After 50 years of trying, physicists have tuned in to the radio waves emitted by fireballs streaking through Earth’s atmosphere.

A meteor with a tail as bright, or brighter, than Venus is known as a fireball – the Chelyabinsk meteor that broke apart over Russia early last year is an example. At its brightest, the Chelyabinsk fireball appeared brighter than the sun.

Fireballs ionise nearby air as they barrel through Earth’s atmosphere, generating a super-bright plasma trail. In 1958, Gerald Hawkins, then at Boston University, predicted that this plasma should produce radio waves as it cools. But hunts for these radio emissions were inconclusive at best.

Now we know that Hawkins was right. Kenneth Obenberger at the University of New Mexico in Albuquerque and his colleagues were searching for mysterious events called radio bursts in data from the , an observatory in New Mexico. Radio bursts show up as points of radiation in images. But to the team’s surprise, analysis of 11,000 hours of data included evidence of 10 low-frequency radio bursts that appeared smudged across the sky.

Meteor radio

The shapes of the smudges were reminiscent of fireballs streaming through the sky. So the team looked at data from a NASA survey telescope that records meteors and that scans some of the same parts of the sky as the radio array. Each of the elongated radio events correlated in time and space with known fireballs, says Obenberger.

“It’s the first detection that is believable because it’s based on imaging,” says at the SETI Institute and NASA Ames Research Center in California’s Silicon Valley. “It’s a new way of looking at meteors.”

The team still needs to work out the exact physical mechanism that causes fireballs to emit these specific low-frequency signals. Solving the puzzle could help improve our understanding of other mysterious events that create plasmas in Earth’s atmosphere, such as lightning strikes and ball lightning, says David Meisel, executive director of the American Meteor Society in Geneseo, New York.

Journal reference: The Astrophysical Journal Letters,

P5, an international group of distinguished experts, has issued its long-awaited report mapping the next 10 to 20 years of US particle physics research. The US Department of Energy’s will now deliberate on its conclusions on future strategies in five key research areas: the Higgs boson, neutrinos, dark matter, dark energy and yet-to-be-discovered particles.

Given budget constraints, the P5 panel concludes that the best way for the US to contribute to particle physics globally is to develop and maintain a world-leading neutrino programme. If the Department of Energy and National Science Foundation follow funding proposals outlined in the report, then the US will see an upturn in neutrino research – with the (Fermilab) at the forefront.

Neutrinos are elusive particles that rarely interact with matter. In 2011, they made the headlines when an experiment suggested they might travel faster than light. Physicists now think they travel almost exactly at the universe’s speed limit but no faster.

The new report recommends pursuing greater international collaboration with , due to begin in 2024. It also advises refocusing Fermilab activities toward improving its accelerator complex to significantly boost the energy of neutrino beams from the LBNE.

Three flavours

“Fermilab has the capability to produce the most powerful beams of neutrinos,” said P5 member , a theoretical physicist at the lab. These beams are the tool that could allow the precision measurements necessary to solve the mystery of the neutrino’s mass – a property that could help answer burning questions about our early universe, but whose value remains obscure despite 60 years of measurements. Part of the mystery stems from the fact that neutrinos come in three flavours, each thought to have a different mass.

“Neutrinos have been around for a long time but our ability to study them has been limited,” says Lykken. “Only in recent years have we had the ability to design experiments to study neutrino properties.”

Detectors looking at neutrinos from space, like IceCube and Super Kamiokande, are poor tools for measuring neutrino mass, says Lykken, because the particles arrive in an uncontrolled way. LBNE has put funds aside to help build a large liquid-argon detector as a target for the lab’s high-energy neutrino beams.

If neutrino experiments see a growth in the US, that would almost certainly mean that other experiments have their funding delayed, or worse. For example, one of the report’s suggestions is to delay the design of third-generation direct dark matter detectors.

“In the end, you can’t afford to do everything with the limited budget and so we were forced to make some painful decisions,” says Lykken.

The Pinwheel Galaxy is a darker place than we suspected. Other large spiral galaxies, such as the Milky Way, boast star-speckled outer shells called stellar haloes. But the edges of the Pinwheel Galaxy are mysteriously barren, putting a wrinkle in one of the most widely held theories of galaxy growth.

We think galaxies get bigger either via colliding and merging with large neighbours or by snacking on dwarf galaxies that fall into their gravitational grasp. But big collisions tend to mangle galaxies, and spirals take several billion years to settle into their orderly shapes. So we think most of the spirals we see today grew by gobbling up nearby dwarfs.

This process rips the dwarf galaxies apart and, over billions of years, they leave behind a faint halo of orphaned stars that surrounds the larger galaxy. We have seen such haloes around the Milky Way and our closest large neighbour Andromeda, and simulations suggest that they should be common around spirals across the universe.

Starving galaxy

Until now, though, we had not been able to weigh galaxies other than the Milky Way and Andromeda with enough precision to measure their stellar haloes. Using an array of eight telephoto lenses, at Yale University and his colleagues were able to measure the visible mass distribution in and around the Pinwheel Galaxy, which lies about 21 million light years away.

Their measurements revealed that its outer halo is strangely devoid of stars. This implies that the spiral galaxy has somehow grown large without feeding on many dwarfs, says van Dokkum.

“There are only a few galaxies that have been studied down to this limit, so it’s hard to say how typical this is,” says at Columbia University in New York. But theorists may be in trouble if galaxies without haloes turn out to be common.

“If we find that the Pinwheel is not an exception, there is something fundamentally wrong,” says van Dokkum. The team plans to use the array to study more galaxies and look for their haloes.

“The fact that they’ve developed this scheme for really being able to map the outskirts of galaxies much farther than people have been able to do before is itself a very nifty thing,” says at Ohio State University in Columbus. “And then this finding of a galaxy without a halo is a very interesting first result.”

Journal reference: Astrophysical Journal Letters,