8 Mass Unaccounted
JPL, Pasadena · Thursday, 1 February – Thursday, 19 April 2029.
The sheet with the empty column came in at 2:40 on the first of February, from Greenbelt, with nothing in the body.
Priya had built it so the column could only be filled from Pasadena. Dana admired that before she resented it.
To fill it she needed a mass, and not one mass but a mass for every row, because the object in September was not the object in January. It had lost about 3% of its mass a month since October, which was what 2% a month of light came to. The only reason anyone knew that was forty orbits of Hubble with one name on the proposal, and the only person outside Washington she had come close to telling was Kessler, and she had never answered him.
She went over it the way she’d go over a budget line she meant to defend. Priya was a NASA postdoc on a Goddard badge, as American as she was. The column was about mass. Nothing in it said braking, or L5, or August. The September memo had put the orbit under the banner and said nothing about the light curve, because in September there hadn’t been one. Every piece of that would hold up in the windowless room, and none of it was why she opened the spreadsheet. Priya had sent her nineteen numbers, then twenty-six, then a column she couldn’t fill herself, and in four months nobody else had sent Dana anything she didn’t already have.
For iron, at the size the thermal fits gave and a shape a good deal flatter than a ball, the mass came out at 1.7 × 10¹³ kg. She took it down month by month along the dimming, put a mass against every launch and arrival, and divided.
The kicks were small. Priya’s tracks, run back to the object, left it at 20 to 40 m/s, and a 15 m mover of a few thousand tonnes going that fast moved 1.7 × 10¹³ kg by a few micrometers per second. The column filled with 10⁻⁶. The largest was 8.7 × 10⁻⁶ m/s, on an outbound in December, and the inbound arrivals pushed the other way often enough that the running sum wandered around zero.
She sat back from it. Things the size of a house left the object and kept speeding up for days, and others came back slowing down, and this was the column that said they hardly mattered. Glen would say debris. Kessler would say coincident. She thought of something at L5 making small hot things and sending them out to the belt and getting some of them back, and didn’t write that down either. Her own orbit since the crossing was good to about a centimeter per second over months and showed no push at all, which was what it ought to show. Three orders of magnitude sat between what the movers could do to it and what she could measure.
She attached the filled sheet, the mass table and the masked light curve with the comparison stars on the same axes, typed nothing, and sent it.
Version seven came on the twenty-sixth of February. Priya had added a column.
It sat at the far right where parent ΔV had been, and it was mostly 99s.
| month | detected | fleet_est | carried_kg/s | loss_kg/s | mass unaccounted % |
|---|---|---|---|---|---|
| 2028-09 | 2 | 15 | 5.8e1 | 1.9e5 | 99.97 |
| 2028-10 | 4 | 46 | 1.8e2 | 1.9e5 | 99.91 |
| 2028-11 | 5 | 85 | 3.3e2 | 1.8e5 | 99.82 |
| 2028-12 | 8 | 146 | 5.6e2 | 1.6e5 | 99.65 |
| 2029-01 | 9 | 215 | 8.3e2 | 1.4e5 | 99.41 |
Dana read the assumptions line. Ten thousand tonnes was a solid iron ball thirteen meters across, with nothing inside it but cargo. A round trip a month was faster than any of Priya’s tracks could close. Every number Priya had a choice about, she had chosen against herself.
She called Greenbelt, which she had done once before.
“You made the payloads stupid,” Dana said.
“On purpose. If I make them sane it’s nines all the way across.” Priya sounded like she was eating. “Even if I’m missing seven out of eight of everything, they’re carrying under 1%. Probably a lot under.”
“The recovery could be worse than an eighth for the cold ones.”
“Then there are more of them and they’re carrying the same per mover. I tried that. It’s in tab four. You’d need ten thousand of them before it’s even ten percent, and the sky around it would be lit up.” A pause and a wrapper. “Where’s the rest going?”
“I don’t know.”
“It’s your mass,” Priya said, not unkindly, and hung up first.
By the second week of March she had stopped doing it on the screen. She did it in pencil on engineering paper, the green kind with the grid on the back, because on paper she could see which step she had skipped.
The object was losing something like 10⁵ kg a second. Priya’s movers, on Priya’s stupid payloads, were taking under a thousand kilograms of that. The question was the other ninety-nine-and-change.
Gone as dust or gas came first, and she tried hard to keep it alive. The December stack was a clean point with nothing around it to the limit, and the February one was cleaner. Iron vapor would have shown in the spectrum, and the spectrum was flat. And anything thrown off at speed pushed back. She worked it on the margin: 10⁵ kg a second leaving at even 10 m/s was 10⁶ newtons on 1.7 × 10¹³ kg, which over a month came to 15 cm/s, ten times what her orbit could see. She tried to save it by making it isotropic, so the pushes cancelled. Isotropic at that rate would have been a coma Catalina would have flagged in one night. She tried letting it go slower than escape speed, about 1.5 m/s off a body that size, and that finished the step. Material that left that slowly didn’t leave. It was still there, or near enough to there that no telescope could separate it from the core.
So it was still there and it was unseen, and she wrote those two words on either side of a line.
Unseen didn’t mean too small to split. At 1 au Hubble couldn’t separate anything within thirty-odd kilometers of the core, so the whole pile was inside the point. Unseen meant it added nothing to the point: no light, no heat. Surveyor sat at L1, 1.5 million kilometers sunward, and Hubble went round in low orbit, and from L5 both of them looked from within a fraction of a degree of the same place. There was no instrument anywhere that saw the object from a different side.
Then the light. Anything sitting in sunlight beside the core added light to the point, and the point was getting fainter. A little less than 2% a month now. She tried to keep it in the sun and make it black. A layer of soot at a few percent albedo would add almost nothing in the visible. But a few percent albedo meant ninety-odd percent absorbed, and it would sit at 300 K in the Surveyor infrared, a second blackbody nobody had seen. Surveyor still looked at it four times a day, and Priya had just been through every frame. She’d have put a second blackbody in a column.
So it was dark, and dark at L5 meant in the core’s shadow.
Dark wasn’t cold. Whatever had come off a core that hot would still be warm, and warm iron in Surveyor’s view would have been a second heat source beside the first. Surveyor saw one warm face and nothing next to it. So the core hid it from Earth’s direction too.
She drew it. The core a circle 2 cm across for 2 km. A line toward the Sun, a line toward Earth, 60° apart, because that was what L5 was. The shadow was a band behind the core along the sunward line, the blind spot a band behind it along the Earth line, each as wide as the core. Where they overlapped was a wedge on the far side, pointing away from both. For a core of radius 1 km the wedge ran out to 2 km from the center.
That put it within about a kilometer of the far face, on the night side. Loose material wouldn’t stay in a wedge. That close to the core it would orbit in a few hours and smear into a ring. Something was keeping it where it was. Something like 2 × 10¹² kg since September, in the one place in the sky nobody could look.
She went through it again the next week from the bottom, cold, and again the week after, and at each step looked for the assumption she had walked past. She found two. Neither moved the wedge.
She wrote the memo on the eleventh of April. It was a page.
Everything since February, in order, with Priya’s movers as a Goddard reprocessing of Surveyor frames, without Priya’s name. One line at the bottom: the region could be observed by an instrument off the Sun–Earth line, and none existed. ESA’s Vigil, the one that would sit at L5 and look, was years from the pad.
JPL interoffice memorandum, CUI//NOFORN across the top and bottom in red, controlled by the Planetary Defense Coordination Office. The same distribution as September. She checked the nine addresses against September’s one at a time. One of them sat in the Eisenhower building, across West Executive Avenue from the White House, and belonged to the man with the yellow legal pad.
She sent it at 9:40 on a Wednesday.
By Monday there had been no read receipt she could see, no reply, no meeting request with a dial-in, no polite note from headquarters that it had been passed along. In September the same list had put her on a plane to Washington within the month.
She checked the secure inbox the way she had once checked the Minor Planet Center’s new-object page, first thing and again after lunch.
Glen put his head in on Monday the sixteenth.
“Have you heard anything about the paper?”
“No.”
“Nothing? It’s been seven months. I’ve got a student who wants to cite it.” He leaned on the doorframe. “Somebody must have said something to you.”
“I haven’t heard anything about anything, Glen.”
He decided it wasn’t a joke he wanted.
On Thursday the nineteenth she was in Kessler’s spring 2028 thermal fits for a dull reason. Priya wanted the spin state to put launch directions in the object’s own frame, and the only attitude anyone had ever measured was in Kessler’s own fits, never in the note, built out of thirteen weeks of Surveyor infrared he had fitted one at a time in May.
She had his files. He had sent them the hour she asked.
He had fitted a two-face model in a frame tied to the Sun and Earth, because the thing held attitude and anything else would have been silly. Week by week a warm cap, its center inferred from how the heat came round the limb, with an error ellipse. She had read the warm face a year ago as heat leaking out of the far side of something that had been hot once. She hadn’t looked at the direction since.
She took the pencil drawing out of the folder and turned it to match his convention, Sun at zero, Earth at 60°. Her wedge pointed along the far bisector, at 210°.
Nobody had ever seen that face, only the edge of its heat, and his ellipses were thirty and forty degrees across. All thirteen sat on the far side. Mean 209, plus or minus 10.
She laid the green sheet over his plot against the window, and the point of her wedge sat inside every one of his ellipses.