If you want to photograph something in space, what better way than to have a spacecraft take the picture? The Swift Telescope – better known for its study of high-energy outbursts and cosmic explosions – was able to observe the flyby of 2005 YU55, the asteroid that came within 324,600 kilometers (201,700 miles) of Earth this week, and captured its tumbling, rapid motion across the sky.
“Swift’s ultraviolet and X-ray capability gives scientists a unique perspective on comets and asteroids, expanding the spectral window beyond the radio, infrared and optical observations so well handled by big ground-based facilities,” said Sergio Campana, a Swift team member at Brera Observatory in Merate, Italy.” Campana requested that the spacecraft train its telescopes on the asteroid as a target of opportunity.
This isn’t the first time Swift has made observations of passing comets and asteroids. All told, the spacecraft has observed ten asteroids, including Vesta — now being studied close-up by NASA’s Dawn spacecraft — and Scheila, which brightened unexpectedly in late 2010 after colliding with a much smaller asteroid.
Even though 2005 YU55 poses no threat of a collision with Earth for at least the next century, this close flyby was a great opportunity to study an asteroid from a fairly close vantage point. Telescopes on Earth were trained on the asteroid’s orbital path, and now these observations by Swift will help in understanding the details of this asteroid and future hazardous asteroids as well.
“We observed the asteroid with Swift’s Ultraviolet/Optical and X-ray telescopes but, as expected, we saw it only in the UV,” said Dennis Bodewits, a Swift team member at the University of Maryland in College Park.
The challenge with 2005 YU55 was its rapid motion across the sky, which was much too fast for Swift to track. Instead, the team trained the spacecraft’s optics at two locations along the asteroid’s predicted path and let it streak through the field. The first exposure began a few hours after the asteroid’s closest approach and fastest sky motion — before 9 p.m. EST on Nov. 8 — but detected only a weak signal.
Six hours later, around 3 a.m. EST on Nov. 9, Swift began an exposure that captured the asteroid sweeping through the Great Square of the constellation Pegasus. The 11th-magnitude rock was then 333,000 miles away and moving at 24,300 mph, about an hour after its closest approach to the moon.
That exposure gave the Swift team more than a streak through the stars. “A novel feature of Swift is the ability to go into a mode tracking the arrival of every photon captured by the instrument. With that information, we can reconstruct the asteroid as a point source moving through the Ultraviolet/Optical Telescope’s field of view,” said Neil Gehrels, lead scientist for Swift at NASA’s Goddard Space Flight Center in Greenbelt, Md.
The 27-minute-long image was effectively sliced into short 10-second-long exposures, which then were combined into a movie. This allows scientists to study short-term brightness variations caused by the object’s rotation.
The result is a movie of 2005 YU55 at ultraviolet wavelengths unobtainable from ground-based telescopes. For planetary scientists, this movie is a treasure trove of data that will help them better understand how this asteroid is put together, information that may help make predictions of its motion more secure for centuries to come.