Two satellites came close to colliding in orbit Wednesday, highlighting the potential hazard posed by debris in space. The derelict Poppy VII-B and IRAS spacecraft made their closest approach – predicted to have been within 47 meters – at 23:39 UTC high above the US state of Pennsylvania. Had a collision occurred, it would have left a field of debris in low Earth orbit.
There has only been one accidental collision between two intact satellites in different orbits. This occurred eleven years ago, involving a then-operational satellite in the Iridium communications network and a defunct Russian communications satellite. In contrast, both of the satellites involved in Wednesday’s near-miss had been inactive for at least 36 years.
Poppy VII-B is a military satellite that was launched by the United States in the 1960s, weighing about 83 kilograms (183 pounds). The Infrared Astronomical Satellite (IRAS) is a retired scientific satellite, which conducted the first infrared all-sky astronomical survey in 1983. It had a mass at launch of 1,073 kilograms (2,366 lb).
The close approach between the two satellites occurred about 900 kilometers (559 miles, 486 nautical miles) above the United States, near the city of Pittsburgh. Had a collision occurred it would have posed no danger to anyone on the ground; conservation of momentum means that any debris would have fanned out in orbit, and even if or when fragments began to reenter the atmosphere, they would have burned up before getting anywhere near the ground. Had the satellites collided, the biggest effect would have been an increase in the amount of small debris objects in orbit, slightly increasing the risk of damage to other satellites from subsequent collisions.
@18SPCS confirmed the 2 inactive satellites (IRSA & GGSE-4) crossed paths without incident. 18th SPCS monitors space debris 24/7/365 & issues conjunction notifications every day to all Nations to support space flight safety. #KeepSpaceSafe pic.twitter.com/vijr6KH7hC
— U.S. Space Command (@US_SpaceCom) January 30, 2020
Satellites are involved in collisions from time-to-time. However, these usually involve either tiny pieces of debris that can damage satellites, but not necessarily destroy them, or pairs of satellites which had intentionally been operating in close proximity coming into unplanned contact. Most anti-satellite (ASAT) weapons also rely on collisions, either between two spacecraft in different orbits or an orbiting satellite and a suborbital interceptor.
Anti-satellite tests are not generally considered collisions, but instead intentional shootdowns, but they contribute to the same problem of orbital debris that both causes and results from collisions. China, the United States and India have all conducted anti-satellite tests in the last fifteen years. China’s 2007 test, which shot down the Fengyun-1C weather satellite, remains the single largest source of debris in orbit to date, with thousands of fragments created.
The only previous collision involving intact satellites that came together by chance took place on 11 February 2009. The satellites involved were Kosmos 2251 satellite, a Russian Strela-2M communications satellite which had been launched in 1993, and the Iridium 33 spacecraft which had been launched in 1997 for US operator Iridium. These satellites were much closer in size than the two which came close on Wednesday, with Kosmos 2251 weighing in at about 800 kilograms (1,760 pounds) and Iridium 33 about 689 kilograms (1,519 lb). The 2009 collision is the second-worst cause of debris in orbit behind the Chinese ASAT test.
There have been several instances of satellites colliding at low relative speed while operating in the same orbit – for example during rendezvous and docking operations. The best-known example is the collision of the Progress M-34 logistics spacecraft with the Mir space station in June 1997 while it was being used to demonstrate a manual docking technique. While the collision caused significant damage to Mir it did not produce the same cloud of debris which can result from high-speed collisions.
Debris in Earth orbit poses a risk to other satellites because at orbital velocity closing speeds even a small fragment can do significant damage – especially if it hits a sensitive area of a spacecraft such as an instrument.
Micrometeoroid/Orbital Debris (MMOD) impacts – which may be of natural or man-made origin – have been observed on uncrewed satellites and crewed missions including the Space Shuttle and International Space Station. Where possible, operational spacecraft will be maneuvered to avoid close approaches with other satellites or known large pieces of debris if there is even a slight chance of an impact occurring.
Wednesday’s near-miss involved two long-defunct satellites: Poppy VII-B, a US military satellite from the 1960s, and the Infrared Astronomical Satellite (IRAS) which was part of a NASA-led multinational science mission in the 1980s. Since neither satellite was functioning, it would not have been possible for either to maneuver to avoid the potential impact.
Ironically, the space journeys of both the satellites that came close to colliding on Wednesday began – fifteen and a half years apart – at the same point on Earth’s surface: the west pad of Space Launch Complex 2 (SLC-2W) at California’s Vandenberg Air Force Base.
Poppy VII-B is the older of the two satellites, deployed by the final Thor SLV-2 Agena-D rocket – the last version of the Thor-Agena not to use solid rocket boosters – as part of a multi-satellite launch on 31 May 1967. The satellite was part of “Program C”, a partnership between the National Reconnaissance Office (NRO) and Naval Research Laboratory (NRL) to build electronic signals intelligence (ELINT) satellites following-on from the NRL’s earlier Galactic Radiation and Background (GRAB) missions. Also known as “Poppy” or “Dyno”, the project was kept classified until its acknowledgment by the NRO in 2005.
The Poppy program was initially intended to focus on a collection of signals from Soviet radar installations, helping to pinpoint the country’s air defenses. After the first few launches, Poppy satellites began launching in groups of two or three which allowed them to triangulate the origin of radio sources. Poppy VII-B formed part of a trio with the Poppy VII-C and VII-D satellites; Poppy VII-A, which was built to an earlier design, was also launched on the same rocket.
As Poppy continued it gained a greater emphasis on detecting and tracking ships from their radio signals. The project later evolved into White Cloud or Parcae, the first generation of the Naval Ocean Surveillance Satellite (NOSS), a naval signals intelligence program run by the NRO which continues to this day.
In addition to its intelligence-gathering role, Poppy VII-B carried Gravity Gradient Stabilization Experiment 4 (GGSE-4). This was the fourth in a series of five experiments hosted aboard Poppy satellites aimed at testing techniques for maintaining the spacecraft’s attitude using Earth’s gravitational field. Unlike Poppy, the GGSE project was not classified so the name GGSE-4 was used to refer to the whole satellite prior to Poppy’s declassification. Poppy VII-B is also known by its Naval Research Laboratory payload number, NRL PL-152.
The Poppy VII-B satellite has a 12-sided, roughly ovoid, shape described by the NRO as “multifaceted”. Power came from solar panels mounted on its surface. The spacecraft measures about 69 centimeters (27 inches) in diameter and weighed 83 kilograms (183 pounds). Due to the secondary mission – to study gravity-gradient stabilization – it may have deployed a boom that would have increased its cross-section. The length of the satellite’s service remains classified. However, it was long dead before Wednesday’s conjunction with IRAS.
IRAS was conceived as a partnership between NASA, the Netherlands Agency for Aerospace Programmes (NIVR) and the Science and Engineering Research Council (SERC) of the United Kingdom.
It was one of the most significant space telescope missions of its era: during a ten-month mission, it became the first satellite to perform an all-sky survey at infrared wavelengths. In doing so, IRAS observed over 96% of the sky, characterizing over a quarter of a million sources of infrared light in the cosmos. As well as conducting its survey, IRAS was also used for targeted observations of specific objects or celestial fields, using longer exposures to capture its objectives in more detail.
A Delta 3910 rocket carried IRAS to orbit on 26 January 1983 – 25 January at the launch site – in a mission which also carried the Plasma Interaction Experiment 2 (PIX-2), a package that remained bolted to the Delta rocket’s upper stage. IRAS separated into low Earth orbit shortly after launch.
The IRAS satellite is far larger than Poppy VII-B, measuring 3.60 meters (11.8 feet) in length, with a diameter of 2.05 meters (6.73 feet). The solar panel that was deployed from the satellite’s body to power it while operational gives it a span of 3.24 meters (10.6 feet). IRAS had a mass at launch of 1,073 kilograms (2,366 lb) although this would have decreased as consumables were expended during its mission.
The IRAS satellite’s lifespan was limited by its supply of coolant. To observe at deep infrared wavelengths, the optics of satellite-based telescopes need to be kept at cryogenic temperatures. IRAS carried a cryostat with 73 kilograms (161 pounds) of liquid helium which was allowed to evaporate, helping to cool the instruments. This allowed the telescope’s mirrors to be kept at about 4 Kelvin (-269 degrees Celsius, -452 degrees Fahrenheit), while the focal plane was cooled to 3 kelvin (-270 degrees Celsius, -454 degrees Fahrenheit).
On 21 November 1983, IRAS ran out of liquid helium and was unable to continue infrared observations. The satellite was decommissioned shortly afterward. As no procedures were in place to deorbit the spacecraft – typical for a mission of its era – the spacecraft was left in orbit to decay naturally over time.
A copy of the satellite catalog maintained by Jonathan McDowell shows the original orbit of Poppy VII-B to have been around 915 by 926 kilometers (569 x 575 miles, 494 x 500 nautical miles) with 70.0 degrees inclination, as of 4 July 1967. By contrast, IRAS was in an 893 by 911 kilometer (555 x 566 mi, 482 x 492 NM) orbit, inclined at 99.1 degrees, while in service on 4 March 1983. By 29 January 2020, Poppy’s orbit had decayed to 902 by 917 kilometers (560 x 570 mi, 487 x 495 NM) while IRAS was measured to be in an 886 by 912 kilometer (551 x 567 mi, 478 x 492 NM) orbit.
While Wednesday’s conjunction involved two old satellites which had been in orbit for many years, and ultimately passed without incident, it highlights the growing problem of debris in low Earth orbit. Many defunct satellites have simply been left in orbit – either through lack of means to deorbit them or through failures that prevented this from being put into action. The upper stages of the rockets that put them there – particularly those with solid-fuelled upper stages or older designs not capable of restarting to perform a deorbit maneuver – also remain in space.
In recent years the increased launch of multiple small satellites – such as CubeSats used for scientific or commercial research and vast constellations of low-orbit communications satellites have raised more concerns about congestion in low Earth orbit – and although most responsible operators have effective plans to mitigate long term debris – either by operating in low enough orbits that the spacecraft will quickly decay or by developing active and passive techniques to deorbit them – larger numbers of satellites will always increase the chance of collisions.
The potential collision of Poppy VII-B and IRAS illustrated the hazard posed by these large objects, but the cloud of smaller debris it could have produced would have been more hazardous to future satellite operations. Smaller objects are by nature harder to track and identify, preventing advance warnings being generated for operators of active satellites. At orbital velocity, tiny objects can cause huge amounts of damage. Ultimately the space surrounding Earth is still very big, so additional debris from a single collision in of itself wouldn’t pose a significantly increased risk. However, in time the debris would have spread out around a range of altitudes popular for Earth observation and scientific missions.
Final update prior to close approach: 47 meter predicted miss distance, w/ increased separation in the cross-track direction. Next scheduled radar passes for both objects to occur approximately two hours after the event. pic.twitter.com/Y07Rh9dR26
— LeoLabs, Inc. (@LeoLabs_Space) January 29, 2020
Many satellites in sun-synchronous polar orbits (SSO) operate at altitudes between 600 and 900 kilometers. Spacecraft use these orbits to ensure they pass over points on the Earth’s surface at the same time of day relative to the position of the Sun. This is useful for ensuring images of the surface are taken with the same angle of illumination, or for providing weather forecasts at the same time every day. Small satellite missions have also been deployed into near-sun-synchronous orbits as secondary payloads on SSO launches.
Scientists are concerned that the increasing presence of debris in orbit could eventually lead to an event known as an ablation cascade, or Kessler syndrome. This is a worst-case scenario where debris caused by a collision could set off a chain of subsequent collisions, eventually leaving so much debris in orbit as to make satellite operation impossible. This is not an imminent threat, however – Wednesday’s events highlight the need to address the threat sooner rather than later.
Space scientists are already grappling with the problem of debris, both by ensuring adequate end-of-life plans for new satellites as they are launched and by investigating ways to capture and safely dispose of debris already in orbit. Regulations in both the United States and European Union call for newly-launched spacecraft and upper stages to either end their missions in designated disposal orbits, be recovered or deorbited, or to be left in an orbit from which they will decay naturally within a maximum of 25 years. These criteria were adopted by the International Organization for Standardization (ISO) in 2007 as ISO-24113. However, there is currently no enforceable international law or treaty governing satellite debris.
A number of CubeSat missions in recent years have tested deployable devices such as sails, intended to speed up natural orbital decay once the satellite has completed its mission. Other satellites with active propulsion systems may keep enough reserve propellant for a deorbit burn, ensuring they re-enter the atmosphere in a targeted location – such as an uninhabited region of the southern Pacific Ocean that has become known as the Spacecraft Cemetery. In some cases a spacecraft that can’t deorbit itself completely might still lower its orbit, relying on the greater drag at lower altitudes to decay faster.
End-of-life disposal of spacecraft is reliant on the satellite still being under control at the end of its mission. In the event of a sudden failure, it may not be possible to put these plans into action.
Upper stages, too, are increasingly being disposed of in more safe ways. Many rockets launching to low Earth orbit with restartable upper stages will schedule an additional burn after spacecraft separation to deorbit the upper stage. This may not be possible on launches to higher orbits, where more of the rocket’s performance might be needed to inject the satellite and the increased orbital velocity might make a deorbit burn less practical. In some cases, instead of deorbiting upper stages from these missions, the stage has instead been boosted to escape velocity, sending it away from the Earth where it can pose no hazard to other spacecraft. Even if this is not possible, operators often try to vent or deplete all remaining propellant to reduce the risk of an explosion later on.
The advent of fully reusable rockets, such as Starship which is currently under development by SpaceX, will also help to mitigate orbital debris as it will give companies economic reasons to ensure parts of their rockets are not left in orbit.
In 2018 the University of Surrey launched RemoveDEBRIS, a CubeSat experiment that was carried to the International Space Station and later deployed into low Earth orbit. The satellite demonstrated techniques for actively removing debris from orbit, releasing a target subsatellite and deploying a net to capture it.
A second experiment tested a harpoon which could also be used to capture a debris object. At the end of the mission, RemoveDEBRIS was expected to extend a drag sail to hasten its own decay from orbit. However, this failed to deploy. Despite the final part of the test failing, RemoveDEBRIS demonstrated techniques that could form the backbone of future efforts to clean up large pieces of debris from orbit before collision can occur.
While it is fortunate that the close conjunction between Poppy VII-B and IRAS did not result in a collision, it shows the importance of using space sustainably. While collisions of satellites remain exceptionally rare events, it is important to remember that they can happen, and have done so in the past.
All organizations who launch or operate satellites must take responsibility for the hardware they place into orbit and ensure it does not pose a future hazard even after they have finished using it.