A team of scientists seems to have discovered a previously hidden genetic cause of Alzheimer’s. In a new study Monday, the researchers found strong evidence that people carrying two copies of a genetic variation already tied to Alzheimer’s risk are practically destined to develop the neurodegenerative disorder as they get older. As much as 2% of the general population may have the same mutation, suggesting that the genetic risk of Alzheimer’s is larger than currently assumed.

Alzheimer’s currently affects around 7 million Americans. It’s a complex condition that can have many different risk factors behind it, including age, cardiovascular disease, and genetics. There are known rare mutations that almost always cause someone to develop Alzheimer’s at a much younger age than usual, while other mutations appear to raise the risk of the classic form of Alzheimer’s, which typically starts occurring after age 65. One of these latter mutations affects the apolipoprotein E gene, or APOE, and is known as APOE4.

About a quarter of the population carries at least one copy of APOE4, and the variant is commonly studied as an important aspect of Alzheimer’s risk by scientists. Often, these studies don’t distinguish between people having one or both copies of the gene, but some research has suggested that these dual carriers, also known as APOE4 homozygotes, have a much higher risk of Alzheimer’s than others.

A large team of researchers from Spain and the U.S. sought to settle the question. To do so, they analyzed brain donor data from the National Alzheimer’s Coordinating Center (NACC) as well as data from five other large-scale studies that tracked people’s biomarkers related to Alzheimer’s, including their APOE4 status. All told, the analysis included over 13,000 people.

In the NACC data, the researchers found that nearly everyone with two APOE4 genes showed medium to high levels of brain changes associated with Alzheimer’s at the time of their death. For comparison, the same was only true for 50% of those with APOE3, the most common APOE variant, and which isn’t thought to affect Alzheimer’s risk. In the biomarker data, the team similarly found that almost everyone with two APOE4 copies had abnormal levels of amyloid beta in their spinal fluid (a potential early sign of the disease) by age 65, while 75% had positive amyloid scans. By age 80, almost 90% of these carriers had all of the biomarkers associated with amyloid and tau (another protein key to Alzheimer’s) that the researchers were able to track.

Not everyone with these changes will show clinical symptoms of Alzheimer’s before they die. But the findings, published Monday in the journal Nature Medicine, provide a clear illustration of almost-complete penetrance, the authors say—the odds of a genetic mutation causing a specific trait. In this case, those with two APOE4 genes appear near certain to develop at least the early signs of Alzheimer’s by the time they reach their mid-60s. Given that level of certainty, it’s more accurate to classify this mutation as representing a distinct, “genetic form” of Alzheimer’s, the researchers argue. They also note that 2% of the population is thought to have two APOE4 copies, which would make this form of Alzheimer’s one of the most common diseases tied to a single gene.

The findings, assuming they’re validated by other researchers, could lead to important changes in how we study Alzheimer’s. For starters, it should lead to a broader definition of genetic Alzheimer’s, with the APOE4 form recognized as usually causing Alzheimer’s at an older age than other genetic causes of it. Given the much higher danger associated with two APOE4 copies, the researchers say, future studies should also not bunch them together with single copy carriers. And simply knowing about this heightened risk should hopefully help scientists better understand how Alzheimer’s can happen, which might one day lead to more effective treatments for it.

“In conclusion, our study provides compelling evidence to propose that APOE4 homozygotes [i.e. two APOE4 alleles, or copies] represent a distinct, genetically determined form of [Alzheimer’s disease], which has important implications for public health, genetic counseling of carriers and future research directions,” they wrote.

An experimental satellite that launched in 1974 disappeared from ground-based sensors in the 1990s, only to be found again this week. Some defunct satellites or debris can often go missing for years, presenting hazards within an increasingly crowded Earth orbit. But, how exactly do objects disappear in space?

The Infra-Red Calibration Balloon (S73-7) satellite was part of the United States Air Force’s Space Test Program. After launching on April 10, 1974, a large reconnaissance satellite, called KH-9 Hexagon, ejected the 26-inch-wide (66-centimeter-wide) satellite, boosting it to a 500 mile (800 kilometers) circular orbit.

The tiny satellite was supposed to inflate in orbit and serve as a calibration target for remote sensing equipment. Its deployment, however, failed, and it became another piece of space junk. When looking over the satellite’s archival data, Jonathan McDowell, an astrophysicist from the Harvard-Smithsonian Center for Astrophysics, found that it had gone missing once before, with radars tracking it in the 1970s before it disappeared, and then again in the 1990s before it disappeared once more.

The satellite was rediscovered earlier this week after being untracked for the past 25 years, according to tracking data from the Space Force’s 18th Space Defense Squadron. “The problem is that it possibly has a very low radar cross section,” McDowell told Gizmodo over the phone. “And maybe the thing that they’re tracking is a dispenser or a piece of the balloon that didn’t deploy right, so it’s not metal and doesn’t show up well on radar.”

Ground-based radar and optical sensors are keeping track of more than 20,000 objects in orbit, and that can get quite tricky. There is a global network of sensors that feeds information to an up-to-date catalogue of satellites, but the majority of objects don’t transmit their identities. Instead, the sensors depend on identifying the orbit of a moving object and matching it with the designated orbit of a satellite.

“It’s basically like air traffic control,” McDowell said. “All this stuff is whizzing around and if you’re going to try flying through that, you want to know where the hazards are.”

After a satellite is launched, engineers on the ground know roughly where it’s going to be and at what altitude it’s going to be drifting. If an object is found in that designated area, then they can rewind that orbit and see if it meets with the orbit of when the satellite was last seen.

“If you’ve got a recent orbital data set, and there’s not too many things that are similar orbit, it’s probably an easy match,” McDowell said. “But if it’s a very crowded bit of parameter space, and you haven’t seen it for a while, then it’s not so easy to match up.”

Tracking satellites in geostationary orbit—a circular orbit directly above the equator—can be challenging because there are no radars positioned to monitor objects precisely on the equator. “There’s actually a hole in the tracking,” McDowell said. “If you hug the equator, you can hide from the tracking.”

If a satellite also carries out an unexpected maneuver, then engineers are forced to hunt for it in Earth orbit. “If you don’t know exactly where the maneuver was, you may have trouble locating it,” McDowell said. “If I rewind the orbit of an object and fast forward for the missing object, do they meet and is the point where they meet where the maneuver happened?”

Most things that end up go missing in space are either defunct satellites or broken up fragments of debris. The Department of Defense’s global Space Surveillance Network is currently tracking more than 27,000 objects in orbit, the majority of which are spent rocket boosters, and operational and dead satellites.

As Earth orbit gets more crowded with a growing number of satellite constellations and rocket launches, it’s become more crucial to keep track of all these objects.

“If you’re missing one or two objects, that’s not a huge risk,” McDowell said. “But you want to do as good a job as you can.”

For more spaceflight in your life, follow us on X and bookmark Gizmodo’s dedicated Spaceflight page.