sbrobert
By Robert Bernstein
Space ROCKS! A Brief Tour of the Minor Bodies Cluttering Our Solar System. That was the title of the latest Humanist Society talk given by Joey Chatelain, a postdoctoral researcher at Las Cumbres Observatory.
Here are all of my photos as well as links to Chatelain’s slides and his entire PowerPoint presentation (which he has very generously shared with us): http://swt.org/hssb/2019-1019-
Las Cumbres Observatory (LCO) is a global robotic telescope network. LCO has telescopes all over the world, so the night sky can be observed 24/7. Their world headquarters is here in Goleta where they run their operations and maintain a prototype telescope.
“Minor Body” is a vague term used by astronomers to refer to objects smaller than a planet but larger than dust!
Here Chatelain showed some common subtypes of Minor Bodies:
Artificial Satellites can sometimes be mistaken for potentially dangerous asteroids since they are so close to Earth. This includes secret government satellites which are occasionally discovered and reported by astronomers who think they are natural. When this happens, the astronomers will typically receive an email informing them that the object is “not of interest” and should be ignored.
TNOs are Trans Neptunian Objects. They are past Neptune and are big balls of ice like Pluto. If Pluto were to come close to Earth’s orbit it would look and act like a giant comet!
Asteroids can usually be thought of as roundish mountains in space. They vary from a couple of meters up to a thousand kilometers or so across. Here is a Dawn spacecraft image of 4 Vesta 1807FA (commonly just called “Vesta”).
The Dawn Spacecraft went on to stop at the asteroid Ceres. This is very unusual for a space mission. Before Dawn, typical space missions either flew by a target once or entered orbit and then stayed there indefinitely. This is because it takes a lot of fuel to slow down enough to enter orbit, and even more to leave orbit once you have arrived.
Chatelain offered a brief summary of naming conventions in this diagram:
The provisional designation contains the year it was discovered followed by letters and numbers based on the half month it was discovered in and the order of discovery. An object might be “discovered” multiple times and have multiple provisional designations.
If an object has a well-defined orbit, it will get the next number in an ongoing sequence. The International Astronomical Union (IAU) can also give a name, often classical in nature. The discoverer can suggest a name, but there are guidelines they usually have to follow.
The Minor Planet Center (MPC) is currently tracking about 785,000 objects. Most of these discovered since 2000.
Astronomers discover new asteroids by taking many images of the sky over many hours or days. These images can be compared to each other and asteroids can be detected because they appear to move across the field with respect to the background stars.
H designates the “magnitude” of an object in the Solar System. It is a logarithmic scale where a higher number means a dimmer object. A magnitude 1 object is exactly 100 times brighter than a magnitude 6 object. It depends on how reflective the object is and how big it is.
Vesta is 3.2 in magnitude. Vesta is 525km in diameter. It is 0.004 the mass of the Moon. Its albedo (reflectiveness) is 0.422. Yet this small object contains 12% of the mass of the entire “Main Belt” of asteroids! Ceres is the largest object in the Main Belt at 939km across.
Unlike in the Star Wars and other science fiction movies, there is not that much stuff in the Asteroid Belt. If you were flying through it in a spacecraft you would not be dodging rocks!
About 100 tons of stuff hits the Earth’s atmosphere each day, but most of it burns up before it hits the surface. Most of this material is actually dust, but some of it can be seen as meteors.
Whether an impact builds up an asteroid in space or breaks it up depends on the relative velocity of the two objects.
Chatelain went on with an “Anatomy of the Solar System”. Starting with Near Earth Objects (NEOs).
The Tunguska blast is a famous impact of an NEO that hit Siberia in 1908, leveling about 80 million trees.
It was said to have been heard all the way in London! That meteor was estimated to be about 40 meters in diameter.
Atiras are entirely inside Earth’s orbit and will never hit us.
Atens occasionally cross the Earth’s orbit, but that does not necessarily mean it poses a risk. But the ones that are a risk are a real danger: They often come from the direction of the Sun, which means they are difficult to see with telescopes!
Apollos are mostly outside our orbit but occasionally cross our orbit. These we can watch with telescopes.
Amors are always outside Earth’s orbit and pose no danger.
Ryugu is an asteroid we have met “in person” via the Hayabusu 2 spacecraft from Japan. It was launched in 2014 and arrived in 2018. In 2020 it will return with a sample.
Ryugu is 865 meters across. Hayabusu blasted it and collected what came off.
NASA went to Bennu with the Osiris-REX spacecraft, arriving on December 3, 2018. Bennu is about half the size of Ryugu. But Bennu is like a boulder field making it very difficult to find a safe place to collect a sample. But there are four possible landing sites. NASA hopes to bring samples back to Earth in March 2021.
Psyche is the next NASA asteroid mission which will visit Psyche. Not such an imaginative mission name! Psyche may be a core of a differentiated planetesimal made mostly of iron and nickel. It is 226km across. A popular news story claimed it has enough gold to make each person on Earth a billionaire. Economically that is meaningless as the price of gold would drop if the supply increased that much.
But in addition to the collapse of the price of gold, there is also a huge logistical problem. It would likely cost more than the gold is worth (and more than the entire world’s GDP for a century) to mine and retrieve all the material on such a large asteroid so far away. There is also a lot of gold in the center of the Earth, substantially more than is on Psyche, but there is no way to extract it. The Psyche mission has no sample return at all.
Beyond the Main Belt of asteroids are two groups of asteroids tied to the orbit of Jupiter. One set is the “Hilda” asteroids which form a triangular arrangement of objects inside the orbit of Jupiter. Chatelain said there are about 3,000 Hildas and they are in 3/2 resonances with Jupiter. Meaning they orbit the Sun three times for each time Jupiter orbits the Sun twice.
The other set of Jupiter-related asteroids are the “Trojans”.
These objects are stuck near the L4 and L5 Lagrange points of Jupiter and the Sun. These are stable points of gravitational energy minima where things can collect.
Lucy is a NASA mission that plans to visit the Trojans. The mission has a planned 2021 launch and 2027 arrival.
Chatelain went on to briefly describe the idea of “Regular Satellites.” These are natural satellites that orbit the planets of the Solar System in regular, circular orbits all in the same plane as the planet’s rotation. The quantity of named Regular Satellites is as follows:
Earth – 1
Mars – 2
Jupiter – 8
Saturn – 23
Uranus – 18
Neptune – 7
There are lots more Irregular Satellites. These are far from the host planets and they are not in traditional regular orbits. Many of these moons orbit backwards with respect to their planets and other regular satellites. The Hubble telescope and NASA missions have allowed discovery of many moons that were previously unknown. I asked if Earth has any and he said our Moon destabilizes their orbits. So, we get temporary ones.
He went on to talk about the Outer Solar System which begins at Jupiter. He talked of Centaurs which are “half comet, half asteroid”. These likely icy objects exist between the orbits of Jupiter and Neptune and are unstable, as they will eventually get too close to a massive planet and either be tossed towards the sun or pushed out of the Solar System.
Past them are the TNOs. Plutinos are in 3/2 resonance with Neptune. Including Pluto.
Pluto is one of the largest TNOs but has much more in common with the other TNOs than it does with the major planets. Its moon Charon is almost as big as Pluto itself.
He showed an image of a TNO temporarily called Ultima Thule which is a bit like two pancakes stuck together in the shape of a snowman.
There are about 3500 comets that have been recorded but it is possible that many millions more exist on the very edges of the Solar System. The nucleus of a comet is the bright central part made of ice and rock. The ice sublimates directly to vapor taking grains of dust with it. The solar wind creates two tails, one of dust, one of gas.
The European Space Agency (ESA) sent the Rosetta mission to explore the Churyumov–Gerasimenko comet which is about 4km across. It returned amazing data and beautiful pictures of an active comet, but the lander landed in a dark crevasse and quickly lost power.
Because comets come from all directions rather than just within the plane of the Solar System, it is theorized that they come from the Oort Cloud which is a bunch of material in a spherical shell very far out from the sun. About 10,000 to 100,000 Astronomical Units (AU) out. One AU is the distance from the Earth to the Sun.
As this is a significant fraction of the way to the next star it does not take much for a passing star to dislodge an Oort Cloud object. It might take 10,000 years for such an object to get close enough to the Sun to become a comet.
Chatelain also talked of “Interstellar Interlopers” which are not bound to the Sun. They are just passing by. Two of these were seen in the past year.
People wanted to know more about the danger of an asteroid to Earth. He said that a small one like Tunguska might be too small to see until it is too late to deflect. It is big enough to destroy a city so our only option would be to evacuate if we knew where it was going to hit.
Something hundreds of meters across is big enough to wipe out an entire state. We probably would see it further away, giving us a chance to deflect it. We probably need about a decade to do that. As far as deflection methods, he enjoys the idea of painting one side white with billions of ping-pong balls or white pebbles. The Sun will reflect off of that and push it out of the way. Of course this would require a lot of time and advanced planning.
Using a nuclear weapon to blast it to pieces is a terrible idea. All those pieces would just hit us anyway. But such a weapon could be detonated near the asteroid. The effect would be to heat one side of the asteroid enough to make that area vaporize and act like a rocket for a few seconds. That would cause a deflection.
It is a tricky geopolitical issue. If we deflect it enough to miss Santa Barbara, it might hit another city!
Chatelain explained the origin of the Main Belt asteroids. They were formed from the original “solar nebula” that formed the rest of the planets.
The talk contained many of the new insights we have learned since the time I learned astronomy in school! Huge thanks to Chatelain, Las Cumbres Observatory, NASA and everyone else who is working to expand our knowledge of the universe we live in!
