Warkworth Observatory role in Rosetta mission

The Rosetta space probe is currently orbiting the 'rubber duck' Comet 67P/Churyumov-Garasimenko.
ESA

The Rosetta space probe is currently orbiting the 'rubber duck' Comet 67P/Churyumov-Garasimenko.

The Warkworth Radio Astrononomical Observatory in north Auckland has joined 12 other radio telescopes from around the world to focus on the Rosetta space craft as it orbits Comet 67P/Churyumov-Garasimenko on its journey around the sun.

The project by researchers from the European radio astronomy group JIVE (Joint Institute for VLBI in Europe) and ESA (European Space Agency) is aimed at figuring out how heavy the comet is. This will help to confirm what it's made of.

While it may be the size of Mt Everest, it's lighter than it looks.

Rosetta selfie with Comet 67P/Churyumov-Garasimenko at 16km.
Delwyn Dickey

Rosetta selfie with Comet 67P/Churyumov-Garasimenko at 16km.

"The mass of this comet is known with very poor accuracy - 10 billion tonnes plus or minus 1 billion tonnes," Auckland University of Technology's Institute for Radio Astronomy and Space Research director and Warkworth Observatory head Professor Sergei Gulyaev says.

"It provides a very low density of the comet of something like 0.4 g per cubic centimetre – surprisingly low."

This means it is much lighter than if it was made of solid ice and suggests it is hollow or very porous.

Professor Sergei Gulyaev, director of AUT's IRASR and head of the Warkworth Observatory.
Delwyn Dickey

Professor Sergei Gulyaev, director of AUT's IRASR and head of the Warkworth Observatory.

One of the aims of the lander Philae was to drill into the comet surface and analyse the material. This would tell scientists what it was made of.

Organic molecules, essential for life on earth, were detected by Philae on the comet surface, but a rough landing meant Philae ended up on an angle, its solar panels partially in shade. This saw it shut down after 60 hours to preserve power.

To get as much science done before the shut down, the lander was directed to drill into the surface, but that seems to have failed.

It's possible to tell the comet's mass by observing small changes in Rosetta's speed as it orbits the comet.

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The heavier an object is the more gravitational force it exerts on an orbiting object and the faster it goes.

Having a broad array of radio telescopes on earth called VLBI or Very Long Base Line Interferometry, ranging across several continents all gathering signals from Rosetta at the same time, means it's possible to determine its exact position with respect to the comet and detect the speed changes using the Doppler Effect.

The Doppler Effect causes the change in the pitch of motor noise as vehicles pass us. The length of the sound wave shortens as the car speeds toward us and lengthens as it moves away

 "What we measure are Doppler shifts of frequency of a signal radiated by Rosetta," Gulyaev says.

"As Rosetta orbits the comet, it changes its velocity and shows acceleration in the gravitational field of the comet. VLBI allows measuring precise position of the spacecraft."

Knowing the mass of Rosetta, having its position, velocity and acceleration as it orbits the comet, means they can find the precise mass of the comet.

The comet will lose some of its material each time it sweeps in close to the sun and warms up. This causes the streaming tail we're familiar with.

"This particular comet has a period of 6.5 years, and it will be interesting to see what will be its mass when it makes a full orbit around the sun – how much mass it loses when orbiting the sun, therefore how long it can exist and what is its lifecycle," Gulyaev says. 

After its orbit was altered 56 years ago following a close encounter with Jupiter, Comet 67P now comes much closer to the sun at 190 million kilometres, which is likely to shorten its life.

It was past its closest point to the sun in August this year and is now heading back out into space.

The ESA Rosetta mission will end in September 2016, possibly with an attempt to land on the comet.

 - Stuff.co.nz

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