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EghtesadOnline: The most accomplished planet-hunting machine of all time will seek out strange new worlds no more. NASA decommissioned the Kepler space telescope over the weekend, beaming "goodnight" commands to the sun-orbiting observatory.

Kepler’s official end came as no surprise. NASA announced on Oct. 30 that Kepler's science work was done, because the spacecraft had run out of fuel. Mission team members said then that decommissioning commands would likely be sent within a few weeks, reported.

"Kepler's team disabled the safety modes that could inadvertently turn systems back on, and severed communications by shutting down the transmitters," NASA officials wrote in a statement on Nov. 16. "Because the spacecraft is slowly spinning, the Kepler team had to carefully time the commands so that instructions would reach the spacecraft during periods of viable communication."

The telescope could not feasibly be refueled and returned to action. The spacecraft orbits the sun, not Earth, and is currently about 151 million kilometers from our planet, Financial Tribune reported.

The final commands were sent from Kepler's operations center at the University of Colorado Boulder's Laboratory for Atmospheric and Space Physics, NASA officials said. The commands got to the spacecraft via NASA’s Deep Space Network based in Canberra, the system of big radio dishes the space agency uses to keep in touch with its far-flung probes.


Goodnight Commands

The Kepler Space Telescope officially went to sleep for good. On Nov. 15, the death anniversary of its namesake German astronomer Johannes Kepler, its ground team sent it a series of "goodnight" commands.

The Twitter account NASAKepler on Nov. 16 wrote, “Farewell, planet hunter. Last night, the @NASAKepler space telescope received its final set of commands to disconnect communications with Earth. The ‘goodnight’ commands were sent on the anniversary of the death of the mission's namesake, Johannes Kepler.”

Richard Stephenson a scientist working at Canberra Deep Space Communication Complex also twitted, “Canberra DSS43 had the honor of transmitting the good night sequence and see the @NASAKepler transmitter wink out for the last time… Thanks for the memories Kepler.”


Kepler’s Mission

The $600 million Kepler mission was launched in March 2009, tasked with determining how common Earth-like planets are throughout the Milky Way galaxy. The spacecraft found alien worlds via the transit method.

When a planet crosses in front of its star as viewed by an observer, the event is called a transit. Transits by terrestrial planets produce a small change in a star's brightness of about one in 10,000 (100 parts per million, ppm), lasting for 2 to 16 hours. This change must be absolutely periodic if it is caused by a planet.

The amount of change in the star’s brightness depends on the sizes of the star and the planet; and the duration of the transit depends on the planet's distance from the star and the star's mass. Since the star’s mass and size can be determined from spectroscopic observations, therefore the planet's size and distance can be calculated. 

Kepler first conducted such observations by staring at more than 150,000 stars simultaneously. Then, in 2013, the second of the spacecraft's four orientation-maintaining reaction wheels failed, ending the original mission. Kepler transitioned to a new mission called K2 in 2014, after team members figured out how to stabilize the observatory with the aid of sunlight pressure.


Greatest Kepler Discoveries

Kepler has discovered 2,682 exoplanets to date, 355 of which were found during the K2 phase. An exoplanet or extrasolar planet is a planet outside the Sun's solar system. That grand number represents about 70% of all known alien worlds.

During K2, Kepler studied a variety of cosmic objects and phenomena, from comets and asteroids in our own solar system to faraway supernova explosions, over the course of different campaigns. However, planet-hunting always remained a significant task of the mission.

Kepler's observations over both of its missions suggest that planets outnumber stars in the Milky Way and that potentially Earth-like worlds are common. Indeed, about 20% of Sun-like stars in our galaxy appear to host rocky planets in the habitable zone, the range of distances where liquid water could exist on a world's surface.

But the mission's legacy extends to other fields as well. For example, Kepler's precise brightness measurements—which the telescope has completed for more than 500,000 stars—are helping astronomers better understand the inner workings of stars. And the instrument's supernova observations could shed considerable light on some of the most dramatic events in the universe.

Even though Kepler has closed its eyes, discoveries from the mission should keep rolling in for years to come since about 2,900 "candidate" exoplanets detected by the spacecraft still need to be further studied.


NASA Space Telescope Kepler