Archive for December, 2009

WISE: Deployments coming soon

Posted in Astronomy, Mission Operations, Observatories on 12/24/2009 by clairdeluna

It looks like the WISE observatory is happily working through the early checkout phase. During the first few days/weeks of a mission, the engineers verify the basic functionality of a newly launched spacecraft, essentially making sure nothing broke during the extreme environment associated with launch.

During launch, the observatory experiences extreme vibration, acoustic, thermal and space (vacuum) environments either simultaneously or in rapid succession. Prior to launch we test each of these individual environments as best we can, but nothing can really prepare for the intensity of the launch. As a result, the post-launch checkout is a very slow, careful process that exercises each bit of spacecraft functionality one tiny step at a time. That way, if anything has broken, it can be identified early, troubleshot, and either recovered, or changed to a redundant piece of hardware. NASA prefers for all spacecraft to be fully redundant (that is, have a spare bit of hardware for everything in case the primary one breaks) but the reality is that full redundancy is cost prohibitive.

Instruments are often not fully redundant. Almost every single instrument on a NASA mission is one-of-a-kind hardware, designed specifically and solely for the science goal of that mission. Instrument designers do their best to provide redundancy where possible. But there are always parts on the instrument that, should they fail, will end the mission for that instrument.

In addition, many missions have hardware that must be deployed. For example, a solar array may need to be driven from a folded to an extended position, or a door may need to be opened to provide a sun-shade to an instrument. For WISE, the sole instrument on board has a cover protecting the cryogenically cooled interior. The cover keeps out water that would freeze to the interior surfaces, possibly obscuring the detectors. This water originates on Earth, but is carried into space on various surfaces of the observatory, such as within the thermal blankets protecting portions of the spacecraft. Once the observatory has had sufficient time in space for the water to have sublimated away (outgas time), the cover can be removed. In this case, the cover deployment currently scheduled for December 29th will be carried out by firing three explosive fasteners.

Should a deployment fail to occur, it can often mean a mission-ending situation. There is no redundancy in the WISE cover (i.e. you cannot remove an alternate cover and get science data). As a result, this is a very critical activity. In addition, ground testing typically does not exercise the firing of pyrotechnics. Rather, the signal used to release the nuts can be tested prior to installation, and the response of the nuts to such a signal can be tested on duplicate hardware. The upshot is that this deployment is likely the first and only time the entire system has ever been tried all together.

It may seem like this is a foolish method, but unfortunately some systems are simply untestable on the ground. Spacecraft engineers work to test such hardware as thoroughly as possible. However, when deployment time comes, we are all holding our breath until the pyros blow correctly and the cover comes off…

It’s Alive…..Alive!!!

Posted in Astronomy, Mission Operations, Observatories on 12/22/2009 by clairdeluna

Finally, after much consternation among solar scientists everywhere (okay, that’s a bit of an exaggeration) the Sun has awaked from one of the deepest solar minima in recent history. You can see a total of five, FIVE, active regions in this extreme ultraviolet picture from SOHO:

The SOHO observatory has monitored solar activity since 1995, sitting at the L1 Lagrangian point between the Earth and the Sun. This stable position, four times farther than the Moon’s orbit, offers an unimpeded view of the solar disk. The science return from this observatory has been incredible, drastically increasing our understanding of the star that powers our planet and lives.

This feat is even more amazing considering SOHO’s history. In 1998, a problem on the observatory ended with the failure of all gyroscopes. For a mission designed around the ability to self-stabilize at the L1 gravitational minimum, such a loss seems mission-ending. However, operations engineers learn early that it’s necessary to work with what is available, even though the situation isn’t optimal. In this case, an amazing effort to rescue the SOHO observatory ensued, and the spacecraft engineers devised a method to use the observatory successfully without gyroscopic stabilization.

The extreme environment of space eventually degrades or destroys the spacecraft we launch. But until we are forced to accept that an observatory is irretrievable, operations engineers will work to keep the mission going. As a result, most missions end due to funding cuts, not hardware failure.

Words have meaning: Delta-V

Posted in Mission Operations, Word Meanings on 12/14/2009 by clairdeluna

Over at PlaneTalking, Ben Sandilands reports on “Delta V,” a joint venture between Virgin Blue and Delta Air Lines. This caught my eye, because “delta-v” means something completely different in mission operations.

For spacecraft in orbit, there are several forces that can alter the trajectory over time. Atmospheric drag slows the spacecraft, causing the orbit to decay. Pressure from the solar wind can push the spacecraft to lower altitudes. Even the gravitational effects from other bodies (most notably the Moon, Sun, and Jupiter) can affect spacecraft orbits. At times it is necessary to modify that orbit to compensate for these effects. This is accomplished by firing the thrusters to change either the altitude or inclination of the orbit. A change in altitude is accomplished by changing the velocity of the spacecraft, hence the term delta-v. Delta is the mathematical symbol for “change in,” and v stands for “velocity.”

Delta-v maneuvers are a regular part of operations management for certain low-earth orbiting satellites. In fact, any spacecraft that requires a particular orientation with respect to the body it is orbiting (like geosynchronous satellites), or that needs to maintain formation flying with other nearby satellites, will need delta-v maneuvers from time to time. The need for regular delta-v maneuvers increases the payload size by necessitating the inclusion of a propulsion subsystem, and increasing the propellent load the satellite must carry. Recent safe-ocean disposal requirements have made propulsion a must on new NASA spacecraft to allow for a controlled de-orbit (as was performed several years ago with TRMM).

Such maneuvers are also used to help guide interplanetary missions to their targets. Small changes in velocity early in the trajectory translate into large shifts in position at the target location. Even a mission with a single destination (like MRO) requires multiple delta-v maneuvers and a skilled navigation engineer. The more complex the flight plan, the more maneuvers will be required. Luckily, when dealing with such large distances, the amount of propellant needed for these adjustments is reasonably small.

In unmanned space missions, a common propellant is hydrazine, which was used in all three missions I have launched. This highly volatile substance ignites immediately upon release, and does not require an oxidizer to produce thrust. So, while you often hear such maneuvers referred to as “burns,” this particular fuel is not actually burning (oxidizing). For missions where delta-v maneuvers are needed solely to de-orbit the spacecraft, the propulsion system is often left “dry,” remaining untested until the mission is complete. This reduces risk that a failure of the system could terminate the mission prematurely.

WISE Launching!

Posted in Launches, Observatories on 12/14/2009 by clairdeluna

Go check out the launch on NASA TV! (Now that the launch is complete, this link is no longer showing the WISE launch. However, you can watch the replay below.)

Update: Once again, despite my constant expectations, the rocket didn’t blow up! Don’t believe me? Here’s the replay…

Words have meaning: Hold (Hold Hold)

Posted in Launches, Mission Operations, Observatories, Word Meanings on 12/13/2009 by clairdeluna

The last few minutes of a launch are very critical. It is at this time that systems on the launch vehichle, and (for some missions) the payload, are switched from using an external power source to using their internal batteries. For Delta launches, this switch usually occurs four minutes before launch, and gives the final insight into the complete health of the system. At this point, if any data indicates a possible problem it is necessary to quickly stop the launch.

In mission operations, words that indicate a specific, time-critical action must be very precise. In this situation, the words used by an engineer to the launch director to stop the launch are:

Preparing to launch a multi-million to multi-billion dollar observatory requires practice. So the last six months prior to launch are filled with practices. “Mission rehearsals” that exercise the post-launch and activation process are very complicated, and are run repeatedly by the mission operations team. “Launch rehearsals” that practice the pre-launch process are performed jointly with the launch vehicle operations team several times before launch. During launch rehearsals, one of the hardest things for me to learn was never to say the word “hold.”

Saying “hold hold hold” only has consequences if you say it on the particular communications channel (voice loop) that is connected to the launch operations team, and I never had any reason to be on that loop. Even so, I spend the last few months finding alternative words to use. “Please carry this for me,” rather than “Please hold this.” “Can you wait a minute,” rather than “Can you hold on.” And the list goes on…

It seems like such an innocuous word, hold, until you realize that with that one word you have the power to stop a launch. And right before launch, even a one day delay is a LOT of manpower, and money. But that’s not nearly as painful as the investigation that would follow a failed launch.

You hope you never have to say “hold hold hold.” But if something’s not right, it’s absolutely the right thing to do…

Words have meaning: (Launch) Window

Posted in Mission Operations, Word Meanings on 12/11/2009 by clairdeluna

Word today that WISE launch is now scheduled for Monday, December 14, 2009 (6:09:33 – 6:23:51 a.m. PST ). For those of you paying attention, you may notice that while the date changes, the time of the launch is still the same. This time range is the “launch window.”

The timing for the launch window can be as short as minutes, and as long as weeks. It all depends on where you are trying to put your spacecraft. For any mission that must operate in a particular orientation with respect to the Earth (this would be pretty much anything that directly orbits the Earth), the window is on the order of minutes. That’s because the Earth turns relatively quickly, rotating 15 degrees per hour. The more precise the needed orbit, the shorter the window becomes. It’s hard to compensate for an orbital offset once you’re in space.

Missions with more distant targets, like the moon, other planets, comets, or one of the Lagrangian points, have a better chance of compensating for changes in launch time. Small perturbations of the path early on translate into much larger shifts in position after they’ve travelled millions of miles. This means the timing of the launch window is typically a function of the relative positions of bodies in the solar system. In fact, more important than knowing where the planets/moons/etc. are now, is knowing where they WILL be at the time you want the spacecraft to rendezvous with them.

After all, if they miss, there’s no turning back…

(Hat tip to palmerin for the ability to sleep in!)

WISE Launch Slips to Saturday

Posted in Launches, Mission Operations, Observatories on 12/11/2009 by clairdeluna

Note: As palmerin mentions below, the launch is now scheduled for Monday. Enjoy the free weekend! I bet the mission team is frustrated!

The WISE launch, originally scheduled for yesterday, has slipped to Saturday morning. The current WISE mission launch page indicates that the reason for the slip is a “problem with a booster steering engine.” Such a problem could range from relatively minor (two sensors giving incompatible readings) to serious (some hardware failure could make the booster unsteerable). But with a 24 hour hold, it’s probably a minor issue.

When your launch window is less than 14 minutes (6:09:33 – 6:23:51 a.m. PST for Saturday), you only get one shot at launching. The typical Delta-II recycle time (how long it takes to get a second try) is about 20 minutes. I’ve seen it done in 17 minutes, and that was very fast. If the problem requires more than a few moments to figure out, the launch is usually scrubbed (postponed) and rescheduled for a later time. How long they wait depends on the severity of the problem. The minimum time is however long it takes to get to the next launch window that will put the spacecraft into the proper orbit. For a mission like WISE, that time is probably 24 hours, hence the 24-hour delay.

Sadly, that means those of us who are launch junkies have to actually drag ourselves out of bed on Saturday morning if we want to watch the launch. But hey, at least we aren’t gonna have to work the weekend!