A Russian space telescope conceived during the Cold War is set to launch on Monday. When it reaches an orbit that will extend almost as far as the moon, the RadioAstron mission will sync up with radio antennas on the ground, effectively forming the biggest telescope yet built, with a "dish" spanning almost 30 times the Earth's diameter.
RadioAstron's roots extend back more than three decades, but the mission lost momentum when the Soviet Union collapsed in 1991. "For 20 years it was always five years away," says collaborator Ken Kellermann of the National Radio Astronomy Observatory in Charlottesville, Virginia.
Now, at long last, the spacecraft is poised to launch from Kazakhstan's Baikonur cosmodrome at 0231 GMT on Monday.
At 10-metres, RadioAstron's antenna is small compared to Earth's largest radio telescopes, which span 100 metres or more. But when its signals are combined with those of telescopes on the ground a technique called interferometry, the resulting observations are as sharp as those produced by a single telescope with a dish as wide as the maximum distance between the component antennas.
Eagle eye
This strategy has been used for decades to create radio telescopes the size of the Earth, and in 1997 the Japanese Space Agency launched the first space telescope dedicated to radio interferometry, HALCA.
With an orbit that will extend more than 10 times as far from Earth as HALCA, out to some 350,000 kilometres, RadioAstron promises to capture detail that is more than 10 times as fine. At its best, RadioAstron will be able to resolve points separated by an angle of just 7 microarcseconds, about 10,000 times the resolution of the Hubble Space Telescope.
"There has never been a radio telescope that has been sent so far from the Earth," says Yuri Kovalev, a team leader at the Lebedev Physical Institute's Astro Space Center in Moscow, Russia, which is managing the mission.
Unfolding petals
If all goes well, a Zenit-2SB rocket will help carry the spacecraft into an oblong orbit that will extend from 10,000 kilometres to more than 300,000 kilometres from Earth. Once in orbit, 27 "petals" made of carbon fibre will unfold to create the 10-metre-wide antenna. Over the course of the telescope's five-year mission, the moon's gravity will tug on the telescope, pulling it up to 390,000 kilometres from Earth.
After a few months of check-out, the team will begin to coordinate observations with telescopes on the ground, including two 100-metre radio telescopes in Green Bank, West Virginia, and Effelsberg, Germany, and the 305-metre Arecibo telescope in Puerto Rico.
RadioAstron will zoom in with unprecedented detail on objects such as the nearby galaxy M87, which is spewing relativistic particles from a colossal black hole at its core. By some estimates, the telescope could be used to image near the black hole's event horizon the boundary around which nothing can escape the black hole's gravity. This could reveal new information about how supermassive black holes accelerate matter to near light speed.
Precision cosmology
The telescope will also be able to register the radio waves emitted by water masers, clouds of water molecules that emit microwave radiation, in the discs of galaxies. This motion can be used to study the rotation rate of the galaxies and measure their distance from Earth. When combined with observations of how fast the galaxies are moving, astronomers can use the galaxy distances to calculate the present-day expansion rate of space and the effect of dark energy. RadioAstron may be able to pinpoint the masers' positions more precisely than previous measurements, says Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The telescope will also use the lighthouse-like radio emission from pulsars, the spinning remains of exploded stars, to reveal how dust and gas is distributed around the stars.
The project faces some challenges, chief among them the flood of data some 144 megabits per second that the dish will collect. "There's so much data coming to RadioAstron that you can't store it on board. The data needs to be transported continuously to the ground," says Kellermann, who co-chairs RadioAstron's International Advisory Committee.
So far, only one antenna, a 22-metre dish in the town of Pushchino, south of Moscow, has been set up to receive signals from the spacecraft. Unless other receiver stations can be set up, a good fraction of data the telescope will collect will be lost. The team hopes more receiving stations will be set up as the mission moves forward.
