The NASA STEREO mission (my mission!) is visiting L4 and L5. These are the Lagrange points are where the Earth and Sun gravitationally balance each other out.  SOHO hangs out at L1, between Earth and Sun (but very near Earth).  L4 and L5 are about 60 degrees ahead and behind the Earth's position in its orbit.L4 L5 illustration

Since L4/L5 gather material, we expect more dust. Dust, of course, can damage our detectors should there be a significant fluke hit directly on a camera. Already, we 'see'-- as in, get hit by-- regions of dust in Earth's orbit. STEREO B suffers more than its sibling A, as B's HI detector is oriented in the ram direction. And no, windshield wipers won't help.

To summarize the article: the twin STEREO satellites (one leading Earth's orbit, one trailing) are going to pass through the L4/L5 regions. There is thought there's unformed planet material, or perhaps the remains of an early proto-planet nicknamed Theia. In one scenario, Theia was perturbed from L4 enough to slam into the Earth, knocking off a chunk of material to create our Moon!

Whether Theia exists, the L4 and L5 points are attraction wells for interesting space clutter. Early planet material from before the Earth formed, later material that drifted in, dust, entire asteroids, there is a lot there. We don't know what, we just know that a) there should be stuff there and b) when we rolled STEREO for a 6-hour peak, we saw stuff. What stuff, we'll find out when we get closer.

STEREO is going there by happy chance. Ostensibly STEREO is a pair of sun-observing satellites in interesting 1AU orbits. Already, though, the wide-field HI detectors have shown us wide starfields and even a comet being stripped by a CME, and the coronagraph have shown us Jupiter and, of course, the sungrazer comet observations started by LASCO.

L4 and L5 are stable Lagrangian points. Unlike L2, L3 or even L1, where SOHO hangs out, they don't require fuel to stay there, just luck. Venus in particular can disturb material out of L4/L5, but material can also accumulate there. They are, in a word, interesting. And risky. Stuart Clark in New Scientist called them gravity holes harboring 'planetary assassins'.

How to handle the L4/L5 passage has been debated. As the earlier articles show, Mike Kaiser at GSFC (STEREO's PI) considered burning most of STEREO's fuel to remain at L4/L5, but that had too much of an impact on the overall STEREO science. L4/L5 are broad enough that STEREO will pass there around Sept-Oct of this year, so there will be time to examine them.

The only instruments really able to see are those aforementioned wide-field HI detectors.

To view L4/L5, STEREO will rotate 180 degrees so the HI (normally pointing 'out') look along the orbit. Informal discussion within the team suggests we'll do this for a half a day several times a month, both approaching and while in the L4/L5 points. During the roll, the in-situ detectors on STEREO (PLASTIC and IMPACT) simply don't work (they expect a certain field orientation), so this is a trade-off. One detector wins, some other science is put on hold. It's a fair trade.

You can spin this story several ways. The tourism bureau just tells us STEREO will visit L4/L5. The engineers talk about risks due to impacts and dusts. The project managers decide to temporarily repurpose this solar mission for planetary work. The scientists muse about what might be lurking in L4/L5. The science writers find speculation on planet-killers.

However we talk about it, for now we plan and wait. Soon, L4 and L5's secrets will be ours.

Alex, the daytime astronomer
Posting every Tuesday and Friday, or track me as @skyday on twitter