The wonderful photos of the planets, moons, and Sun taken by space probes must surely be one of the great accomplishments of humanity’s first steps into space. And most of them are available online, free for non-commercial use.
One of the less-known space programs is STEREO (Solar TErrestrial RElations Observatory), a rather contrived acronym, but a fine description of what the program does—it makes stereo photographs of the Sun’s surface using two spacecraft, separated by well over 100 million miles. (Compare that to the normal separation of 63-65 mm between the two lenses of a stereo camera, the same as the interocular distance between an adult’s eyes.)
The spacecraft are approximately in Earth’s orbit, but one is considerably ahead of the Earth and one behind— so they’re rather unimaginatively named STEREO (Ahead) and STEREO (Behind). Both take photographs of the Sun at several different wavelengths of light, including four wavelengths of extreme ultraviolet that are absorbed by Earth’s atmosphere and thus can only be studied from space. The cameras and other scientific instruments on board the two spacecraft are part of an effort to better understand the Sun’s coronal mass ejections.
Understanding coronal mass ejections (CMEs) is important because these powerful eruptions can blow up to 10 billion tons of the Sun’s atmosphere into interplanetary space at speeds of approximately 1 million mph (1.6 million kph). CMEs can trigger severe magnetic storms, damaging satellites and even disabling electrical power lines on Earth. CMEs are also extremely hazardous to astronauts performing extravehicular activities outside the protection of the Space Shuttle or the International Space Station.
In this photo from the STEREO (Ahead) spacecraft in late September, we see a massive solar prominence erupting, suspended in the Sun’s twisted magnetic fields. The prominence is many times the size of planet Earth and was recorded in extreme ultraviolet light emitted by ionized helium. A sequence of these photographs has been made into a QuickTime movie that shows the prominence lifting away from the Sun’s surface and unfurling into space over the course of several hours.
Moving out in the solar system, the still-active Deep Impact spacecraft, which sent an impact probe to hit the nucleus of Comet Tempel 1 back in July 2005, was aimed back at Earth from a distance of 31 million miles/ 50 million km to make a series of photographs of the Moon making a transit across the Earth. This false- color version is an infrared-green-blue image using filters at 850 nm (infrared), 550 nm (green), and 450 nm (blue).
Even farther from Earth, orbiting Mars, the High Resolution Imaging Science Experiment camera on NASA’s Mars Reconnaissance Orbiter is sharp enough to make this stunning picture of Earth and its Moon, from a distance of about 142 million kilometers (88 million miles). In the photo, Earth has a diameter of about 90 pixels and the Moon’s diameter is about 24 pixels. You can see the west coast outline of South America at lower right, although clouds are the dominant feature. But for me, the main effect of the picture is to emphasize how far apart the two bodies are, compared to their sizes.
Also on the Mars Reconnaissance Orbiter is CRISM, the Compact Reconnaissance Imaging Spectrometer for Mars. CRISM can identify minerals on the Martian surface— including those that formed in the presence of water—by “reading” more than 500 colors in the sunlight reflected from the surface. CRISM images strongly suggest Mars once had vast lakes, flowing rivers, and a variety of other wet environments that had the potential to support life. CRISM chief scientist Scott Murchie, from the Johns Hopkins University Applied Physics Laboratory in Maryland, said, “The big surprise from these new results is how pervasive and long-lasting Mars’ water was, and how diverse the wet environments were.”
This color-enhanced image shows the delta in Jezero Crater, which once held a lake. Clay-like minerals (shown in green) were carried into the lake by ancient rivers, forming the delta. Because clays tend to trap and preserve organic matter, the delta is a good place to look for signs of ancient life.