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.