# Why has the Earth-Sun libration point L1 been chosen over L2 for NEOCam to detect new NEOs?

above: Profoundly not-to-scale illustration of NEOCam in an orbit around the Sun-Earth libration point L1, about 1.5 million kilometers from Earth. Presumably Sun-shield and Earth-shield block light (both infrared and visible) from the Sun and the Earth in order for the instrument to work at cold temperature necessary to detect the faint infrared light radiated from NEOs.

above: Infrared astronomer Amy Mainzer illustrates how asteroids warmed by the sun will stand out more brightly in the infrared compared to reflected visible light from the sun. One coffee cup is black the other white in the false-color infrared thermal image. From here.

Question: Why not L2? It receives about 4% less radiation from the sun ($1/r^2$) and would only need one shield to block heating from both the Earth and the Sun at the same time, and the optical data receivers on Earth would be looking through a dark, night-time sky instead of a bright daytime sky. These are probably mild advantages at best of course, but both L1 and L2 must have been originally considered and L1 was selected. So what were the benefits of L1 over L2 — why was L2 less suitable for the mission?

In either case NEOCam would be in an orbit around the libration point, not at it, so its solar panels would still receive sunlight at L2, though also receiving 4% less light.

The following is from the JPL NEOCam web page about the orbit:

NEOCam's orbit has been carefully designed to maximize scientific discovery while minimizing cost, complexity, and risk. Similar to NASA's SOHO and Genesis missions, NEOCam will occupy a region of space fairly close to the Earth (in astronomical terms) called the Earth-Sun L1 Lagrange point. This vantage point at L1, which is about four times further away than the Moon and interior to the Earth along the Earth-Sun line, allows NEOCam to view a large fraction of the Earth's orbit at any given time, and the sunshade (based on the 1983 IRAS mission) allows it to look close to the Sun.

This region of space is ideal for NEOCam. It will allow the observatory to maintain a nearly constant distance from Earth (about 1 million kilometers): far enough away to provide a stable, cold environment, yet close enough to support the high-speed radio communications needed to send NEOCam's large-format images back to Earth. The return of these large-format images will allow astronomers to detect even the faintest asteroids and comets with great sensitivity.

We want to find NEOs that are inside Earth's orbit, like Atens, and telescopes don't like looking close to the Sun. So the more inside the orbit of the Earth you can get, the more new NEOs you will find without having to look at the Sun.

Ideally you'd like a NEOCAM near the orbit of Venus. Then you'd be able to catch 'em all. But at E-S L1, you'll find most of the threats to Earth.

• It's a 2% difference - a delta of 3 million kilometers out of 150 million. Is that really the reason? – uhoh Dec 3 '16 at 0:30
• Speaking of "near the orbit of Venus", it was watching a short video of Ed Lu that got me here. – uhoh Dec 3 '16 at 0:33
• Since the interest is in asteroids that cross or come close to crossing Earth's orbit, getting inside Earth's orbit as opposed to outside can make a significant difference. As a simple example, if you imagine that you can't look closer than 90° to the Sun, then if you were at L2, you would be able to see no asteroids that just cross Earth's orbit from inside. From L1, you could see all of them. Of course, you can look closer than 90° to the Sun, but you can see from the simple example that your coverage of such asteroids can be a strong function of whether you are are L1 vs. L2. – Mark Adler Dec 3 '16 at 1:08
• OK I see. So far all I know about the angular extent of the Sun shade is that small not-to-scale image above. But there it looks like roughly 45° in which case the effect is sill real but less. Of course it only takes one big NEO to "destroy the world", so it would be silly if we all died because NEOCam was put in the "wrong" orbit. Then again this is quite a compelling argument to put it even closer as you point out. – uhoh Dec 3 '16 at 1:21
• Does being in the position to see the sun-facing side of the NEOs have anything to do with it as well? – StarWeaver Dec 23 '16 at 14:02