Category Archives: Exoplanets

Astronomy, Exoplanets, Space Systems

Update on efforts to revive the Kepler spacecraft

The amazing successful exoplanet finding spacecraft Kepler went out of action last May when one of its reaction wheels (basically an electric powered gyroscope) ceased to function properly. The spacecraft has four reaction wheels and needs at least three to point its telescope with sufficient stability and accuracy to carry out the extremely precise measurements of the light of stars. An exoplanet is detected by the slight dimming of a star’s light when a planet transits across the star’s face as seen by the spacecraft.

One reaction wheel had already failed (i.e. friction on the spinning wheel grew too high) and so the failure of the second seemed to ring the death knell of the observatory’s exoplanet searching. However, the team began an effort to determine if one or both of the reaction wheels could be revived to a level of performance that could allow a return to observations for at least some scientific goals if not for exoplanet searching.

Here is a good summary of the status of the recovery efforts: More Efforts to Revive Kepler Space Telescope Mission Planned – Coalition for Space Exploration

And here is the complete statement from the Kepler team:

Kepler Mission Manager Update: Pointing Test

The team has continued exploratory recovery testing of Reaction Wheel 4 (RW4). On Thursday, July 25, 2013, the wheel spun in both directions in response to commands.

While both RW4 and RW2 have spun bi-directionally, friction levels remain higher than would be considered good for an operational wheel. However, it will be important to characterize the stability of the friction over time. A constant friction level may be correctable in the spacecraft’s attitude control system, whereas a variable friction level will likely render the wheels unusable.

With the demonstration that both wheels will still move, and the measurement of their friction levels, the functional testing of the reaction wheels is now complete.  The next step will be a system-level performance test to see if the wheels can adequately control spacecraft pointing.

Reaction wheel
High-precision pointing of the Kepler spacecraft is controlled by reaction wheels,
which are small electric motors mounted on the spacecraft that control
the three axes of motion: up/down, forward/back and left/right. Image Credit: Ball Aerospace

The team is preparing for the next test using RW2. Friction levels on RW4, the wheel that failed in May, are higher and no additional testing is planned at this time. The pointing test involves determining the performance of the wheel as part of the spacecraft system. The test will be conducted in three stages.

The first stage of the pointing test will determine if the spacecraft can sustain coarse-point mode using RW1, 2 and 3. Coarse-point mode is regularly used during normal operations, but has insufficient pointing accuracy to deliver the high-precision photometry necessary for exoplanet detection. During coarse-point the star trackers measure the pointing accuracy of the spacecraft. When using wheels to control the spacecraft, pointing is typically controlled to within an arcsecond, with a fault declared if the pointing error exceeds a quarter of a degree. This degree of pointing accuracy would be equivalent to keeping an imaginary Kepler telescope pointed at a theatre-size movie screen in New York City’s Central Park from San Francisco.

In the first stage, testing will demonstrate whether or not operation with RW2 can keep the spacecraft from entering safe mode. A safe mode is a self-protective measure that the spacecraft takes when an unexpected event occurs, such as elevated friction levels in the wheels.

In the second stage, testing will investigate RW2’s ability to help control the spacecraft pointing with enough accuracy to transmit science data to the ground using NASA’s Deep Space Network. If RW2 can sustain coarse-point in stage 1, the second stage of the test will be to point the high-gain antenna to Earth and downlink the data currently stored aboard. This requires that the pointing be controlled more tightly than simply avoiding safe mode, yet does not require the very fine control needed to return to science data collection.

The Kepler spacecraft
The spacecraft provides the power, pointing and telemetry for the photometer.
Other than the four reaction wheels used to maintain the precision pointing
and an ejectable cover, there are no other moving or deployable parts on
the spacecraft. Image Credit: NASA Ames/Ball Aerospace

The final stage of the test will determine if RW2 can achieve and maintain fine-point, the operating mode for collecting science data. During fine-point the fine-guidance sensors measure the spacecraft pointing. When using wheels to control the spacecraft, pointing is controlled to within a few milliarcseconds. Using our imaginary Kepler telescope example, this degree of pointing accuracy would be equivalent to pointing at a soccer ball in New York City’s Central Park from San Francisco.

The team anticipates beginning the pointing performance testing on Thursday, August 8, 2013 and will continue into the following week if all goes well. A determination of whether Kepler can return to exoplanet data collection is expected a couple weeks after these pointing tests are complete.

As engineers explore recovery of the spacecraft, scientists continue to analyze the existing data. Earlier this week the team delivered their findings for 1,236 new Kepler Objects of Interest (KOIs) to the NASA Exoplanet Archive. The new KOIs were found by searching the observational data from Quarters 1 to Quarter 12. Of the 1,236 new KOIs, 274 were judged to be planet candidates, while many others were determined to be false positives. These newly announced Kepler planet candidates bring the current count to 3,548. Some of these new planet candidates are small and some reside in the habitable zone of their stars, but much work remains to be done to verify these results.

Also announced this week is the Kepler Science Conference II Nov. 4-8, 2013 at NASA Ames Research Center at Moffett Field, Calif. Registration is now open.

Regards,
Roger

Activism, Astronomy, Exoplanets

Planetary Society sponsoring Alpha Centaur exoplanet search

The Planetary Society is supporting a project using a ground based telescope to detect planets around the two stars of the Alpha Centauri system, the closest stars to our own solar system:

Astronomy, Education, Exoplanets

A new resource guide for exoplanets

Prof. Andrew Fraknoi of the Foothill College Astronomy Department sends this announcement about a new resource guide for the study of exoplanets:

A new annotated guide to written, web, and audio-visual resources for teaching about planets orbiting other stars is now available.  Materials in the guide to this rapidly-changing branch of astronomy include video and audio files of lectures and interviews with leading scientists in the field, phone and tablet apps, a citizen-science web site, popular-level books and articles, and more.

Published by the NASA Astrophysics Education and Outreach Forum and the Astronomical Society of the Pacific, the guide can be found as a PDF file at:

http://www.astrosociety.org/education/astronomy-resource-guides/the-search-for-planets-around-other-stars/

 

Astronomy, Exoplanets

Hubble determines the color of an exoplanet for the first time

The Hubble telescope has determined the true color of an exoplanet for the first time:

Hubble spots azure blue planet

Astronomers using the NASA/ESA Hubble Space Telescope have, for the first time, determined the true colour of a planet orbiting another star. If seen up close this planet, known as HD 189733b, would be a deep azure blue, reminiscent of Earth’s colour as seen from space.

But that’s where the similarities end. This “deep blue dot” is a huge gas giant orbiting very close to its host star. The planet’s atmosphere is scorching with a temperature of over 1000 degrees Celsius, and it rains glass, sideways, in howling 7000 kilometre-per-hour winds [1].

At a distance of 63 light-years from us, this turbulent alien world is one of the nearest exoplanets to Earth that can be seen crossing the face of its star. It has been intensively studied by Hubble and other telescopes, and its atmosphere has been found to be dramatically changeable and exotic, with hazes and violent flares (heic0720, heic1209). Now, this planet is the subject of an important first: the first measurement of an exoplanet’s visible colour.

“This planet has been studied well in the past, both by ourselves and other teams,” says Frédéric Pont of the University of Exeter, UK, leader of the Hubble observing programme and an author of this new paper. “But measuring its colour is a real first — we can actually imagine what this planet would look like if we were able to look at it directly.”

In order to measure what this planet would look like to our eyes, the astronomers measured how much light was reflected off the surface of HD 189733b — a property known as albedo [2].

HD 189733b is faint and close to its star. To isolate the planet’s light from this starlight, the team used Hubble’s Space Telescope Imaging Spectrograph (STIS) to peer at the system before, during, and after the planet passed behind its host star as it orbited. As it slipped behind its star, the light reflected from the planet was temporarily blocked from view, and the amount of light observed from the system dropped. But this technique also shows how the light changes in other ways — for example, its colour [3].

“We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star,” explains Tom Evans of the University of Oxford, UK, first author of the paper. “From this, we can gather that the planet is blue, because the signal remained constant at the other colours we measured.”

The planet’s azure blue colour does not come from the reflection of a tropical ocean, but is due to a hazy, turbulent atmosphere thought to be laced with silicate particles, which scatter blue light [4]. Earlier observations using different methods have reported evidence for scattering of blue light on the planet, but these most recent Hubble observations give robust confirming evidence, say the researchers.

HD 189733b presented a favourable case for these kinds of measurements as it belongs to a class of planets known as “hot Jupiters”. These massive planets are similar in size to the gas giants in the Solar System, but instead lie very close to their parent star — this size and proximity to their star make them perfect subjects for exoplanet hunting. We know that hot Jupiters are numerous throughout the Universe. As we do not have one close to home in our own Solar System, studies of planets like HD 189733b are important to help us understand these dramatic objects.

“It’s difficult to know exactly what causes the colour of a planet’s atmosphere, even for planets in the Solar System,” says Pont [5]. “But these new observations add another piece to the puzzle over the nature and atmosphere of HD 189733b. We are slowly painting a more complete picture of this exotic planet.”

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Artist’s impression of HD 188733b:

Artists impression of HD 189733b

This illustration shows a “hot Jupiter” planet known as HD 189733b orbiting its star, HD 189733. The NASA/ESA Hubble Space Telescope measured the actual visible light colour of the planet, which is deep blue. This colour is not due to the presence of oceans, but is caused by the effects of a scorching atmosphere where silicate particles melt to make “raindrops” of glass that scatter blue light more than red light.

The planet HD 189733b was discovered in 2005 and is so close to its star that it is gravitationally “tidally locked”, so that one side always faces the star and the other side is always dark.

Because the planet is only 63 light-years from Earth, a visitor would see many of the same stars we see in our nighttime sky, though the constellation patterns would be different. Our Sun and the nearest star to our Sun, Alpha Centauri, appear as two faint stars near the centre of the image.

Credit:

NASA, ESA, and G. Bacon (AURA/STScI)

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This video shows where the Hubble telescope finds this star: