Hiding Among The Hill Spheres

Bright Spring sunlight wakes me earlier than I’d like. I get to the office before I need to, but there’s Jeremy waiting at the door. “Morning, Jeremy. What gets you here so soon after dawn?”

“Good morning, Mr Moire. I didn’t sleep well last night, still thinking about that micro black hole. Okay, I know now that terrorists or military or corporate types couldn’t bring it near Earth, but maybe it comes by itself. What if it’s one of those asteroids with a weird orbit that intersects Earth’s orbit? Could we even see it coming? Aren’t we still in danger of all those tides and quakes and maybe it’d hollow out the Earth? How would the planetary defense people handle it?”

“For so early in the day you’re in fine form, Jeremy. Let’s take your barrage one topic at a time, starting with the bad news. We know this particular object would radiate very weakly and in the far infrared, which is already a challenge to detect. It’s only two micrometers wide. If it were to cross the Moon’s orbit, its image then would be about a nanoarcsecond across. Our astrometers are proud to resolve two white‑light images a few milliarcseconds apart using a 30‑meter telescope. Resolution in the far‑IR would be about 200 times worse. So, we couldn’t see it at a useful distance. But the bad news gets worse.”

“How could it get worse?”

“Suppose we could detect the beast. What would we do about it? Planetary defense people have proposed lots of strategies against a marauding asteroid — catch it in a big net, pilot it away with rocket engines mounted on the surface, even blast it with A‑bombs or H‑bombs. Black holes aren’t solid so none of those would work. The DART mission tried using kinetic energy, whacking an asteroid’s moonlet to divert the moonlet‑asteroid system. It worked better than anyone expected it to, but only because the moonlet was a rubble pile that broke up easily. The material it threw away acted as reaction mass for a poorly controlled rubble rocket. Black holes don’t break up.”

“You’re not making getting to sleep any easier for me.”

“Understood. Here’s the good news — the odds of us encountering anything like that are gazillions‑to‑one against. Consider the probabilities. If your beast exists I don’t think it would be an asteroid or even from the Kuiper Belt. Something as exotic as a primordial black hole or a mostly‑evaporated stellar black hole couldn’t have been part of the Solar System’s initial dust cloud, therefore it wouldn’t have been gathered into the Solar System’s ecliptic plane. It could have been part of the Oort cloud debris or maybe even flown in on a hyperbolic orbit from far, far away like ‘Oumuamua did. Its orbit could be along any of an infinite number of orientations away from Earth’s orbit. But it gets better.”

“I’ll take all the improvement you can give me.”

“Its orbital period is probably thousands of years long or never.”

“What difference does that make?”

“You’ve got to be in the right place at the right time to collide. Earth is 4.5 billion years old. Something with a 100‑year orbit would have had millions of chances to pass through a spot we happen to occupy. An outsider like ‘Oumuamua would have only one. We can even figure odds on that. It’s like a horseshoe game where close enough is good enough. The object doesn’t have to hit Earth right off, it only has to pierce our Hill Sphere.”

“Hill Sphere?”

“A Hill Sphere is a mathematical abstract like an Event Horizon. Inside a planet’s Sphere any nearby object feels a greater attraction to the planet than to its star. Velocities permitting, a collision may ensue. The Sphere’s radius depends only on the average planet–star distance and the planet and star masses. Earth’s Hill Sphere radius is 1.5 million kilometers. Visualize Hill Spheres crowded all along Earth’s orbit. If the interloper traverses any Sphere other than the one we’re in, we survive. It has 1 chance out of 471 . Multiply 471 by 100 spheres sunward and an infinity outward. We’ve got a guaranteed win.”

“I’ll sleep better tonight.”

~~ Rich Olcott

Myopic Astronomy

Cathleen goes into full-on professor mode. “OK folks, settle down for the final portion of “IR, Spitzer and The Universe,” our memorial symposium for the Spitzer Space Telescope which NASA retired on January 30. Jim’s brought us up to speed about what infra-red is and how we work with it. Newt’s given us background on the Spitzer and its fellow Great Observatories. Now it’s my turn to show some of what Astronomy has learned from Spitzer. Thousands of papers have been published from Spitzer data so I’ll just skim a few highlights, from the Solar System, the Milky Way, and the cosmological distance.”

“Ah, Chinese landscape perspective,” murmurs the maybe-an-Art-major.

“Care to expand on that?” Cathleen’s a seasoned teacher, knows how to maintain audience engagement by accepting interruptions and then using them to further her her own presentation.

“You show detail views of the foreground, the middle distance and the far distance, maybe with clouds or something separating them to emphasize the in‑between gaps.”

“Yes, that’s my plan. Astronomically, the foreground would be the asteroids that come closer to the Earth than the Moon does. Typically they reflect about as much light as charcoal so our visible-light telescopes mostly can’t find them. But even though asteroids are as cold as interplanetary space that’s still above absolute zero. The objects glow with infra-red light that Spitzer was designed to see. It found hundreds of Near-Earth Objects as small as 6 meters across. That data helped spark disaster movies and even official conversations about defending us from asteroid collisions.”

<A clique in the back of the room> “Hoo-ahh, Space Force!

Some interruptions she doesn’t accept. “Pipe down back there! Right, so further out in the Solar System, Spitzer‘s ability to detect glowing dust was key to discovering a weird new ring around Saturn. Thanks to centuries of visible‑range telescope work, everyone knows the picture of Saturn and its ring system. The rings together form an annulus, an extremely thin circular disk with a big round hole in the middle. The annulus is bright because it’s mostly made of ice particles. The annulus rotates to match Saturn’s spin. The planet’s rotational axis and the annulus are both tilted by about 27° relative to Saturn’s orbit. None of that applies to what Spitzer found.”

Vinnie’s voice rings out. “It’s made of dust instead of ice, right ?”

Cathleen recognizes that voice. “Good shot, Vinnie, but the differences don’t stop there. The dust ring is less a disk than a doughnut, about 200 thousand times thicker than the icy rings and about 125 times wider than the outermost ice ring. But the weirdest part is that the doughnut rotates opposite to the planet and it’s in Saturn’s orbital plane, not tilted to it. It’s like the formation’s only accidentally related to Saturn. In fact, we believe that the doughnut and its companion moon Phoebe came late to Saturn from somewhere else.”

She takes a moment for a sip of coffee. “Now for the middle distance, which for our purpose is the stars of the Milky Way. Spitzer snared a few headliners out there, like TRAPPIST-1, that star with seven planets going around it. Visible-range brightness monitoring suggested there was a solar system there but Spitzer actually detected light from individual planets. Then there’s Tabby’s Star with its weird dimming patterns. Spitzer tracked the star’s infra‑red radiance while NASA’s Swift Observatory tracked the star’s emissions in the ultra‑violet range. The dimming percentages didn’t match, which ruled out darkening due to something opaque like an alien construction project. Thanks to Spitzer we’re pretty sure the variation’s just patchy dust clouds.”

Spitzer view of the Trifid Nebula
Credit: NASA/JPL-Caltech/J. Rho (SSC/Caltech)

<from the crowd in general> “Awww.”

“I know, right? Anyway, Spitzer‘s real specialty is inspecting warm dust, so no surprise, it found lots of baby stars embedded in their dusty matrix. Here’s an example. This image contains 30 massive stars and about 120 smaller ones. Each one has grown by eating the dust in its immediate vicinity and having lit up it’s now blowing a bubble in the adjacent dust.” <suddenly her cellphone rings> “Oh, sorry, this is a call I’ve got to take. Talk among yourselves, I’ll be right back.”

~~ Rich Olcott