DSCOVR Returns First Full Image of Sunlit Earth
The Deep Space Climate Observatory (DSCOVR) spacecraft was launched on a SpaceX Falcon 9 back in February, the first SpaceX launch destined for somewhere beyond Earth orbit. To be precise, DSCOVR was bound for the Earth-Sun L1 point, one of a series of Lagrangian points. Every orbit has Lagrangian points, which are defined by special geometric and gravitational relationships between the orbiting body (the Earth, in this case) and the body being orbited (the Sun).
To understand the importance of the L1 point, keep in mind a simple rule of orbits: bodies that orbit a central object more closely will have shorter periods -- that is, the time it takes to complete one orbit -- than those that orbit further out. The Earth takes one year to complete one orbit around the Sun. Venus, which orbits the Sun more closely, takes only 225 days. Mars, which orbits the Sun at a greater distance than Earth, takes 687 days.
This is potentially a problem if you want to plan a mission like DSCOVR's. The purpose of DSCOVR is twofold: 1) continually image the Earth in multiple spectra, including visible light, for climate science applications; 2) monitor incoming material from the Sun to provide advance warning of potentially hazardous solar storms. But if you design a spacecraft to do this and place it in any old orbit between the Earth and the Sun, orbital mechanics will ruin your day pretty quickly. Because your spacecraft orbits the Sun more closely, it will travel around its orbit more quickly, and pretty soon it will be way out of alignment, well ahead of the Earth, and no longer able to image our planet or monitor incoming solar material.
This is where the L1 point comes in. In any orbital relationship, the L1 point lies directly between the two bodies, where their gravitational attractions balance in such a way that any object at the L1 point will have the same period as the orbiting body. In other words, objects at the Earth-Sun L1 point will orbit the Sun with the same period as Earth, remaining, from our perspective, stationary between us and the Sun. This animation depicts a different spacecraft at the Earth-Sun L1, but the principle is the same:
Animation of the Solar and Heliospheric Observatory (SOHO) spacecraft in its orbit at the Sun-Earth L1 point. Note that due to its position at L1 it remains aligned between the Sun and the Earth rather than racing ahead of the Earth in its orbit. The small oval orbit that SOHO has is called a "halo orbit." This is used because it's the most stable way to keep a spacecraft in the vicinity of the L1 point. DSCOVR uses a similar halo orbit. Credit: NASA/JPL
So DSCOVR was sent to the Earth-Sun L1 point, approximately 1 million miles from Earth, which enabled it to keep a continuous eye on Earth's weather, as well as monitor the solar environment with vigilance. It reached its destination in early June. Yesterday, it sent back its first data from the Earth Polychromatic Imaging Camera (EPIC), which observes the Earth in infrared, visible light, and ultraviolet. The visible light portions were combined to produce this gorgeous image of the sunlit Earth (click here for the full resolution):
Visible light image of the sunlit Earth, taken by DSCOVR's EPIC instrument. Credit: NASA
Speaking of Earth-Sun L1 spacecraft and fantastic imagery, check out SOHO's web page for a ton of observations of our Sun, including animations of all the recent data like this one (bright object moving to the left is Mercury):
Credit:SOHO/ESA/NASA