There are roughly 25,000 asteroids near Earth large enough to level a city. Scientists have found fewer than half of them.
That gap — between what's out there and what we've catalogued — is the quiet center of planetary defense work. No known threat is imminent. But NASA's own estimates put the count of so-called "city killers" (objects 140 meters and larger, capable of regional rather than global destruction) well above what current surveys have located. Congress directed NASA to find 90 percent of these objects by 2020. We're still short.
That's not a failure of effort. It's a funding and hardware problem — and understanding it clarifies why planetary defense missions punch so far above their budget weight.
The Middle Range Is What Actually Worries Scientists
The public mental model of asteroid risk tends to cluster at the extremes: small space rocks that burn up harmlessly, or extinction-level impactors from the movies. The actual concern lives in between.
"Small stuff is hitting us all the time, so we're not so much worried about that. And we're not so worried about the large ones from the movies because we know where they are," said Dr. Kelly Fast, NASA's acting planetary defense officer, at a recent conference in Arizona. "It's the ones in between, about 140 meters and larger, that could really do regional rather than global damage and we don't know where they are."
The detection problem is partly geometric. Many of these mid-size asteroids approach from the direction of the Sun, making ground-based observation difficult or impossible until they're already close. That's why the survey mandate from 2005 remains unfinished — not because no one is looking, but because the tools available from Earth have real blind spots.
Nancy Chabot, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory, put the operational logic plainly: "You can't do anything about the asteroids if you don't know where they are."
What DART Proved — and What Hera Is There to Measure
The DART mission demonstrated in 2022 that a kinetic impactor could meaningfully change an asteroid's orbit. That was the proof of concept. But a deflection test without follow-up measurement is like firing a shot and never checking where it landed.
That's ESA's Hera mission. Currently en route to the Didymos binary asteroid system — the same target DART hit — Hera will characterize the crater DART left, measure the mass and internal structure of Dimorphos, and assess exactly how much the orbit changed and why. The spacecraft carries two CubeSats, Juventas and Milani, designed to conduct close-proximity science that the main spacecraft can't safely attempt alone.
This is the part that doesn't get enough credit. DART was the headline. Hera is the science. Without it, we'd know that kinetic deflection can work, but not how well or under what conditions — which is precisely what you need to plan an actual planetary defense response. The mission's budget is modest relative to what it returns: a calibrated deflection model that makes every future response more reliable.
The Telescope Gap Is the Next Bottleneck
Detection remains the binding constraint. A deflection capability you can't deploy in time is no capability at all. The 2013 Chelyabinsk event — a roughly 20-meter object that injured over a thousand people in Russia — arrived without warning. Katie Kumamoto of Lawrence Livermore National Laboratory cited it as a reminder that even objects well below the "city killer" threshold can cause serious harm, and that the detection pipeline has real gaps at the smaller end too.
NASA's proposed NEO Surveyor space telescope — designed specifically to find the objects that ground-based surveys miss — is the obvious next piece of infrastructure. Its status in the current budget environment is worth watching closely. SpaceNews reported that NASA administrator Jared Isaacman recently defended the agency's budget proposal amid steep cuts, though the specifics of which programs survive that process will shape planetary defense capacity for the next decade.
The pattern here is consistent: planetary defense missions are cheap relative to the risk they address, technically tractable, and internationally supported. What they require is sustained political will to fund the survey infrastructure before the detection gap closes itself the hard way.
Also this week:
Artemis II is the dominant story in human spaceflight right now. SpaceNews confirmed the mission is ready to fly its lunar free-return trajectory — the first crewed flight beyond low Earth orbit since Apollo 17. NASA's science team is actively monitoring solar activity to protect the crew from radiation exposure during the transit. Watch for the launch window confirmation and the ESA European Service Module performance — Europe's contribution to Artemis is its propulsion system, and this will be its first crewed test.
Comet 3I/ATLAS is getting attention from ESA's Juice spacecraft, which captured images of the object using its JANUS science camera. ESA published initial findings this week. An interstellar comet observed by a spacecraft already in transit is a genuinely unusual alignment — worth following as more characterization data comes in.