It's one of the first things kids learn when they start to turn their heads skyward: Never look directly at the sun. Even if there's a solar eclipse you're dying to see, don't do it.
It turns out the Hubble Space Telescope operates under the same guidelines –- it too can’t look directly at the medium-sized star in the center of our solar system.
This isn't normally something astronomers want to do, but the upcoming transit of Venus is too exciting to miss with Hubble's amazing eyes in the sky. So to protect its cameras, the orbiting telescope is going to use perhaps the neatest way to observe the transit: with the moon as a mirror.
A transit happens when Venus passes between Earth and the sun. On June 5 or 6 this year — the exact time and date depends on where you are in the world — Venus will be visible as a small black circle crossing the disk of the sun.
Transits are rare because of the two planets' relative orbits, but that also makes them predictable. They come in pairs eight years apart, and the time between pairs alternates between 121.5 years and 105.5 years. The upcoming transit is the pair of one that occurred in 2004. The next transits won’t come until 2117 and 2125.
Astronomers aren't just interested in observing the transit; they plan to use the rare cosmic occurrence to analyze the sunlight passing through Venus' atmosphere to capture the fingerprint of its makeup. As sunlight passes through a planet's atmosphere, certain wavelengths are absorbed. Scientists can read the absorption spectrum that makes it all the way through and determine the atmosphere’s composition. Sunlight will pass through Venus' atmosphere and end up on the moon, and that's where Hubble will be focusing.
But scientists already know what Venus' atmosphere is made of, so rather than taking another set of measurements, Hubble will gather data that serves as a benchmark for the future study of exoplanets — i.e., planets orbiting other stars.
Scientists learn about exoplanets by spectroscopic analysis of the star's light passing through a planet's atmosphere. If Hubble can get an accurate reading of Venus, the method will help scientists search for and discover other Earth-like planets, all in the name of finding life on other worlds to help us better understand our own.
Hubble will use its advanced Camera for Surveys, Wide Field Camera 3 and Space Telescope Imaging Spectrograph to view the transit in wavelengths ranging from ultraviolet to near-infrared light as well as perform spectroscopic analysis. In all, it will observe the transit for seven hours. Astronomers need such a long observation because the spectral signatures are quite faint, with only 1/100,000th of the sunlight filtering through Venus' atmosphere to be reflected off the moon. The transit is also a one-shot deal, so the more precise and complete the observation the better.
Astronomers have already done a dry run of the observation. They've picked the spot on the moon that Hubble will focus on, the Tycho crater. In January, they ran a test to make sure Hubble can capture and refocus on the same spot for the full seven hours, since the Earth blocks Hubble's view of the moon for 40 minutes of each 96-minute orbit. The rehearsal was a success.
This isn’t the first time scientist have used the transit of Venus to help answer the big questions about our planet and the universe. Astronomer Jeremiah Horrocks observed the 1639 transit and used his measurements to complete Johannes Kepler’s calculation predicting the pattern of transits. Edmund Halley figured that transits could be used to determine the distance from the Earth to the sun, also called an astronomical unit. By tracking the time of one transit from different locations around the world, astronomers could triangulate the distance. A global cooperative of astronomers observed the 1761 and 1769 transits and found the Earth-sun distance to be 95 million miles; another global group observed the 1874 and 1882 transits and found a distance of 92 million miles, which is very close to the modern measurement of 93 million miles.
Transits of Venus are not only rare, they’re fascinating and historically important. It’s worth taking time out of your day to see, safely of course, though Hubble’s lunar reflection is going to be a much cooler way to watch it than the pinhole eclipse cameras you used to make in grade school.
Photo: Venus transiting the sun in 2004 as observed by NASA's Transition Region and Coronal Explorer. Credit: NASA.