Help Shape the Future of Space Exploration

Join The Planetary Society Now  arrow.png

Join our eNewsletter for updates & action alerts

    Please leave this field empty

Headshot of Emily Lakdawalla

LCROSS: Hitting those tantalizing purple pixels

Posted by Emily Lakdawalla

16-09-2009 12:32 CDT


After I posted a report on the selection of crater Cabeus A as the target for the upcoming LCROSS impact into the Moon, I received an email with several questions about the target. The chosen site is not the center of Cabeus A. Rather it's a spot on the floor of a smaller crater on the rim of Cabeus A, where there are two "tantalizing purple pixels" (as my commenter called them) in a Lunar Prospector map of subsurface hydrogen. The email expressed curiosity about how large an area those pixels represented, and what the targeting accuracy of LCROSS was. Unsaid was the question: how sure is the LCROSS team that they'll hit the spot where Lunar Prospector saw hydrogen?

Water concentrations near the lunar south pole


Water concentrations near the lunar south pole
The crater Cabeus A is the target site for the LCROSS impact. Lunar Prospector data suggests that an area near the crater's northern rim may contain as much as 2% water in the soil.
So I sent a bunch of questions of my own to LCROSS project scientist Tony Colaprete, who got right back to me with answers. The crater on the rim of Cabeus A that contains those tantalizing purple pixels is about 15 kilometers across. The area covered by those purple pixels is "about 5-6 kilometers across; the area we are targeting is about 3.5 kilometers across," Colaprete told me. That 3.5 kilometers is the targeting circle for the Centaur. "The actual LCROSS performance is considerably better than this," he said. "The 3.5 kilometer diameter is meant also to take into account uncertainties in the data sets used for [target] selection." That is, they're going to zero in on that target accurately enough that the positional accuracy of the maps produced from earlier data sets is a significant contributor to their uncertainty about where they'll hit.

Three and a half kilometers is a pretty small target circle. For comparison, the Phoenix landing ellipse was 100 kilometers long and 19 kilometers wide. There are a great many reasons why it's easier to target a spot on the Moon accurately than it is to target a spot on Mars accurately, chief among them being Mars' pesky atmosphere. (It's variability in the density and height of Mars' changeable atmosphere on any given day that makes Mars landing ellipses so very long in the down-track direction.)

LCROSS still has a long way to fly before its impact next month. It was 687,000 kilometers away as of its last positional Tweet -- that's almost double the average Earth-Moon distance. "We are getting near the end and things are looking good!" Colaprete told me.

In related news, Lunar Reconnaissance Orbiter successfully achieved its mission orbit at 50 kilometers altitude yesterday; the science mission has begun. There is going to be a press briefing on NASA TV tomorrow at 10 am my time with early results from the mission. I don't know if they'll have an image from LROC from the science orbit or not -- it may be too soon. But I'm looking forward to seeing a lot of eye candy tomorrow.

See other posts from September 2009


Or read more blog entries about:


Leave a Comment:

You must be logged in to submit a comment. Log in now.
Facebook Twitter Email RSS AddThis

Blog Search

Planetary Defense

An asteroid or comet headed for Earth is the only large-scale natural disaster we can prevent. Working together to fund our Shoemaker NEO Grants for astronomers, we can help save the world.


Featured Images

LightSail 2 and Prox-1
Bill Nye at LightSail 2 pre-ship review
LightSail 2 pre-ship review team photo
Swirling maelstrom
More Images

Featured Video

Class 9: Saturn, Uranus, and Neptune

Watch Now

Space in Images

Pretty pictures and
awe-inspiring science.

See More

Join The Planetary Society

Let’s explore the cosmos together!

Become a Member

Connect With Us

Facebook, Twitter, YouTube and more…
Continue the conversation with our online community!