 9 had hit us, life today might not exist, or at least the kind of life we live today. We should be thankful for Jupiter to vacuum up the majority of the small bodies wandering our Solar System, protecting our planet from catastrophe.

Star Wars and Physics

First of all, let me point out that I love Star Wars. I am old enough that the first three movies came out after I was born, so I grew up with the legacy of Luke, Leia, Han, Chewbacca, and all the rest. Everyone has already talked about how Han Solo's line about making the Kessel Run in less than 12 parsecs is inaccurate since a parsec is a distance and if the Kessel Run is less than 12 parsecs in distance, then big deal. But that is not what I want to talk about. I'm not going to talk about lightsabers or the force, since they are part of the mythos.

I want to talk about when ships are docking on the Death Star.

In almost every scene I've seen of the Millennium Falcon or the Imperial Shuttle landing in one of the cargo bays on the Death Star, I notice two things.

One, they never show a force field being deactivated, which is not that big a deal since a force field technically is invisible.

Two, when the Millenium Falcon or the Imperial Shuttle is coming in and landing, there are people in the cargo bay. This is important because if the force field is down, the cargo bay will have no atmosphere in it. Granted, the stormtroopers or whomever could have some sort of scuba gear or self-contained breathing in their armor, but when the Millennium Falcon leaves in Episode IV, Luke, Leia, Han, and Chewbacca run to the ship without any difficulty breathing. But they leave without deactivating a force field. So did the Millennium Falcon just fly through a force field? Is there something across the opening of the cargo bay that prevents the atmosphere from leaving but allows a ship to go through? That seems a little far-fetched, but again, it is science fiction.

So is there a force field across the opening of the cargo bay to hold in the atmosphere? If there isn't, how are the stormtroopers able to breathe and for that matter, our heroes when they are leaving? If there is, how is the Millennium Falcon or the Imperial Shuttle able to travel through the force field without a problem?

The Rings of Jupiter

A side view of Jupiter's Rings from Galileo spacecraft

Image Credit:

NASA, JPL, Galileo Project, (NOAO), J. Burns (Cornell) et al.



Saturn is not the only planet with rings. All the Jovian planets have some sort of ring system, though Saturn's rings are the most impressive. Jupiter's rings are so thinly distributed that they were not discovered until 1979 when Voyage I discovered them on its journey through the Solar System.

Jupiter's rings are not as bright as Saturn's rings, which are mostly composed of ice compounds. Jupiter's rings are dark and reddish which tells us that the material making up the rings are of a rocky origin. The ring is located within the Roche limit, the distance from Jupiter where gravity would rip apart a poorly consolidated moon, asteroid, or comet wandering inside that distance. The origin of the ring is probably a moon or asteroid that wandered too close to Jupiter.

 

The rings of Jupiter would have been dissipated long ago if the ring was not continuously replenished by the moons near the rings. The moons get hit by small meteorites, impacting the surface of the moon, and blowing dust out into orbit. This dust then gets incorporated into the rings. We know this by looking at the distribution of dust particles in the rings and see that the portions of the rings near moons are more densely packed with dust than those sections farther away. For example, near the moons Amalthea and Metis, the ring is densest between these moons. Amalthea orbits just outside the ring and Metis, just within the rings diameter.

 

Besides the main ring seen above, there is a much thinner ring outside Amalthea's orbit. This ring is called the gossamer ring because it is so thin. Again, by noting that this ring is densest near Amalthea and Thebe (orbiting farther out from Amalthea), we can conclude that micrometeorite impacts keep the rings intact. Also, Amalthea, Metis, and Thebe are also shepherd satellites of the rings, which will be explain more in detail when we talk about Saturn, its rings, and its moons.

The Banded Atmosphere of Jupiter

Infrared and Visible Light Image of the Bands of Jupiter 

Image Credit:

NASA

One of the most striking features when you look at images of Jupiter is that the atmosphere is not homogeneous, but banded. The Jovian planets are the only planets that display this feature, though Jupiter's is the most pronounced.

The bands are regions of high pressure, rising gas and low pressure, sinking gas separated by high winds. The bands that have high pressure, rising gas are called zones. The zones are bright but cold. They are made up of ice particles that are either ammonia or water ice. The darker bands, called belts, are low pressure, thin clouds that are sinking towards the planet. The belts do not reflect as well as the zones, so appear darker in comparison. The high winds separating the bands are called jets, travel along lines of latitude (east-west), and blow in either the prograde direction or retrograde direction. The prograde jets travel along with the direction of rotation of the Jupiter (from west to east) and the retrograde jets travel against the direction of rotation (east to west). The prograde jets are the transitions from zones to belts (as you move away from Jupiter's equator) and the retrograde jets transition from belts to zones..

The zones and belts are distinct on the surface of Jupiter. Just like Earth, Jupiter has distinct temperature regions based on their location on the planet. They are named as follows:

• The Equatorial Zone: as the name implies, this is the zone at the equator. Its range is from 7°S to 7°N.
• The North and South Equatorial Belts: the belts just north and south of the Equatorial Zone. They are reddish in color and range from 7°N (S) to 18°N (S).
• The North and South Tropical Zones: the South Tropical Zone is where the Great Red Spot is located
• The North and South Temperate Belts: the South Temperate Belt contains Oval BA (Red Jr.)
• The North and South Temperate Zones
• The N-N and S-S Temperate Belts
• The N-N and S-S Temperate Zones
• The North and South Polar Regions

A description of the belts and zones

Image Credit:

Sky and Telescope



 

The jets between the zones and belts travel at over 100 m/s (360 kph or 224 mph) and keep the bands from mixing.

NASA to Resume Launching Astronauts to Space

NASA announced yesterday that it will resume launching astronauts into space using two American companies, Boeing and SpaceX. This will be done to stop our reliance on the Russians to launch astronauts to and from the International Space Station.

Boeing will be using its CST-100 spacecraft and SpaceX will be using the Crew Dragon spacecraft. NASA is currently using SpaceX's Dragon spacecraft to send supplies to the ISS.

This is good news for NASA as it may be the first step to making space travel privatized in the US instead of making space travel exclusively for military and the government.

News of the announcement is here on the NASA website.

The Great Red Spot

Jupiter's Great Red Spot just above a smaller, white storm

Image Credit:

NASA

 

Jupiter's Great Red Spot is, by far, the largest storm in the solar system. It is located in the southern hemisphere of Jupiter and has been around since it was first officially discovered in 1668. There is evidence however that it may have been discovered in 1635. At over 340 years old, it is the oldest known storm in the solar system.

Since it's original discovery, it has undergone changes in its size, being anywhere from 2 to 3 Earth diameters. It has ranged from 24,000 km to 40,000 km on its long axis (along lines of latitude) and 12,000 km to 14,000 km on its short axis. Since 1904, the Great Red Spot has decreased to half its longitudinal extant (its long axis is half as long as it was in 1904), and it is estimated if the trend continues, the Great Red Spot will become more circular by 2040. This is assuming that the storm is finally petering out, or if this is just a natural cycle of the Great Red Spot.

Comparison of Earth to Jupiter. Lower right of Jupiter is Great Red Spot

Image Credit:

NASA

The spot itself does not change latitude, but has been in the same location relative to the equator since its discovery. Unlike storms on Earth, the Great Red Spot is constrained to its latitude because of the banded atmosphere, and the sharp boundaries between the bands.

Voyager imaging also showed that the Great Red Spot is colder than the surrounding gas, which implies that the center is higher in altitude than the rest of the atmosphere. If you recall from the post on sunspots, the center of a sunspot is colder than the rest of the Sun, and sits below the surface. However, atmospheric science tells us that the cooler the gas, the higher the gas is. The Great Red Spot is 8 km above the surrounding gas.

The Great Red Spot has been observed to rotate counter-clockwise (anticlockwise) in about 14 Jovian Days (140 Earth hours). The speeds at the edge of the spot have been clocked at 432 km/hr (270 mph). On Earth, a Category 5 hurricane achieves wind speeds of 158 mph and an F6 tornado reaches 300 mph. In both cases, however, the atmosphere blown around by the hurricane or tornado is much less dense than the gases on Jupiter. Also, F6 tornadoes only are about 2 miles in diameter, which is still pretty big, and do not last as long. Granted, if you've been unlucky enough to have been through a tornado (and an F1 at that), they do seem to last forever. Imagine having to experience a Category 5 hurricane or an F6 tornado for over 300 years. If we were to equate an F6 tornado to the size of the Great Red Spot, the F6 tornado would span the entire US from coast to coast.

Scientists do not know why its red, but it is believed to be due to organic molecules, sulfur compounds, or phosphorus compounds. The shade of red has changed over time from pink to deep red and back again, so it is likely that the color is due to a combination of all three compounds.

The Great Red Spot has a little brother (sister?) called Oval BA or Red Jr., which is south of the Great Red Spot, and a little farther to the west. Like the Great Red Spot, Oval BA is red and has persisted for many years, being discovered in 2000 after three small white storms collided and merged.

The Southern Hemisphere of Jupiter.

Image Credit:

NASA and Christopher Go

Callisto

Image Credit:

NASA/JPL/DLR

The farthest of the Galilean moons, and the second largest of all Jovian moons, Callisto has the smallest density of the four major moons of Jupiter of 1.81 g/cm³. From the density, we know that Callisto is mostly water and ice. It has a radius of 2410 km, giving it a total mass of 0.018 Earth masses. Orbiting at 1.883 million km from Jupiter, it takes 16.7 Earth days or 40 Jupiter days. Like Ganymede, Io, and Europa, it is tidally locked to Jupiter, so it rotates on its axis once ever 40 Jupiter days, as well. It does not share a resonance with Io, Europa, and Ganymede, which, if you recall, is 1:2:4. Callisto's resonance would be 9.4 if you would want to know that.

Since it is the farthest Galilean moon, it does not go as intense tidal friction as do the others. Because of this, it is not as active as the other three, and is not as well differentiated even though it is larger than both Io and Europa. It may have a liquid ocean below the ice crust, but it is less likely to harbor life than Europa.

The Four Galilean Moons of Jupiter. Clockwise from top left:

Io, Europa, Callisto, and Ganymede. Note how Callisto is the only one that has a uniform interior.

Image Credit:

NASA/JPL

 

Since it is not as strongly affected by tidal forces, Callisto's surface is slightly older than the surfaces of Io, Europa, and Ganymede. It is dark, dusty, and pockmarked with multiple craters. The dark and dusty material is from millennia of asteroid and comets impacts, disintegrating and covering the surface.

Callisto, however, is the best bet for human presence in the Jupiter system. Since it is farther from Jupiter than the rest, the magnetic field of Jupiter is less intense at the Callisto than the other Galilean moons. Also, the radioactivity from the moon itself is not as strong since the moon is geologically inactive. The thin atmosphere contains both carbon dioxide and molecular oxygen which can be used by the colonists or scientists on the surface, and with the liquid ocean below the crust, water is in abundance, as well. Though it is farther from Jupiter, it is still well within Jupiter's gravity well, so any asteroids or comets coming nearby will most likely be capture by Jupiter and crash into Jupiter (see Comet Shoemaker-Levy 9).

Callisto, like the other Galilean moons, formed at roughly the same time as Jupiter, but they were far enough away to be prevented from being accreted by the newly formed planet. They, obviously, are still under the gravitational influence of Jupiter itself.

Ganymede

Surface of Ganymede

Image Credit:

USGS Astrogeology Science Center/Wheaton/ASU/NASA/JPL-Caltech

 



Ganymede is the third closest Galilean Moon, and the largest of all Jupiter's moons. In fact, it is actually larger than Mercury and our Moon, and only three quarters the size of Mars. It has a density of only 1.9 g/cm³, about two times the density of water, implying that Ganymede is composed mostly of water and ice, though it does contain some rocky material and a small iron core. The iron core is inferred from Ganymede's size. It is large enough that in the past, radioactivity in the interior made the iron and rock molten, allowing for the heavier iron to sink to the center of the moon. Its ice crust is about 500 km thick, but it does have a 5-km thick liquid water layer about 170 km below the surface. Despite it being larger than Europa, its ice crust is too thick to allow life to exist, so we believe.

 

 Ganymede orbits at approximately 1.07 million km from Jupiter, giving it an orbital period of 7.15 days (twice that of Europa and four times that of Io). Its radius is 41.3% that of Earth, but its mass is only 0.025% of Earth, as Earth is mostly iron, nickel, and rocky material while Ganymede, as mentioned above, is mostly ice and water. Much like the Moon as it orbits the Earth, Ganymede is tidally locked to Jupiter. For that matter, so are Io, Europa, and Callisto. This means that for every orbit the Galilean moons make around Jupiter, each one rotates on its axis once. So much like the same face of the Moon is facing Earth, the same faces of Io, Europa, Ganymede, and Callisto are pointed to Jupiter.

 

Tidally locked moon orbiting a planet

Small line pointed to planet at all times

 

 

Ganymede has a surface that is not uniform. About a third of the surface is old, dark, and heavily cratered. We know that this makes the surface old as the early Solar System was heavily bombarded by asteroids and comets, though there are some impacts today, but not on the scale seen when the Sun was just being born. A way to look at this is to compare the surface of the Earth with that of the Moon. The Earth has a very active surface, from earthquakes, volcanoes, flowing water, and the weather, where the Moon is not active at all. A larger percentage of the surface of the Moon is cratered compared to the Earth. We know that we are not being impacted as much now as the Earth was in the beginning, as we do not see large meteors every day. However, Ganymede also has a large percentage of its surface that is much younger. Much like Io and Europa are affected by tidal forces from Jupiter and the other Galilean moons, Ganymede is as well. The tidal forces can cause fractures in the surface crust, allowing liquid water to come up to the surface and flow. This flowing water creates grooves on the surface, which flow through the craters. If the grooves formed before the craters, then the grooves would be broken, but we see the grooves as continuous.

"Old" surface vs. "New" surface

The left side of the image is the older surface of Ganymede, dark and cratered. The right side is younger, with less craters and grooves running along the surface

Image Credit:

NASA/JPL/DLR

 

Europa

Galileo Images of Europa

Image Credit:

NASA/JPL/DLR

 

Europa is the fourth largest satellite of Jupiter, the smallest of the fours Galilean moons, and the second closest to Jupiter of the Galilean moons. It orbits 671,100 km from Jupiter and takes only 3.55 Earth days to go around Jupiter (twice that of Io). It has a density of 3.013 g/cm³, making it about 3 times that of water. It does have a lower density than Io, which will be explained in the next paragraph. It's radius is about 1561 km, making it smaller than the Moon, which is about 1738 km in radius.

Europa has a surface covered with ice and is believed to have subsurface liquid water under the ice. The water is kept liquid by two processes, the insulation from the ice and from tidal forces heating the interior, much like tidal forces keep the interior of Io molten. If life exists anywhere else in our Solar System, this may be the most likely place to look for life. However, any mission we send to Europa to find life would have to be very careful to be sterilized as any microbes hitching a ride from Earth would contaminate any detection techniques used on Europa to search for life.

Just recently, Europa was found possibly to experience plate tectonics, much like Earth. Slabs of ice may slide under each other, creating "Europa-quakes", much like earthquakes experienced on Earth as continental plate slide under and above each other. More information about plate tectonics can be found on space.com's article about Europan plate tectonics.

Artist's Concept of Plate Tectonics on Europa

Credit: Noah Kroese, I.NK

Close-up view of possible plate spreading on the surface of Europa

Credit: NASA/JPL

 

Io

Mosaic of the Voyager Missions

Image Credit: 

NASA/JPL

 

Io is the third largest Galilean moon and the closest to Jupiter. It has vey little water and ice, making it the dryest body in the Solar System. It is mostly rocky material which is reflected in its density, 3.55 g/cm³, comparable to the density of the Moon. It is has a larger diameter than the Moon, with an average radius of 1816 km (the Moon has a radius of 1737 km). It only takes 1.77 days to orbit Jupiter, at a semi-major axis distance of 421,700 km. Io has a very hot surface even though it is at the same distance from the Sun as Jupiter. Why?

The quick and simple answer is that tidal forces from Jupiter's immense gravity pull and squeeze Io, keeping the interior hot. As Io orbits Jupiter, those differential forces from Jupiter, as well as the other Galilean moons, keep the interior hot and molten. As we know from Earth, molten interiors lead to volcanoes on the surface and Io has plenty of those. At least 150 active volcanoes have been observed on Io, erupting continuously.

Jupiter also has a much stronger magnetic field than Earth, so large, that Io itself orbits within its confines. The magnetic field lines actually capture ionized particles from the solar wind also ionizes atoms in the thin atmosphere of Io. This ionizing radiates the surface and helps keep it hot even at its extreme distance from the Sun.

The combination of the volancism and the magnetic field of Jupiter gives Io one of the youngest surfaces in the Solar System. The surface is estimated to be only a million years old and making it a place that would not be fun to visit. Io does have an atmosphere, making it one of the few satellites in the Solar System to have an atmosphere. The atmosphere is composed of mainly sulfur compounds, so would not be a pleasant place to smell, even if you could stand the heat and didn't have to breathe.

A comparable place to Io, and Io is in fact the inspiration for this place, is Mustafar in the Star Wars universe. At the end of Episode III: Revenge of the Sith, Obi-Wan and Anakin fight on the surface of Mustafar which is covered in lava lakes and has a lot of volcanic activity. Mustafar is compressed and pulled in the same way as Io, but with two gas giant planets in neighboring orbits providing the gravity, rather than a gas giant and fellow satellites.

Eruption on the surface of Io

Image Credit:

NASA/JPL/University of Arizona

Plume on the limb of Io

Image Credit:

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Galilean Moons

Callisto, Ganymede, Europa, and Io

From Astronomy Online

 

 

The Galilean moons are the four largest moons orbiting Jupiter (though they are not the closest). They were first discovered by Galileo Galilei (and independently, by Simon Marius) in 1609 and 1610 after Galileo turned his telescope towards Jupiter. He noticed these bright dots near Jupiter and as he followed Jupiter in the sky over the course of weeks, he observed those dots staying with Jupiter as it moved in the sky. He correctly concluded that these dots, which he first called stars, were gravitationally bound to Jupiter. He was the first to discover satellites orbiting another body other than the Earth. As we learned in the post about Galileo, there could be objects that did not orbit the Earth. This put another nail into the coffin, successfully debunking the theory that everything orbited the Earth, the Geocentric model of the Solar System.

 

When Galileo discovered these four moons, he wanted to name them for his sponsor, Cosimo de Medici, calling them Cosmian stars, or the Medici family, the Medician stars. He labeled them Jupiter I, Jupiter II, Jupiter III, and Jupiter IV based on increasing distance from Jupiter. This convention held up until the mid 20th century until moons closer to Jupiter were discovered. The four names we use today (Io, Europa, Ganymede, and Callisto), were actually proposed by Johannes Kepler and adopted by Simon Marius who had also observed the moons. Galileo did not like the names and kept his convention, never using the names we use today.

 

All four moons are bigger than any dwarf planet, including Pluto. In fact, Ganymede has a larger diameter, though a smaller mass, than Mercury. The three inner Galilean moons also experience a 4:2:1 resonance. For every four orbits around Jupiter completed by Io, Europa completes two orbits and Ganymede completes one. Callisto has an orbital period of 9.4 times that of Io, so it doesn't really fit the pattern.

 

All the moons are actually visible with amateur telescopes and binoculars, as long as none are obscured by Jupiter's face. The next four posts will go into more detail about these fascinating moons.

The Moons of Jupiter

"Jupiter family" by NASA/JPL - PIA01481This image or video was catalogued by Jet Propulsion Laboratory of the United States National Aeronautics and Space Administration (NASA) under Photo ID: PIA01481. Licensed under Public domain via Wikimedia Commons

 

Jupiter has 67 known natural satellites, with 51 of them having diameters of 20 km or less. Many of these moons are probably captured asteroids that got caught in Jupiter's massive gravitational field. Most of these moons have only been discovered since 1975 with improvements in telescopes and the Pioneer and Voyager missions.

Jupiter's moons are generally divided into groups based on proximity to Jupiter, composition of the moon, and other orbital characteristics.

These groups are:

• The Inner Group: as the name implies, these are the inner most moons of Jupiter. There are four of these moons are they are all less than 200 km in diameter with semi-major axes less than 200,000 km from Jupiter. They have generally low eccentricities and have inclinations close to 0°, i.e. their orbits are almost directly above the equator of Jupiter.
• The Galilean moons: These are probably the most well-known of all of Jupiter's moons. These are the four moons discovered by Galileo in 1609 or 1610 after Galileo pointed his telescope towards Jupiter. He noticed these small objects moving along with Jupiter as Jupiter went around the Sun. These four moons will be discussed in more detail later on.
• Themisto is a single moon that does not belong to any group. It is farther away from Jupiter than the Galilean moons but closer to Jupiter than the next group, the Himalia group. Themisto is located approximately halfway between Callisto and the innermost Himalia moon, Leda. It has a semi-major axis of 7.39 million km, an eccentricity of 0.2006, and an inclination to Jupiter's rotation of 47.48°.
• The Himalia group: This group is named after the largest member, Himalia. There are five known satellites in this group with orbits ranging from 11.15 million km to 11.75 million km. They orbit at an inclination of 26.6° to 28.3° and have eccentricities of 0.11 to 0.25. These moons have compositions similar to C-type asteroids, which lead astronomers to believe that this group is made up of a captured asteroid that was ripped apart by tidal forces from Jupiter's gravity. Any moons that are found in this group will have a name ending with "-a".
• Carpo is another group made up of a single moon. It has an orbit at the inner edge of the next group, the Ananke group, but its orbital parameters are different. It has an inclination of 55°, a semi-major axis of 17.15 million km, and an eccentricity of 0.4316.
• The Ananke group: This group is named after the largest satellite in the group, Ananke. Theses asteroids range in eccentricities from 0.02 to 0.28 (much less than Carpo), semi-major axes from 19.3 million km to 22.7 million km, and inclinations from 145.7° to 154.8°. From the inclination, and if you remember from the post about a day on Venus, the inclinations of these satellites tell you that these satellites orbit retrograde. Looking down on Jupiter from the north, Jupiter rotates counter-clockwise (or for you Europeans, anti-clockwise), but these satellites orbit clockwise around Jupiter. Any satellites found in this group will have a name ending in "-e".
  
• The Carme group: Named after the largest satellite, Carme. They have semi-major axes of 22.9 million km to 24.1 million km and eccentricities between 0.23 to 0.27. The inclinations range from 164.9° to 165.5°, which means these also orbit retrograde around Jupiter. It is believed that these moons formed after a D-type asteroid was captured by Jupiter and broke up. Like the Ananke group, any future Carme group moons will end in an "e". Two exceptions to these moons are Kalyke is much redder than the other asteroids, and Taygete has a much higher eccentricity (e=0.3678).
• The Pasiphae group: These satellites share similar orbital distances but at a slightly lower inclination (144.5° to 158.3°). The largest moon in this group is Pasiphae.

Jupiter

Jupiter: Note the Great Red Spot to the lower right and the shadow of Europa on the lower left

Image is from the Cassini Spacecraft

 

Jupiter is by far the largest planet in the solar system in terms of mass and size. However, it is still less then 99% of the Sun's mass. It has a semi-major axis distance of 5.2 AU which gives it an orbital period of about 11 years. It rotates once on its axis every 10 hours, making it one of the fastest rotating bodies in the Solar System.

Previously, we had talked about Galileo and how he had discovered the first moons around a planet other than our Earth. This discovery helped prove the geocentric theory of the solar system was incorrect (Objects can orbit something other than the Earth). Galileo originally wanted to call them the Medician moons after his sponsors, the Medicis, but it was eventually agreed to name them the Galilean moons and to name them after four of the lovers of Zeus; Io, Europa, Ganymede, and Callisto.

Saturn is no the only planet with rings. In fact, all four Jovian planets have a ring system, though not quite as magnificent as Saturn's rings. Jupiter's rings are not as extensive as the rings around Saturn and were not discovered until 1979 by Voyager 1.

Jupiter is also home to the largest storm in the Solar System. The Great Red Spot is at least 183 years old and may be older. It may have been first observed in 1665, but it isn't clear if it was observed again until 1831. It is between 24,000 to 40,000 km east-west and 12,000 to 14,000 km north-south, making it capable of holding 2 to 3 Earths.

Jupiter also shares its orbit with a two groups of asteroids known as the Greek and Trojan asteroids. These asteroids were discussed previously here. These asteroids, much like the Amor asteroids, will never impact Jupiter. However, Jupiter's gravity is strong enough to capture both asteroids and comets. Many of Jupiter's moons are possibly captured asteroids, and back in 1994, Comet Shoemaker-Levy 9 entered Jupiter's gravitational field, broke up, and collided with Jupiter.

Jupiter has a very thick atmosphere, as the majority of its volume are the many layers of gas. There is a rocky core, approximately Earth-sized, but is only a small fraction of the entire diameter of Jupiter. The combination of its large volume and the majority of the composition of the planet give Jupiter the largest mass of all the planets, but also a low density, just a little bit above that of water.

Apollo Asteroids

A simple schematic of the inner solar system, the yellow star in the middle is the Sun, the gray circle is Mercury, the grayish-yellow circle is Venus, the blue circle is Earth, the red circle is Mars, and the orange circle is Jupiter. The green band between Mars and Jupiter is the asteroid belt. The brown band covering the area around the Earth is the location of Apollo asteroids.

The third and final class of near-Earth asteroids are the Apollo asteroids. The first asteroid to be determined to be an Apollo asteroid is, surprisingly, 1862 Apollo. Okay, maybe not that surprising. Apollo asteroids are defined by three things:

1. They all have semi-major axes of greater than one AU
2. Their perihelions are all less than 1.017 AU (the distance from the Sun to the Earth at aphelion).
3. They have the potential to collide with the Earth, i.e. they are all Earth-crossing asteroids.

There are only 5766 known Apollo asteroids, though more are being found. Of those 5766, 832 are numbered. Remember that a numbered asteroid has a known orbit and can have its position predicted at different times.

The largest known Apollo asteroid is 1866 Sisyphus which has an average diameter of 8.5 km and a mass of approximately 7.7e18 kg (7.7 followed by 17 zeroes for non-math people), 773,900 times smaller than the Earth. Even though it is much smaller, if it even did hit Earth, everyone on Earth would have a really bad day. Luckily, it has been predicted that its closest approach will be on November 24, 2071 (set your calendars) and will only be 0.11581 AU away from Earth (still farther than the Earth-Moon distance). The Chicxulub asteroid which is believed to have killed off the dinosaurs 65 million years ago was comparable in size.

The Chelyabinsk meteor on February 13, 2013 was also a Apollo asteroid. It was estimated to only be 20 meters in diameter with a mass of 13,000 metric tons (13 million kilograms), again much smaller than 1866 Sisyphus.

Though Aten asteroids have the potential to cross Earth's orbit, Apollo asteroids are the ones with which we should be the concerned. Every single Apollo asteroid has the potential to collide with Earth as all of them cross the Earth's orbit. Apollo asteroids, by far, make up the majority of known near-Earth orbits (which is called an observational bias - comment if you have a question on this). As technology improves, possibly hundreds if not thousands more near-Earth asteroids, and by default, Apollo asteroids, will probably be discovered.

Amor Asteroids

A simple schematic of the inner solar system, the yellow star in the middle is the Sun, the gray circle is Mercury, the grayish-yellow circle is Venus, the blue circle is Earth, the red circle is Mars, and the orange circle is Jupiter. The green band between Mars and Jupiter is the asteroid belt. The brown band covering area between the Earth and just outside of Jupiter's orbit are the location of Amor asteroids.

Amor asteroids are near-Earth asteroids with perihelions outside of Earth's orbit, i.e. they never cross the orbit of Earth). However, they can cross the orbit of Mars (and in some cases, Jupiter), so it is believed that Phobos and Deimos may have been Amor asteroids captured by Mars. These class of near-Earth asteroids are named after the first asteroid defined to be an Amor asteroid, 1221 Amor.

Amor asteroids are defined by three things:

1. It must have an orbital period of greater than one year. Since Kepler's third law of planetary motion says that the square of the period of the orbit in years must equal the cube of the semi-major axis of the orbit in AUs, the semi-major axis must be greater than one AU.
2. To be a near-Earth asteroid, recall that the asteroid must come within 0.3 AUs of Earth's orbit. This the is the closest Venus and Earth can theoretically get.
3. To be an Amor asteroid, it cannot come closer to Earth than Earth's aphelion because it cannot cross any part of Earth's orbit. Earth's aphelion is 1.017 AU.

In reality, the third definition trumps the first definition since obviously, 1.017 AU is greater than 1.0 AU. By these definitions, for an Amor asteroid, the semi-major axis must be greater than 1.017 AU  and the perihelion of the asteroid must be between 1.017 AU and 1.3 AU. There are 3729 known asteroids that fall into this category, 580 of which are numbered, and 75 with proper names. The most-well known Amor asteroid is 433 Eros which is the first asteroid to be orbited and landed on. The spacecraft NEAR Shoemaker visited and flew by twice before landing in 2001.

433 Eros rendering from NEAR Shoemaker visit

via Astronomy Picture of the Day



 

Amor asteroids can be further subdivided into four subgroups:

• Amor I are Amor asteroids with semi-major axes between Earth and Mars, i.e. 1.017 AU
• Amor II have semi-major axes between 1.523 AU and 2.12 AU. The average semi-major axis for main asteroid belt asteroids is 2.12 AU. They have perihelions of less than 1.523 AU as all Amor asteroids must have. Again, they may impact Mars.
• Amor III have semi-major axes between 2.12 AU and 3.57 AU. They may be considered main belt asteroids, but have high enough eccentricities to bring them within 0.3 AU of Earth. Some may have high enough eccentricities to bring them close to Jupiter (5.2 AU). Amor asteroid 5370 Taranis is actually a Jupiter crosser.
• Amor IV have the most extreme eccentricities since their semi-major axes are greater than 3.57 AU but still have perihelions between 1.017 AU and 1.30 AU. Their aphelions are greater than 5.2 AU (the size of Jupiter's orbit) and some may even have aphelions nearing Saturn (9.6 AU). All these asteroid not only cross the orbit of Mars, but also cross the orbit of Jupiter. So far, no Saturn crossers in this subgroup have been discovered. Only two Amor IV asteroids have been discovered: 3552 Don Quixote and (85490) 1997 SE5.

Again, these are asteroids that we do not have to worry about as they do not come closer than 0.017 AU of Earth. But they will be concern for any future crewed missions to Mars and beyond.

Note: The Moon is 384,400 km or 0.00257 AU, so there is no danger of these asteroids impacting the Moon, either.