Herschel Spacecraft Won’t “Bomb” the Moon, But GRAIL Will

by Nancy Atkinson on December 11, 2012

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Artist concept of Ebb and Flow, the two GRAIL spacecraft in orbit of the Moon. Credit: NASA

The Herschel space telescope is slated to be decommissioned next March as the observatory’s supply of cryogenic helium will be depleted. One idea for “disposing” of the spacecraft was to have it impact the Moon, a la the LCROSS mission that slammed into the Moon in 2009, and it would kick up volatiles at one of the lunar poles for observation by another spacecraft, such as the Lunar Reconnaissance Orbiter. However, that idea has been nixed in favor of parking Herschel in a heliocentric orbit. But don’t be disappointed if you were hoping for a little lunar fireworks. There will soon be a double-barreled event as the twin GRAIL spacecraft will impact the moon’s surface on December 17, 2012.

NASA will be providing more information about the GRAIL spacecrafts’ impacts at a briefing on Thursday, but the Gravity Recovery and Interior Laboratory (GRAIL) team said last week that they were still formulating ideas for the impact scenario, and looking at the possibility of aiming the crashes so they are within the field-of-view of instruments on LRO. The two spacecraft are running out of fuel – Principal Investigator Maria Zuber said they have to do three maneuvers every day to keep the spacecraft from slamming into the Moon on their own – and earlier this year the duo were lowered from their prime mission orbit of 55 kilometers above the Moon to 23 km, and this week were lowered to 11 km to enable even higher resolution data.

The two spacecraft have been providing unprecedented detail about the Moon’s internal structure as they send radio signals to each other and monitor any changes in distance between the two as they circle the Moon. Changes as small as 50 nanometers per second have been measured, and last week the team detailed how they were able to create the most detailed gravity map of the Moon, as well as make determinations that the Moon’s inner crust is nearly pulverized.

We’ll provide more information about the GRAIL impacts when it becomes available, but preliminary details are that the impacts will take place on Dec. 17 at 19:28 UTC (2:28 p.m. EST).

The impact by LCROSS (Lunar Crater Observation and Sensing Satellite) confirmed the presence of water ice and an array of volatiles in a permanently shadowed crater at the Moon’s South Pole, and it is expected GRAIL would be targeted for similar observations.

Artist’s concept of Herschel at the L2 libration point one million miles from Earth. Credit: ESA

The Herschel team had said earlier this year that because the cryogenic superfluid helium coolant is running out — and the spacecraft needs to be at temperatures as low as 0.3 Kelvin, or minus 459 degrees Fahrenheit to make its observations — one idea of getting rid of the spacecraft would be to impact it on the Moon. This week, they posted on the Herschel website that ‘the lunar impact option is feasible, but carries an additional cost on top of that of the heliocentric orbit option. The ESA Executive has decided that the Herschel spacecraft will be “parked” indefinitely in heliocentric orbit.”

The Herschel operational large halo orbit around L2 is unstable, and so the orbit needs regular “maintenance,” and consequently, after end-of-helium (expected in March 2013), the spacecraft will need to be “parked” somewhere else with no need of orbit maintenance.

Herschel team member Chris North told Universe Today that the mission operators needed to get some engineering tests done to determine if the Moon impact was feasible. “Basically they hand it over to engineers who do things that are considered too risky during the scientific mission itself – e.g. test the attitude control to its limits to see what it can withstand!” North said via email. He added that most people he had spoken with were all for the impact, — having it “go out in a blaze of glory.”

But, surprisingly, the costs for impact are greater than leaving it in a parking orbit for a few hundred years. It’s orbit may have to be maintained again in the future, as some estimates put it at potentially impacting Earth at some point in several hundred years.

And for anyone worried that a lunar impact by the GRAIL spacecraft will “hurt” the Moon, one look at the Moon shows that it has been hit in the past and continues to get impacted by asteroids and meteoroids, with no adverse affect to its orbit.

As LCROSS principal investigator Tony Colaprete said about the LCROSS impact, “What we’re doing with the Moon is something that occurs naturally four times a month on the Moon, whether we’re there or not. The difference with LCROSS is that it is specifically targeted at a certain spot, Cabeus crater,” and that the laws of physics mean there will be a miniscule perturbation.

Even though the Centaur rocket stage that hit the Moon was expect to kick up about 350 tons of lunar regolith, “The impact has about 1 million times less influence on the Moon than a passenger’s eyelash falling to the floor of a 747 jet during flight,” Colaprete said.

The two GRAIL spacecraft are about the size of washing machines, much smaller than the Centaur rocket, so will have less of an impact.

About 

Nancy Atkinson is Universe Today's Senior Editor. She also is the host of the NASA Lunar Science Institute podcast and works with Astronomy Cast. Nancy is also a NASA/JPL Solar System Ambassador.

Martin_Lambert December 11, 2012 at 11:13 PM

Why do we have to dump Herschel?! :( It is a 3.5m mirror in space! After Spitzer run out of coolant it began a “warm mission”, and it is still a very productive telescope (with 85cm mirror)!

Raimo Kangasniemi December 12, 2012 at 3:31 AM

All of Herschel’s instruments need to be kept in very low temperature for them to operate; one of Spitzer’s instruments is still able to partly operate as it tolerates a somewhat higher temperature.

lcrowell December 12, 2012 at 3:06 PM

Herschel is not space servicable. It also turns out that trying to create space servicable spacecraft and the servicing spacecraft that does the work is as expensive or more expensive than just replacing non-servicable spacecraft.

LC

Aqua4U December 12, 2012 at 12:32 AM

Finding a pulverized lunar crust MORE than pays for the price of the Grail mission! Mining ops anyone?

Park Hershel close.. then go out and recharge the liquid helium tanks?

delphinus100 December 12, 2012 at 2:01 AM

Park close to…what? Go out and service it with what?

And its tanks were not designed for in-space replenishing, in any case.

Aqua4U December 12, 2012 at 4:32 PM

Hmm….. how were the tanks originally filled? Was the orifice used to do that then buried deep within the vehicle? I actually don’t know the design, but assume SOMEONE might have had similar thoughts?

delphinus100 December 12, 2012 at 9:44 PM

They were almost certainly filled on earth with the inherent assumption of gravity assist. Not to say that cryofluids can’t be transferred in microgravity, indeed we may expect to see orbital fuel depots for that very purpose…with both the storage and receiving system designed for that from the start. It should be noted that even the LH2 and LOX tanks for the Shuttle fuel cells also could not be refilled in space, had there been a need.

We saw how hard it was to change out elements of the Hubble telescope that were not meant for Shuttle-based servicing in space, vs. the modular components that were so designed. And Hershel was built with no servicing or other post-launch human contact in mind (as is JWST, there being no manned spacecraft that can currently reach its final location…at least with no meaningful cargo, or economically, if you ultimately tried to use Orion/SLS to do so).

Aqua4U December 13, 2012 at 9:40 PM

Key here are turbo molecular gas powered pressurizing accumulators and cryogenic liquid storage ‘tank-age’. I like…. purified foamed lunar glass linings in my vacuum Dewars…. Thanks for asking!

JonHanford December 12, 2012 at 3:25 PM

I wonder if the GRAIL impacts will be on the Earth-facing side of the moon? Judging from the size of the spacecraft, I would think a modest amateur scope(6-8″ aperture) should be able to spot the impacts. People have recorded impacts of meteors less massive than GRAIL, on the unlit portion of the moon, that occurred during meteor showers(IIRC, the Leonids).

TheVeganarchist December 12, 2012 at 3:36 PM

litterbugs!

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