Friday, May 17, 2013

Kepler Loses Another Reaction Wheel.

It was Deja Vu all over again yesterday, as the Kepler team at NASA announced the probable failure of the spacecraft's second reaction wheel. Kepler is a NASA satellite searching for exoplanets (planets outside our solar system) by finding stars with periodic variation in their brightness, which would indicate planets passing in front of them. Kepler needs three working reaction wheels in order to maintain the necessary level of pointing precision for science operations. It lost reaction wheel #2 back in 2012, so it's been operating without a spare ever since. I first wrote about Kepler in January 2013, when Reaction Wheel #4 (the second reaction wheel to exhibit problems) was showing signs of degradation and poor performance. After a ten day rest period, which operators hoped would redistribute lubricant in the reaction wheel and allow it to spin more easily, the Kepler team returned the spacecraft to full operations with the hopes that the wheel could hang in there. Turns out, it couldn't.
The location of Kepler's reaction wheels.
Source: NASA

On Wednesday NASA announced that the wheel seemed to have failed again. During a semi-weekly contact with Kepler, operators found the spacecraft unexpectedly in safe mode, which is what the spacecraft does when it detects something is wrong. In this mode it ceases science operations and faces the sun to gather as much power as possible, allowing the communicating line with Earth to drop in and out as it spins around the Sun. When operators commanded the reaction wheels to stop rotating the vehicle in order to maintain the communication link, reaction wheel 4 stopped spinning, but maintained full torque, meaning that it was trying as hard as it could to spin up but was not moving at all. This behavior likely indicates a structural failure of the wheel bearing. In other words, bad news bears.

NASA says the Kepler mission was originally only 3.5 years, so technically Kepler completed all of its mission goals. However, it is standard for missions like this to low-ball the life expectancy so that they can tell congress it will cost less, and claim 100 percent mission success even if something goes wrong later in the life of the spacecraft. In reality, missions are often extended for 2-5 times the original mission life. When Kepler reached its original 3.5 year mission goal in 2012, funding for the satellite was extended for six more years. There had been more noise than anticipated in Kepler's data, due to worse than expected signal and more noise than anticipated in the spacecraft, so observations were taking longer than originally expected. It was estimated that Kepler would take until 2015 to fulfill all of it's mission goals. So, if Kepler is headed for hospice, it will have reached it's originally planned mission life, but not entirely fulfilled it's mission goals.

But all is not lost! Kepler operators have not given up on attempts to resuscitate the wheel. They plan to try jump starting it by sending start and stop commands in rapid succession, and perhaps spinning it in the opposite direction. If all attempts fail, they may investigate pursuing other science goals using a scanning mode, where 3-axis control is not necessary.

Even if rejuvenation in some form is not possible, the mission is far from a bust. Kepler has made significant contributions to science and the search for planets. It has identified 2740 candidate stars, and led scientists to estimate that there are at least 17 billion Earth-sized exoplanets in the Milky Way Galaxy alone. There is also a huge trove of data from Kepler that has not been analyzed yet, and will provide fodder for research and discovery for years to come. There may very well be data revealing an Earth-sized star in the habitable zone of a far-away planet, just sitting on a server somewhere waiting to be extracted. And Kepler is just the beginning of that search - NASA is already planning the next generation of exoplanet-finding telescopes, starting with TESS, which would launch in 2017.

Wednesday, February 13, 2013

Landsat 8 Launches Successfully!

The title of this post could actually be "Live video of Landsat launch blows my mind". Landsat was launched atop a United Launch Alliance (ULA) Atlas V 401 rocket with live video coverage from aboard the rocket throughout the various stages of the launch streaming to the web for anyone to watch. The best part of the coverage was the end, when a camera on the rocket booster showed the separation of Landsat 8 from the booster, with a backdrop of the rising sun above a crescent Earth. See for yourself, and just try to hold your jaw closed:

Those flashes visible in the lower right hand corner as the satellite drifts away are not UFOs, as some might think, but are likely tumbling space junk catching the light of the rising Sun.

NASA reports that the satellite began communicating with Earth and charging its batteries shortly after separation from the booster rocket, and is doing well. However, the mission has been having issues with its remote ground stations in Fairbanks, AK and Sioux Falls, SD, which pose little threat to the spacecraft and will hopefully be sorted out soon.

The Landsat Data Continuity Mission will be renamed to Landsat 8 and handed off to the US Geological Survey after the satellite has been commissioned. The Landsat program has been imaging the entire globe every 8 days since 1972 at a 30 meter resolution, providing beautiful and informative pictures and enabling scientists to track the evolution of the Earth due to both natural and unnatural causes. Here are a couple of my favorite Landsat pictures:
The shrinking of the Aral Sea due to the diversion of its waters for irrigation in central Asia.
A complete, cloud-free image of Antartica, pieced together from hundreds of images taken over time.

Monday, January 21, 2013

Planetary Resources Reveals Telescope Prototype

An artist's rendition of the Arkyd-100 in orbit.
Planetary Resources, the startup aiming to make billions of dollars mining platinum from asteroids in our solar system, recently revealed their prototype for the Arkyd-100 Space Telescope. A fleet of these telescopes will eventually be launched to search for minable, accessible, asteroids. It is an amazingly small and compact 11 kg satellite with small deployable solar arrays - significantly smaller than the prototype they previously debuted. The telescope is compact for launch, but can be extended several inches once deployed in space, increasing its focal length.

Among the most interesting revelations is the company's plan to use the telescope for laser communications rather than relying on a network of radio dishes. Laser communications is a really neat technique. Basically, instead of sending telemetry and images over modulated longer-wavelength radio waves (like most satellites do to communicate with ground stations on Earth), you use shorter-wavelength waves in the visible spectrum. This has many benefits:
  • Because shorter wavelengths have higher energy, you can transmit information at a much higher density, or data rate.
  • Radio and television stations transmit in the radio wavelengths, so anyone operating a radio transmitter needs a license to avoid interference. The optical spectrum has none of these restrictions.
  • Laser receptors/transmitters on the ground are much smaller and cheaper to build and operate than their radio counterparts.
  • For satellites with imaging payloads, the telescope can be used both as an optic to collect light (take images or receive information) and as a laser to send it (transmit images to the ground). there is no need for a separate radio antenna that takes up room and adds mass to a satellite.
The main drawback of laser communications is that water vapor in Earth's atmosphere absorbs waves in the visible spectrum (that's why it's dark on a cloudy day). Moreover, it requires extremely precise pointing in order to hit receptors on Earth. This is why long-range laser communications are currently only widely used in space-based satellite to satellite communications where water vapor is not an issue. However, if these obstacles can be overcome (by using continual weather analysis to find clear areas with no clouds and arid climates to transmit, and building robust stabilization systems to point the spacecraft), then laser communication can be an incredibly efficient and cheap option. Planetary Resources says it is under contract with NASA to develop this technology for use on future government-funded satellites.

The company says it hopes to sell its satellites to other companies to help fund its ultimate goal of retrieving and selling valuable platinum from an asteroid. I will certainly be watching as their design matures and progresses!

You can watch their informational video, which includes a small tour of their facilities, here.

Friday, January 18, 2013

Kepler Reacts to a Sticky Wheel

In a Kepler Mission Manager Update, NASA announced today that the Kepler mission is temporarily ceasing science operations due to increased friction in reaction wheel #4. Kepler is a NASA satellite that has been searching for Earth-like planets around nearby starts since it launched in 2009. Reaction wheels are used to maneuver satellites and other spacecraft requiring fine, continuous pointing control. A motor spins up the reaction wheel in one direction, which causes the spacecraft to spin in the opposite direction. Spacecraft almost always have 4 reaction wheels: one for each axis of rotation, and one spare. If two or more reaction wheels fail, the spacecraft loses it's ability to attain and hold commanded positions. For a telescope like Kepler, where steady pointing is critical to collecting high quality images of faint stars, this is a mission ending-anomaly.

The type of reaction wheel used on Kepler
Source: Ball Aerospace
This isn't the first road bump for Kepler. The spacecraft experienced problems with reaction wheel #2 back in July 2012, and took steps to mitigate future risk at that time. Among other things, they decided to increase operational temperatures  (likely to keep the lubricant in the wheels warmer);  increase spin-rates (because the faster wheels spin, the less likely they are to get stuck, and the more the lubricant gets distributed nicely), and implement bi-directional rotation (to even out any use-related wear and tear).

Reaction wheel side view
The Kepler operations team is dealing with this latest threat to the mission by putting Kepler in a ten day "rest period" to give the wheel's lubricant time to become more evenly distributed. During this time the wheels will not be used, and a modified safe mode attitude (where the solar arrays are pointed at the sun to keep the spacecraft power-positive) will be maintained using thrusters.

Reaction wheel problems seem to be prevalent lately, especially in high profile missions. In August, Operators for the Dawn Mission also experienced complications or failures with two of its reaction wheels. Dawn had already lost a reaction wheel in June 2010 when it lost a second wheel in August. It is now using an inventive combination of the two remaining reaction wheels and its hydrazine thrusters to point the spacecraft (this solution is not sufficient for the strict pointing requirements of Kepler).

It is interesting to note that the operations teams identified the anomaly differently in each case. Kepler operators noticed that the amount of torque needed to change the spin rate of its reaction wheel was higher than normal during a semiweekly contact with the spacecraft, indicating increased friction in the wheel. Dawn operators became aware of the problem when the spacecraft software detected the problem and initiated an automatic shut down of the wheel.

Monday, September 3, 2012

What (on Earth) is a Space Elevator?

The number one roadblock on the way to space is access to launch vehicles. The cost of launch is by far the most expensive part of a satellite campaign; You can build a high quality, small, capable satellite for 100,000 dollars if you are creative and thrifty, but a ride to space on a rocket costs at least 2-3 million, and even then smaller payloads have almost no control over when and where the rocket launches.

In addition to being outrageously expensive, rockets are also bad for the environment. The Space Shuttle Main Engines burn about a half-million gallons of fuel during lift-off and acceleration, producing 28 tons of carbon dioxide, and, according to Discover Magazine, "23 tons of harmful particulate matter settle around the launch area each liftoff, and nearly 13 tons of hydrochloric acid kill fish and plants within half a mile of the site."  Furthermore, rockets are not reusable.

Besides the cost and environmental damage, rockets are complicated! Rocket engines and their fuel systems are so complex that only three countries have ever put people in orbit. And once you figure out how to do it, the risk of something going wrong is incredibly high. In the last 45 years, 15 people have died during the take-off or re-entry phases of their mission.

It makes you wonder who said "Hmmm. Clearly the easiest, safest way to get to space is to fill a giant tube with explosives, put some people in the tip, and light it on fire." (For the record, it was Jules Verne who, in 1865, wrote the novel that inspired the movement to send people to space in rockets, although he proposed using a giant cannon to escape Earth.) Why not, say, build a really long ladder reaching from Earth into space, and keep climbing until you have escaped the Earth's gravitational pull, then let go? Ouala! - you are in orbit.
Tsiolkovsky, looking old and wise.
(Source: Wikipedia)

In fact, someone did have this idea. In 1885, 30 years after Verne published his novel, Russian scientist Konstantin Tsiolkovsky was inspired by the Eiffel Tower to propose just such a system. His design included a "celestial castle" at the top of a spindle-shaped cable that reached out 22,238 mi above sea level, which is the altitude required for geo-stationary orbit.

Tsiolkovsky never built his elevator (though he is considered a founding father of rocketry), but the basic principles behind space elevator design have not changed since his original proposal. All space elevator designs include a giant cable attached to Earth's equator that reaches up into space. The centripetal force provided by the Earth's rotation keeps the cable extended and stationary above a point on Earth. (Imagine holding on to a long string and spinning around in place - the string flies around you in your orbital plane, extending out from your hand). Designs also include a counter weight at the space end of the cable - like Tsiolkovsky's celestial castle - that provides enough mass to keep the cable straight. (Tie a tennis ball to the end of your string - it now flies out straight from your arm rather than curving away from the direction of motion). Once the cable is in place, a robot can simply climb the cable, starting from Earth. Traveling at 180 mph - the speed of a fast train - a climber could reach GEO orbit in about 5 days. And by the time it got to the end, it will have achieved orbital velocity (taken from the Earth's rotation). It could toss a satellite out the window and call it a day. No rockets required.

So why don't we have a space elevator yet? 
Diagram of a space elevator. The height relative 
to the diameter of the Earth on the diagram is 
to scale. The height of the counterweight varies 
by design and a typical, workable height is 
shown. Source: Wikipedia.
The major difficulty is in finding a sufficiently strong material with which to construct the cable. Tension on the cable increases with distance from Earth, so the cable must grow exponentially wider as it moves away from Earth in order to sustain the force of the cable below it, and provide a centripetal force to the cable and counter-weight above it. (If you use a flimsy string to swing your tennis ball around, it breaks by the tennis ball, not near your hand, right?) The cable must therefore be made of a material with an extremely high specific strength. Currently, the only materials with the requisite strength are carbon nanotubes, which were developed at MSFC in the 90s, and are difficult to create and connect to form a long, low-mass cable while still preserving their strength, though research continues.

Other challenges include handling space-trash collisions at LEO altitudes without damaging the cable, designing a climber that can scale a cable that varies in thickness (some current designs have rollers that use friction to roll up the cable), and deciding what to use for the counter-weight (maybe material that is ferried up from Earth along the cable, or perhaps a captured asteroid?). There are also more complicated concerns, like managing cable oscillations and vibrational nodes as climbers move up and down, or satellites are launched off of the cable at various altitudes. 

Though it is not necessarily on the general public's radar, the space elevator concept is being actively explored by the space community. NASA has long supported the idea, mostly through financing competitions and prizes to encourage innovation in the area. The US-based group Liftport hopes to use crowd-sourced fundraising (via Kickstarter) to design and build a moon-based space elevator, which would allow astronauts to gently descend to the lunar surface from orbit, and then climb away again. Astronauts or robots would launch from Earth in a rocket and rendezvous with the elevator cable at a base station located in the L1 Lagrange point between Earth and the Moon. (Here is a video of a test the group performed 6 years ago using a prototype robot to climb a cable held aloft by giant helium balloons).  In February the Daily Yomiyuri reported that Japanese construction firm Obayashi Corp, (the same company that is currently refurbishing the Golden Gate Bridge), has announced plans to build a space elevator by 2050 that would shuttle 30 people to space at a time, and in November 2011 the New York Times reported that Google has been secretly working away at a space elevator design in its Google X lab in Mountain View. 

With private companies like SpaceX, Virgin Galactic, and Sierra Nevada Corp (to name just a few) entering the market and competing for contracts at a faster pace than ever before, I think it is probable that we will have cheaper, more reliable rockets years before we have a cheap, reliable space elevator. But that is beside the point. What will happen when a weekend trip entails a casual train ride the edge of outer space? Or when designing satellites to be small, light, and compact is no longer necessary? Or when you can stretch out a net to capture passing space trash, or build a permanent science laboratory at any altitude? A space elevator wouldn't just replace rockets, it would open up outer space to a whole new world of possibilities. 

Saturday, August 25, 2012

Mars Science Laboratory Beams Back Landing Video

The HIRISE spacecraft photographed MSL as it was parachuting down to
the Martian surface. Credit: NASA.
I haven't posted about the Mars Curiosity rover yet, because it has been thoroughly covered by other news sources and I've just been enjoying reading about all of its success so far. However, NASA just released the video of Curiosity's descent, as viewed from the spacecraft itself. It is pretty incredible to see such high quality video of the fantastic landing, which was a feat of creativity, sophisticated software, and brilliant systems integration.

Mike Wall, from, describes what you will see in the video:
The high-definition video chronicles the final 2.5 minutes of Curiosity's 7-minute plunge through the Martian atmosphere in real time, starting just after the rover jettisoned its heat shield. The first few seconds show the heat shield falling away toward the red dirt of Gale Crater far below. 
Other milestones follow, such as parachute deploy and ignition of the engines on Curiosity's "sky crane" descent stage, which lowered the 1-ton rover to the Martian surface on cables. Audio from mission control at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., plays over the video, describing the nail-biting action
As Curiosity nears the Martian surface, huge dust clouds billow around the $2.5 billion robot, kicked up by the sky crane's rockets. Then the view clears to show a close-up, static shot of scattered pebbles.

If you haven't seen it yet, watch 7 Minutes of Terror, the movie trailer-esqu video simulation of the landing sequence before watching the real thing.

7 Minutes of Terror:

Real video of MSL descent:

Sunday, August 19, 2012

Proba-1 is Back in Business.

ESA's Proba-1 microsat was built and launched by the ESA as a proof-of-concept for semi-autonomous Earth observation - operators can upload coordinates and the satellite will autonomously image that location. The satellite ended up turning into a heavily-used science tool, but it almost shut down in May due to star tracker degradation. 

Proba-1 captured this image of London's Olympic Park
 neighborhood.   Credit: ESA 
Many small satellites use star trackers to orient themselves. The star trackers point behind the satellite (away from the Sun and Earth) and take pictures of star fields, which they can identify and use to calculate the satellite's orientation.  The CCD camera's attached to Proba-1's star trackers had sustained severe radiation (they were 5 years older than the planned lifetime for Proba-1), and the radiation had created permanent damaged or "hot" pixels which  show up in images as white spots. The star trackers were mistaking hot pixels for stars, and sending incorrect coordinates to the main satellite computer, causing it to error out or mispoint. 

Proba-1 was saved by a software patch written specifically for the satellite, which - once loaded - allowed the star trackers to distinguish between hot pixels and real stars. The star trackers are apparently working "as good as new" now, even with degraded CCDs. The proof is in the pudding - at the right is a farewell shot of Olympic Park that the satellite shot last week.