Friday, July 22, 2016

Hildas As Cyclers


Hilda Asteroids - Red,   Sun Jupiter Trojans - Blue,   Main Belt - Green

The above image was made from screen captures of Scott Manley's beautiful animation Asteroids In Resonance With Jupiter.

Jupiter is the dot off to the left, the sun is the yellow dot in the middle. Within the Main Belt can be seen Mercury, Venus, Earth and Mars. I colored the different asteroid populations so we can tell them apart.

The Sun Jupiter Trojans have a 1 to 1 resonance with Jupiter. They co-rotate with Jupiter. The leading Trojans remain in a neighborhood 60 degrees ahead of Jupiter and the trailing Trojans stay in a neighborhood 60 degrees behind.

The Hildas have have 3 to 2 resonance with Jupiter meaning they circle the sun three times for every two Jupiter orbits. Jupiter's orbital period is about 12 years and the Hildas have 8 year periods.

The Hilda orbits only look triangular in Manley's animation because they're being viewed in a rotating frame. You can see Jupiter remains on the left side of the image. In an inertial frame, a Hilda orbit is an ordinary elliptical orbit with aphelion passing through the Trojans and perihelion passing through the main belt.


I envision the Hilda biomes playing a similar role as Marco Polo's caravans shuttling people and goods between east and west. But the Hildas travel between the Trojans and the Main Belt.

The would be a series of regular fly bys for a Hilda Cycler

1) Main Belt to trailing Trojans — 4 years.
2) Trailing Trojans to Main Belt — 4 years.
3) Main Belt to leading Trojans — 4 years
4) Leading Trojans to Main Belt — 4 years
5) Main Belt to Sun Jupiter L3 — 4 years. But there is no asteroid population at SJL3.
6) From SJL3 to Main Belt 4 years

Then back to step 1). The cycle repeats itself.

So not only can a Hilda be a go between between the Main Belt and Trojans, but it can also move stuff between the trailing and leading Trojan populations. Trailing to leading takes 8 years and leading to trailing takes 16 years.

As can be seen from Manley's animation, there is a steady stream of Hildas traveling the circuit. 

Delta V

The Hildas have a variety of eccentricities. I will look at a Hilda orbit having an eccentricity of .31. That would put the aphelion at 5.2 A.U. and the perihelion at 2.74 A.U. (The perihelion is in Ceres' neighborhood, Ceres' semi-major axis is 2.77 A.U.).

Assuming a circular, coplanar orbit at 2.74 A.U.,  it would take 2.6 km/s to leave a Main Belt Asteroid and board a Hilda.

Assuming a circular, coplanar orbit at 5.2 A.U., it would take 2.2 km/s to depart the Hilda and rendezvous with a Trojan.

However, coplanar orbits is a very optimistic assumption. Asteroids have a large variety of inclinations. Making a 10 degree plane change from a Hilda's orbit can cost 2 to 3 km/s.

Ways to mitigate delta V expense

Many asteroids spin about pretty fast. This plus their shallow gravity wells make them amenable to bean stalks, also known as space elevators. 

"Why would an asteroid need a space elevator?" I'm sometimes asked. The questioner will assert "It's very easy to get off an asteroid's surface, and getting off the body's surface is the only reason for an elevator." Which is wrong, of course.

Speed of a body on an elevator is ωr where ω is angular velocity in radians per time unit and r is distance from center of rotation. If r is large, the elevator can fling a payload at high velocity with regard to the asteroid. It is quite plausible for an asteroid's bean stalk to provide .5 to 1 km/s delta V.

Also an asteroid bean stalk allows rendezvous with an ion propelled space craft. Ion ships have great ISP but minute thrust. Soft landings with an ion craft are not possible on larger asteroids like Ceres, or Vesta.

And ion propelled ships are more viable in the outer system. When a ship's acceleration is a large fraction of the local gravity acceleration, an ion burn is more like a chemical impulsive burn. See General Guidelines for Modeling a Low Thrust Ion Spiral. In the outer Main Belt, the sun's gravity is about 1 millimeter/sec2. Sun's gravity at the Trojans is about .2 millimeters/sec2.

Jupiter's Trojans

Not much is known about Jupiter's Trojans. Japan hopes to launch a mission in the early 2020's. Another proposed mission is Lucy which would also launch in the early 2020's. 

These bodies are on average 5.2 A.U. from the sun and so receive only 1/27 the sunlight earth enjoys. For this reason I am hopeful they are rich in volatile ices. I'd give better than even odds they have lots of water and carbon dioxide ice. Nitrogen compounds like ammonia and cyano compounds are a possibility. Aside from earth, Nitrogen is in short supply throughout the inner solar system and these would be a great export to the Main Belt biomes.

Their numbers are speculation. According to Wikipedia:

Estimates of the total number of Jupiter trojans are based on deep surveys of limited areas of the sky.[1] The L4 swarm is believed to hold between 160–240,000 asteroids with diameters larger than 2 km and about 600,000 with diameters larger than 1 km. If the L5 swarm contains a comparable number of objects, there are more than 1 million Jupiter trojans 1 km in size or larger. For the objects brighter than absolute magnitude 9.0 the population is probably complete. These numbers are similar to that of comparable asteroids in the asteroid belt. The total mass of the Jupiter trojans is estimated at 0.0001 of the mass of Earth or one-fifth of the mass of the asteroid belt.
Two more recent studies indicate, however, that the above numbers may overestimate the number of Jupiter trojans by several-fold. This overestimate is caused by (1) the assumption that all Jupiter trojans have a low albedo of about 0.04, whereas small bodies may actually have an average albedo as high as 0.12;[16] (2) an incorrect assumption about the distribution of Jupiter trojans in the sky. According to the new estimates, the total number of Jupiter trojans with a diameter larger than 2 km is 6.3 ± 1.0×104 and 3.4 ± 0.5×104 in the L4 and L5swarms, respectively. These numbers would be reduced by a factor of 2 if small Jupiter trojans are more reflective than large ones.[16]
The number of Jupiter trojans observed in the L4 swarm is slightly larger than that observed in L5. However, because the brightest Jupiter trojans show little variation in numbers between the two populations, this disparity is probably due to observational bias. However, some models indicate that the L4 swarm may be slightly more stable than the L5 swarm. 
The largest Jupiter trojan is 624 Hektor, which has an average diameter of 203 ± 3.6 km. There are few large Jupiter trojans in comparison to the overall population. With decreasing size, the number of Jupiter trojans grows very quickly down to 84 km, much more so than in the asteroid belt. A diameter of 84 km corresponds to an absolute magnitude of 9.5, assuming an albedo of 0.04. Within the 4.4–40 km range the Jupiter trojans' size distribution resembles that of the main-belt asteroids. An absence of data means that nothing is known about the masses of the smaller Jupiter trojans. The size distribution suggests that the smaller Trojans are the products of collisions by larger Jupiter trojans.

I'd love to see science fiction stores set on 624 Hektor.

This article written in memory of Hilda Alvarez May 5, 1929 - July 20, 2016

5 comments:

Jim Baerg said...

"I'd love to see science fiction stores set on 624 Hektor."

The closest I'm aware of is one of Poul Anderson's _Tales of the Flying Mountains_ "Ramble with a Gamblin' Man", set mostly on 1143 Odysseus, but with a reference to government of the Leading Trojans being based on Hektor.

Hollister David said...

Jim, now that I think about it I recall William Barton had written a story that included a Trojan location. Can't remember the details at the moment.

Chris Wolfe said...

SJL3 sounds like a great place for an observatory. Instantaneous parallax measurements with a baseline anywhere from 9.8 to 10.9 AU would be quite useful in directly measuring distance to astronomical objects. Could also be useful for constraining properties of exoplanets.

The Hildas would be quite isolated outposts, only seeing other people every four years or so. Permanent residents might see three or possibly four orbits over their lifetimes. Rescues would be terribly unlikely in case of problems, so this would be a very hardy and self-sufficient lot.

The main belt objects visited by Hildas would be similarly isolated, particularly since many of them would not fit a simple resonance (that is, may not see the same Hilda more than once a lifetime). On the other hand, if there are a lot of Hildas (and probably a lot of cyclers on inner system trajectories) then any given rock will see new people pretty regularly.

The Trojans could be centers of trade and manufacturing, with relatively frequent visits from a series of Hildas and abundant local resources but without the radiation and impact hazards of Jupiter orbit.

It seems to me that a Hilda outpost would be at a disadvantage in terms of local resources compared to the Trojans and the main belt. Other than the essential life support work there would be little in the way of employment opportunities.
One way around this: colleges. Each leg of the trip would produce a new crop of four-year degree graduates for placement. Families with children might depart for a destination 2 or 3 hops away so their kids graduate by the appropriate arrival date. If these outposts go beyond vocational training then perhaps an environment suitable for research and innovation can be developed, particularly since the results of intellectual pursuits can be traded by radio and may not require a lot of materials to produce. Of course the drawback is that if you need something you don't have and can't build then you'll be waiting a long time. Perhaps that can be mitigated through research cooperatives that can run experiments at a better-equipped facility when needed.
I think this kind of seminomadic / journeyman society would help promote genetic diversity and encourage regular contact between enclaves while (hopefully) reducing stereotypes / prejudices. It would require a fairly strong social safety net for those people who trained four years for a job that doesn't exist when they arrive. Probably every person would have to work in maintenance / life support to pay their way and to be capable of pitching in when necessary later in life. I think the sort of long-term thinking required to efficiently distribute talent and resources in this environment would be a benefit to social stability for the species.

Paŭlo Ebermann said...

It is not really clear: What would be the advantage of traveling with a Hilda compared to a space ship with a similar trajectory? You would still need the same Delta V?

I suppose one advantage would be that you can live of the land (or "the stone") during the travel time, but I didn't find this in your article.

Hollister David said...

Paŭlo Ebermann, I'll use Aldrin's terms for cyclers. He calls the large body people spend most their time traveling in "Castles". Then the vehicles that ferry passengers between the castle and destination (or departure body) are called taxis.

Yes, for the taxis to depart a body and rendezvous with a castle, they would need the as much or more delta V for a normal injection to a transfer orbit.

But the taxi trip between Ceres and a Hilda cycler wouldn't take 4 years. More likely days to weeks. The extensive radiation shielding isn't needed. Having an 5 A.U. aphelion, it's likely a lot of the Hildas have lots of water and other volatiles. So there's likely abundant radiation shielding. Lots of regolith is also good shielding.

And with lots of volatiles also comes oxygen, water, hydrocarbons. It might be possible to "live off the land" as you say.

So in short, taxis need as much or more delta V, but they can be a lot less massive. I believe you're right. That would be worth mentioning in the main body of my article. I'll do so when I have time and energy...