December 4th: The WISE Mission
Podcaster: Roz Brown
Organization: Ball Aerospace & Techologies Corp.
WISE stands for the Widefield Infrared Survey Explorer – a NASA satellite that will carry an infrared-sensitive telescope into space in order to return an “all-sky” survey. Ball Aerospace built the satellite for the WISE mission – which by the way – is scheduled to launch from Vandenberg Air Force Base on Dec. 9. Roz Brown talks with Joan Howard, Ball Aerospace’s program manager for the WISE mission.
WISE Podcast to be aired on 365 Days of Astronomy – Dec. 4, 2009
I’m Roz Brown, Media Relations Manager for Ball Aerospace. Today I’m with Joan Howard, Ball Aerospace’s program manager for the WISE mission.
WISE stands for the Widefield Infrared Survey Explorer – a NASA satellite that will carry an infrared-sensitive telescope into space in order to return an “all-sky” survey. Ball Aerospace built the satellite for the WISE mission – which by the way – is scheduled to launch from Vandenberg Air Force Base in just a few days.
RB: Joan, WISE is an infrared mission, so it uses a different kind of instrument than was used for the Hubble mission, for instance. Tell us how it’s different…..
JH: First, the telescope is much smaller, and it’s longer wavelength infrared instrument. WISE will survey the sky in four wavelengths, so the instrument is a four-channel, super-cooled infrared telescope designed to provide a full-sky, infrared map. Additionally, WISE will measure more than 100,000 asteroids in the solar system, and identify two-thirds of all stars in the solar neighborhood that have not yet been seen — including those closest to the Sun. The other missions focus on selected areas of the sky – so they have better sensitivity but only in small areas.
RB: NASA has procured other sky surveys in past decades. What will WISE do that they didn’t?
JH: WISE sees infrared light, which is light beyond the red part of the rainbow invisible to our eyes. There has not been an infrared survey in more that 20 years and there has been significant progress in technology during that time. Because the atmosphere blocks the infrared, ground based surveys are not able to gather data in these wavelengths. WISE has the ability to survey the entire sky with 500 times more sensitivity than previous sky surveys, and should provide information to astronomers, physicists and the public for decades to come. And it will do all that in six months.
RB: Six months. So the WISE mission is only expected to last about 7 – 10 months? Why is that?
JH: Cryogen, in this case solid hydrogen, boils offs as it cools the instrument. This mission is designed for a six month survey, which covers the entire sky, but current analysis indicates that the cryogen will last for 10 months. You should know that all objects produce infrared light and the warmer they are the more they produce. Additionally, in the expanding universe, distant objects are moving away from us rapidly so their wavelengths are shifted longer. Even something that would be emitting visible light would be visible only in the infrared if moving rapidly away.
The telescope needs to be colder than the objects in space it will observe so that the detectors can see the dim infrared emission from them rather than from the telescope. Our principal investigation is fond of saying that we’re looking for the old, the cold, and the dusty.
RB: So, the commissioning takes a month and the survey of the sky is scheduled for six months and there’s still three months coolant left. What happens in those three months?
JH: WISE is expected to complete a second pass of half the sky. This would increase the sensitivity of that part of the survey, find more near-Earth asteroids, help determine variability of some objects, and the distance and motion of other nearby objects.
RB: How cold will the components be?
JH: The optics will be cooled to less than 20 degrees centigrade above absolute zero. The detectors need to be cooled further to less than 8 Kelvin. The MCT detectors operate at higher T, around 32 Kelvin.
RB: How will you keep it that cold?
JH: We have filled the WISE cryostat – think of it like a giant thermos - with solid hydrogen which surrounds the instrument.
RB: Tell us a little about the Ball Aerospace spacecraft.
JH: The spacecraft is approximately 3 feet tall and 6 in diameter in an octagon shape. So, it’s a relatively small, compact spacecraft. WISE is a single string spacecraft which means that for the most part, there is no redundant hardware. Every part has to work since there is no back-up as in most spacecraft. The design is suited for this type of low earth orbit short lived mission. Although small, it does provide the very stable pointing required as well as power, communications and fault protection.
RB: Nonetheless, it’s called the Widefield Telescope – how do you get the wide-field view?
JH: The wide-field of view comes from the fact that the field of view is nearly a degree – which is much wider than most science instruments. WISE has a small telescope diameter (40 cm) and large detector arrays. There are four infrared sensitive detector arrays of greater than one megapixel each. The resolution of the telescope is six arc seconds in the near infrared and 12 arc seconds in the mid infrared.
RB: Tells us a little about the orbit launch.
JH: The orbit is called Sun-synchronous, because it flies over the dividing line between day and night on Earth, called the terminator. The 523 km circular orbit, inclined by 97.4 degrees, shifts as Earth moves around the Sun so that it can stay over this line. This allows the spacecraft to constantly be in sunlight, so the solar panels will always be pointed at the Sun, providing power to the spacecraft. The spacecraft rotates slowly tail over head once per orbit to keep the telescope always pointing up into space, away from the ground.
RB: What is the importance of this mission?
JH: WISE is a gap-filler mission. It will fill the gap in all-sky surveys in the mid-infrared. WISE will also create a catalog of over 300 million sources that will be of interest to future infrared studies, especially the James Webb Space Telescope scheduled to launch in 2014. WISE may well find the closest star to the sun – we now know only the closest star that emits in the visible wavelength; Visible stars are estimated to be only one-third of the stars in the universe. Because we are mapping the trajectory of so many asteroids, we’ll also provide needed information for those tracking the danger potential.