Australasian Science: Australia's authority on science since 1938

It’s Raining on the Sun

Image of solar filament

A huge magnetic filament erupts on 30 March 2010. Credit: SDO/AIA

By David Reneke

Dave Reneke brings news from the space and astronomy communities around the world.

It came like a bolt out of the blue and lasted only a few seconds, but it opened up new questions and solved an old one about our closest stellar neighbour, the Sun. Scientists working with NASA’s new Solar Dynamics Observatory (SDO) have released the most astonishing movies of the Sun anyone has ever seen, including a massive eruption – one of the biggest in years.

Seasoned researchers who regularly monitor these explosions were gobsmacked! Even by their hard-to-impress standards, this was a whopper. Not only is the footage dramatic but it could solve a longstanding mystery of solar physics: the phenomenon of’ “coronal rain”. The video shows a billion tonnes of magnetised plasma blasting into space while debris from the explosion falls back onto the Sun’s surface.

Astronomers have seen eruptions like this before, but rarely so large and never in such fluid detail. Karel Schrijver of Lockheed Martin’s Solar and Astrophysics Lab says his favourite part of the movie is the coronal rain. “Blobs of plasma are falling back to the surface of the Sun, making bright splashes where they hit,” he explains. “This is a phenomenon I’ve been studying for years.”

It’s not surprising that plasma should fall back to the Sun through simple gravity, but the puzzle of coronal rain is how slowly it seems to fall. The Sun’s gravity should be pulling the material down much faster than it actually moves.

So what’s slowing the descent? For the first time, the unsurpassed spectral resolution of SDO provides an answer.

Unseen by previous observatories, the rain appears to be buoyed by a “cushion” of hot gas somewhere between 1,000,000 and 2,200,000°K. Like a stellar signpost our solar jigsaw puzzle is starting to come together.

Sailing High on Other Worlds
Rovers that can cover a few kilometres have boosted our knowledge of the Moon and Mars, taking close-up images of the surface and analysing rocks and soil. But rovers are slow and tend to get stuck in sand traps or other treacherous terrain. What we need is an “aerobot”. A what?

We’re talking seriously here about astro-sailing on other worlds in a specially constructed blimp held aloft by winds at a constant altitude. With a power plant, propellers and propulsion system controlled by sophisticated robotic intellect, an aerobot could fly itself in a highly autonomous fashion while taking pictures and sampling the atmosphere’s chemistry.

Imagine being able to smoothly glide over regions of Mars that rovers fear, covering thousands of kilometres, floating in the air currents and providing a closer look at volcanoes and ancient lake beds than is possible with orbiting spacecraft.

Ballooning missions to other planets is not a new idea. The Soviets sent two 4-metre balloons operated by battery power to Venus in 1985, which floated high above the planet’s surface. Jeffery Hall is leading a planetary balloon effort at NASA’s Jet Propulsion Laboratory in Pasadena, California, and he too has set his sights on our sister world.

Hall’s team has designed, built and tested two teflon- and mylar-coated prototype balloons that could survive the acid corrosion on Venus. They are also working on two aerobot concepts for Titan even though a NASA mission to Titan is at least 10 years away.

Hall’s prototype 7-metre diameter balloon is designed to carry a 100 kg payload and float at an altitude of 55 km in Venusian acid-laden clouds. Tests show that it should be able to float for 30 days or more, circumnavigating Venus every 4–6 days.

David Reneke is an astronomy educator, writer and broadcaster who represents Australasian Science on more than 60 networked radio stations across Australia. He also produces a range of educational CD-ROMS on astronomy and space exploration for beginners, and runs an astronomy outreach program for schools throughout NSW. Subscribe to David’s free Astro-Space newsletter at